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Smart Nanoparticles for Drug Delivery Application: Development of Versatile Nanocarrier Platforms in Biotechnology and Nanomedicine

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The study of nanostructured drug delivery systems allows the development of novel platforms for the efficient transport and controlled release of drug molecules in the harsh microenvironment of diseased tissues of living systems, thus offering a wide range of functional nanoplatforms for smart application in biotechnology and nanomedicine. This article highlights recent advances of smart nanocarriers composed of organic (including polymeric micelles and vesicles, liposomes, dendrimers, and hydrogels) and inorganic (including quantum dots, gold and mesoporous silica nanoparticles) materials. Despite the remarkable developments of recent synthetic methodologies, most of all nanocarriers’ action is associated with a number of unwanted side effects that diminish their efficient use in biotechnology and nanomedicine applications. This highlights some critical issues in the design and engineering of nanocarrier systems for biotechnology applications, arising from the complex environment and multiform interactions established within the specific biological media.
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Review Article
Smart Nanoparticles for Drug Delivery Application:
Development of Versatile Nanocarrier Platforms in
Biotechnology and Nanomedicine
Domenico Lombardo ,
1
Mikhail A. Kiselev,
2,3,4
and Maria Teresa Caccamo
1
1
Istituto per i Processi Chimico-Fisici, Consiglio Nazionale delle Ricerche, 98158 Messina, Italy
2
Frank Laboratory of Neutron Physics; Joint Institute for Nuclear Research, Dubna, Moscow Region, Russia
3
Lomonosov Moscow State University, Moscow, Russia
4
University of Dubna, Dubna, Moscow Region, Russia
Correspondence should be addressed to Domenico Lombardo; lombardo@me.cnr.it
Received 10 October 2018; Accepted 2 December 2018; Published 27 February 2019
Academic Editor: Ilaria Fratoddi
Copyright © 2019 Domenico Lombardo et al. This is an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is
properly cited.
The study of nanostructured drug delivery systems allows the development of novel platforms for the ecient transport and
controlled release of drug molecules in the harsh microenvironment of diseased tissues of living systems, thus oering a wide
range of functional nanoplatforms for smart application in biotechnology and nanomedicine. This article highlights recent
advances of smart nanocarriers composed of organic (including polymeric micelles and vesicles, liposomes, dendrimers, and
hydrogels) and inorganic (including quantum dots, gold and mesoporous silica nanoparticles) materials. Despite the remarkable
developments of recent synthetic methodologies, most of all nanocarriersaction is associated with a number of unwanted side
eects that diminish their ecient use in biotechnology and nanomedicine applications. This highlights some critical issues in
the design and engineering of nanocarrier systems for biotechnology applications, arising from the complex environment and
multiform interactions established within the specic biological media.
1. Introduction
In the last decades, the development of novel approaches for
the construction of nanoformulations (nanocarriers) for the
ecient transport of drug molecules oers a wide range of
biotechnology applications [1, 2]. Smart nanostructured
materials can deliver drugs to the target sites with reduced
dosage frequency and in a (spatial/temporal) controlled
manner to mitigate the side eects experienced with tradi-
tional therapies. In particular, they allow resolving the main
critical issues encountered with conventional pharmaceuti-
cal treatments such as the nonspecic distribution, rapid
clearance, uncontrollable release of drugs, and low bioavail-
ability [35]. The overall eect is a sensitive reduction in
toxicity and/or adverse reactions. However, despite the
remarkable developments of recent methodologies, most of
all nanocarriersaction is associated with a number of
unwanted side eects that diminish their ecient use in
nanomedicine. This highlights some critical issues in the
design and engineering of nanocarrier systems for biotech-
nology applications, arising from the complex environment
and multiform interactions established within the specic
biological media [68].
In this article, we highlight the recent development of
nanostructured nanocarrier systems for drug delivery appli-
cations with a focus on the main properties and applications
of the main organic nanocarriers (such as polymer-based
micelles, liposomes, and dendrimers) and inorganic nanopar-
ticles (such as carbon nanotubes, gold nanoparticles, and
quantum dots). We analyse the main factors (and parameters)
that strongly inuence the design of nanostructure systems
for the delivery of active drugs and chemotherapeutics.
Hindawi
Journal of Nanomaterials
Volume 2019, Article ID 3702518, 26 pages
https://doi.org/10.1155/2019/3702518
Furthermore, we put into evidence the current status (chal-
lenges and limitations) and emerging approaches of the
nanoplatforms for therapeutic applications.
2. Nanocarriers for Drug Delivery:
Basic Properties
Conventional drug delivery systems of chemotherapeutic
agents present a number of critical issues associated with
the sensitive toxicity, poor specicity, and drug resistance
induction, which sensitively decrease the therapeutic e-
ciency of many drug systems. Nanocarrier-based platforms
are dedicated systems to the transport of chemotherapeutic
active drugs composed of colloidal nanoparticles with submi-
cron size (typically <500 nm) generally characterised by a
high surface area to volume ratio. These nanostructured pro-
totypes have enabled eective delivery of active (including
anticancer) drugs into the diseased tissues. The overall goal
of the employment nanocarriers in drug delivery applications
is to treat a disease eectively with minimum side eects,
thereby aiming at a sensitive improvement of the therapeutic
outcomes by exploiting the (patho-)physiology of a diseased
tissue microenvironment.
Modern smart nanostructured systems can be broadly
divided into organic and inorganic nanocarriers (see
Figure 1), while their physiochemical properties can be tuned
by altering their compositions (organic, inorganic, or hybrid),
dimensions (small or large sizes), shapes (sphere, rod, hyper-
branched, multilamellar, or multilayered structures), and
surface properties (functional groups, surface charge, PEGy-
lation, coating processes, or attachment of targeting moie-
ties). While a number of nanocarrier-based platforms have
been approved for the treatment of various diseases (includ-
ing tumors), many others are in dierent phases of clinical
trials [9, 10]. In the following sections, we will discuss the
main features of the dierent types of nanocarriers.
3. Organic and Polymer-Based Nanocarriers
The organic nanocarriers are carbon-based nanomaterials
that are generally characterised by a high biocompatibility
and improved drug loading capacity. They allow a versatile
control of both morphology and chemical composition,
while their colloidal stability and relatively large size allow
incorporating and carrying a wide combination of dierent
(hydrophilic/hydrophobic) drugs [11, 12]. Depending on
the preparation methods, we can subdivide them into two
main categories, namely, nanostructures that exploit the
self-assembly processes (such as amphiphilic systems) and
those that are obtained by specicsynthesis methods (such
as the dendrimers, hyperbranched polymers, chemical nano-
gels, and carbon nanotubes). It is worth noticing that the new
generation of nanocarriers often is constructed by the suit-
able combination of the two methods by exploiting the supra-
molecular approach [13, 14].
3.1. Polymer-Based Amphiphilic Nanocarriers Obtained by
Self-Assembly Processes. Many drug delivery nanocarrier sys-
tems are formed starting from basic building blocks that
self-assemble under the eects of a number of driving (non-
covalent) soft interactions, including van der Waals interac-
tions, hydrophobic eect, hydrogen bonding, hydration and
electrostatic forces, ππstaking interactions, steric and
depletion interactions, coordination bonding, and solvation
[1315]. In this respect, the amphiphilic macromolecules
provide unique and still eective opportunities for design-
ing novel materials for advanced application in drug deliv-
ery processes. Amphiphilic macromolecules possess both a
hydrophilic portion, which can be uncharged or charged
(anionic, cationic, or zwitterionic) and interacts favourably
with the surrounding water, and a lipophilic (or hydrophobic)
portion, which is usually composed of hydrocarbon chains
that tend to minimize its exposure to water. In water
Inorganic (& metallic)
nanocarriers
Vesicles
Micelles
Mul
liposome
Self-assembly & amphiphiles
Organic (& polymer-based)
nanocarriers
Synthesized
Quantum dot
Gold nanoparticle
Dendrimer Mesoporous
silica NPs
Hard nanoparticles
SPION
Solid lipid NP
Carbon
nanotube
u
l
iposome
Q
u
a
n
t
u
dot
l
d
l
De
n
d
r
i
m
er
M
e
s
o
p
o
r
o
u
s
s
ili
ca
N
Ps
C
a
r
b
o
n
n
n
n
n
anotu
be
Fe
3
O
4
Au
Figure 1: Example of the most employed organic and inorganic nanocarriers for smart application in drug delivery.
2 Journal of Nanomaterials
solutions, the hydration of the hydrophilic component as
well as the collapsing hydrophobic association of the tail(s)
causes a microphase separation with the formation of aggre-
gates, when they exceed a given concentration (critical
micelle concentration (CMC)) [15]. Control over the
amphiphilesshapes (by varying the critical packing factor
parameter Cpp) gives the possibility to develop and manip-
ulate nanostructure architectures (see Figure 2) ranging
from spherical micelles (Cpp 1/3) to cylindrical micelles
(1/3 Cpp 1/2), vesicles (1/2 Cpp 1) and lamellar struc-
tures (Cpp =1) [15], while for larger values (Cpp >1), the
amphiphiles will assemble into invertedphases [14, 15].
Owing to their characteristic structure, micelles and liposomes
(vehicles)oer special protection against degradation and a
wide range of possibilities for targeted functionalization and
combined therapy [2, 13, 14].
3.2. Micelle and Vesicle Nanocarriers from Polymer-Based
Amphiphiles. Polymers are widely used for drug delivery sys-
tems because of their biocompatibility and biodegradability
as well as ease in the design and preparation and ecient
delivery of the therapeutic active agents to the diseased
tissues. The dierent polymers have specic properties that
depend on the chemical-physical characteristics of their
building block, while the versatile modication of their
chemical groups has been employed for the functionalization
and drug conjugation of many polymer-based nanoparticles
[16]. According to Won et al. [17], by controlling the hydro-
philic/hydrophobic balance (by the modulation of the weight
fraction FWof the hydrophilic block), it is possible to obtain a
variety of shapes and morphologies of amphiphilic polymer
nanocarriers in water solution, including spherical micelles
(FW=5570%), spherical vesicles (FW=4555%), and vesi-
cles (FW=2040%).
Micelles-like nanocarriers obtained by the self-assembly
of amphiphilic polymers have attracted much attention for
drug delivery applications [18]. The micelleshydrophobic
core creates a microenvironment for the incorporation of
lipophilic active compounds (drugs), resulting in signi-
cantly enhanced solubility of hydrophobic drugs to achieve
improved bioavailability. At the same time, the hydrophilic
shell provides a stabilizing interface between the hydro-
phobic core and the aqueous medium, with the aim at
enhancing the colloidal stability and inhibiting aggregation
and unwanted interactions with other components. On the
other hand, vesicles prepared from amphiphilic polymers
(called polymersomes) present a characteristic bilayer struc-
ture with an aqueous interior core, which is able to encapsu-
late hydrophilic molecules within the aqueous interior and
also integrate hydrophobic drug molecules within the inter-
nal region of the bilayer membrane.
Recently, (tumor and intracellular microenvironment)
responsive polymersomes with diverse functions, struc-
tures, and self-assembling morphologies have been discussed
[5, 6]. Typical tumor (micro)environments can be utilized to
construct responsive block copolymer-integrated nanoplat-
forms, allowing for a triggered payload release and enhanced
imaging sensitivity. The main endogenous stimulithat
can be used as internal triggers are the (weak) acidic pH, tem-
perature gradients, a variety of specically overexpressed
enzymes, and redox species [5, 6]. The design of the hydro-
philic shell could enhance the colloidal stability of the
drug-loaded micelles in the bloodstream to achieve the long
circulation in the body when the concentration of the poly-
mer is higher than the CMC. Besides, the nanoscaled micelles
with a small size (<200 nm) reduce nonselective uptake by
the reticuloendothelial system (RES) and show the enhanced
permeability and retention (EPR) eect at solid tumor tissue
sites (passive targeting) [18].
Many biodegradable polymers show promising perfor-
mances in the drug delivery applications by providing a high
level of control over the complex structure-function relation-
ship [1519] and a controlled release of drugs by crossing the
physiological (and pathological) barriers of the living sys-
tems. Natural polymers have been widely investigated for
Vesicles
Elongated
micelles
Multilamellar
liposome
Critical packing
parameter
V
Cpp
1/3 Cpp
1
1/3
Cpp
1/2 1/2
Cpp
1
Micelles
Cpp = V/(a0 · Ic)
a0
Ic
Figure 2: Analysis of the critical packing parameter Cpp and relevant shape factors that inuence the amphiphilic nanocarrier morphology.
3Journal of Nanomaterials
drug delivery studies in the past years including chitosan,
dextran, heparin, and hyaluronan [19]. However, recent
research on the design of synthetic polymers to build various
nanostructured delivery platforms is gaining particular atten-
tion in the eld of nanomedicine. Polyesters, polycarbonates,
polyamides, and polypeptides are among the most com-
monly used synthetic polymers.
In the next section, we briey describe the most employed
polymeric species and promising polymer candidates for
the development of nanostructured drug delivery systems.
3.2.1. Poly(lactic Acid) (PLA) and Poly(lactic-co-glycolide)
(PLGA) Copolymers. Among all the commonly used biode-
gradable synthetic polymeric (bio)materials, the most
employed for drug delivery applications are the saturated
poly(α-hydroxy esters), including poly(lactic acid) (PLA),
poly(glycolic acid) (PGA),andpoly(lactic-co-glycolide) (PLGA)
copolymers [2024]. Due to their excellent safety prole, good
biocompatibility, low levels of immunogenicity and toxicity,
and the tuneable rate of biodegradation in vivo, these poly-
mers have been approved by the US Food and Drug Admin-
istration (FDA) and European Medicines Agency (EMA) as
eective carriers for drug delivery in humans.
The biodegradability of PLGA is based on the hydro-
lytic degradation through de-esterication of the polymers
(Figure 3) to generate the lactic and glycolic acid monomeric
components, which are metabolized and then removed by
the body by natural pathways (such as the Krebs cycle).
Their physicochemical and mechanical properties can be
tailored via the selection of the polymer molecular weight,
copolymerization, and functionalization. Polyethylene glycol
(PEG) is the most popular hydrophilic polymer for surface
modication of both (hydrophobic) PLA and PLGA to form
an amphiphilic block copolymer [20, 21]. Their applica-
tions have focused on drug delivery systems mainly involv-
ing nanoparticles, micelles, and hydrogels. Poly(ethylene
glycol)poly(lactic-co-glycolide) (PEGb-PLGA) diblock
copolymer micelles represent one of the most promising
platforms for drug delivery, where the hydrophobic PLGA
core can eciently encapsulate many therapeutic agents,
while the hydrophilic PEG shell prevents the adsorption of
proteins and phagocytes, thus extending the blood circula-
tion periods [23]. Copolymer conformation and critical
packing factor parameter Cpp regulate the morphology of
the self-assembled structures, thus inuencing the specic
biomedical application. Dierent structures with dierent
properties have been used in dierent copolymer combi-
nations including AB diblock type, ABAorBAB
triblock type, and alternating multiblock, multiarmed block,
and star-shaped block types (where A and B are represen-
tative of the PEG hydrophilic and the PLGA hydrophobic
segments, respectively) [2024]. In Figure 4 are reported
the PEG-PLGA diblock (and PEG-PLGA-PEG triblock)
copolymersmicellar structures and (hydrophilic/hydro-
phobic) drug encapsulation characteristics.
PEGPLGA diblock copolymer micelles have been tested
extensively in humans for the incorporation and (controlled)
delivery of small molecule drugs and many hydrophobic
anticancer compounds [2123]. Recent researches evidenced
the development of PLGA nanocarriers for the delivery of
therapeutic biomacromolecules which are able to maintain
their colloidal stability (and to maximize their loading e-
ciency) even in the harsh physiological environment condi-
tion of the diseased tissues [2426].
Chemical conjugation of the PEGPLGA copolymer
facilitates a high drug loading, characterised by a forced
localization of the drug in the inner hydrophobic chains.
Recently, doxorubicin- (DOX-) conjugated PLGAPEG
micellar nanocarriers with a higher DOX loading displayed
a more sustained drug release behavior compared with phys-
ically incorporated DOX in PEGPLGA micelles [25]. More-
over, up to 50% release of conjugated DOXPLGAPEG
micelles was obtained over 2 weeks while a total release of
physically entrapped PEGPLGA micelles took only 3 days.
PEG-PLGA nanocarrier encapsulation of proteins and
peptide drugs, such as insulin, calcitonin, and DNA, has been
reported in several studies [23]. Finally, the suitable combi-
nation of imaging and functionalized nanoparticles has
enabled concurrent diagnosis and therapy of diseased tis-
sues through the development of theranostic nanocarriers.
Recently, a PLGA-PEG-folate theranostic system was com-
bined with dual imaging tracers (namely, near infrared and
19F magnetic resonance imaging) with the chemotherapeutic
agent doxorubicin DOX [27]. The in vitro cytotoxicity assay
also showed that folate-targeted PLGA-PEG nanoparticles
were able to kill cancer cells more eciently than were
non-folate conjugated particles [27].
Finally, various preliminary animal studies have dis-
played the great potential of these PLA and PLGA-based
nanocarriers in the treatment of various diseases including
diabetes, cancer, cardiac disorder, bacterial/viral infection,
autoimmune diseases, and cartilage damage [2024].
HO
CH3CH3
O
O
O
O
O
H
O
R
HO
HO
OH
OH
Glycolic acid
H+
Lactic acid
Poly(lactide-co-glycolide acid (PLGA)
Metabolised by the body
Biodegradable
nm
Figure 3: Biodegradability of the PLGA polymer. Polymer degradation is based on the hydrolysis of the copolymer, followed by the
metabolization by the body.
4 Journal of Nanomaterials
3.2.2. Chitosan. Chitosan is a biodegradable and biocompat-
ible polymer with chemical functional groups (that typically
have positive surface charges) that can be easily modied to
perform specic functions, suitable for a wide range of
potential applications [25]. Chitosan-based nanoparticles
have been investigated in various drug delivery applications,
following dierent (parental and nonparental) routes of
administration, including treatment of dermatologic and gas-
trointestinal diseases, pulmonary diseases, and drug delivery
to the brain and ocular infections [25]. Polymeric micelle
nanoparticles based on amphiphilic chitosan derivatives
obtained by grafting hydrophobic long acyl chains have been
recently prepared via self-aggregation in water [26]. More-
over, self-assembled amphiphilic micelles based on chitosan
(CS) and polycaprolactone (PCL) were produced and used
as carriers of paclitaxel (PTX) to improve its intestinal
pharmacokinetic prole [27]. Experimental results indicated
that chemical modication of chitosan nanoparticles can
improve their targeting and bioavailability. Recent advances
highlight the use of chitosan nanoparticles for tumor tar-
geting [28], imaging and therapy (theranostic) applications
[29], and construction of targeted drug delivery systems. A
chitosan-based nasal formulation of morphine (Rylomi-
neTM) is currently in phase 3 clinical trials in the US
and phase 2 clinical trials (UK and EU).
3.2.3. Temperature-Sensitive Polymeric Nanocarriers. Recent
investigations have focused on stimulus-sensitive (smart)
nanocarriers for drug delivery, due to the possibility to con-
trol the delivery and release of drugs to a specic site at the
desired time. Many prototypes of (internal and external)
stimulus-responsive nanosystems have been developed,
including physical (e.g., temperature, light), chemical (e.g.,
redox, pH), and biological (e.g., enzymes) smart delivery
systems [30, 31].
Temperature is one of the most widely explored stimuli
for drug delivery application in cancer. Thermosensitive
micelles are comprised of polymers having thermoresponsive
blocks, which undergo a sharp change in their aqueous
solution properties [32, 33] that destabilize the micellar
structure thus allowing the controlled triggering of the drug
release [33, 34]. The rst generation of thermosensitive poly-
mers micelles was based on mere hydrophobic interactions
between polymer blocks, while more recently shell or core
crosslinking was introduced, in order to improve their sta-
bility in the circulation after intravenous administration.
Various nanoformulations of drug-loaded micelles based on
thermosensitive polymers have shown promising results
in vitro, as well as in vivo [33, 34]. Many polymers are incom-
pletely soluble below a certain temperature, known as lower
critical solution temperature (LCST), where the polymer
n
yx
m
PEG
hydrophilic PLGA
hydrophobic
PEG
hydrophilic
AAB
PEG-PLGA-PEG triblock copolymer
PEG
hydrophilic PLGA
hydrophobic
AB
PEG-PLGA diblock copolymer
CH2CH2CH
CH3
C
H3COOO
OO
CH2
CO(())
CH2CH2
O
n
yx
mH
CH2CH2CH
CH3
C
H3COOO
OO
CH2
CO(())
(a)
Hydrophobic
core
Hydrophilic
shell
Block copolymer micelle
(b)
Hydrophobic drug
Hydrophlic drug
(c)
Figure 4: Main characteristics of PEGb-PLGA (diblock and triblock) copolymers (a). Micellar self-assembly (b) and
hydrophilic/hydrophobic drug encapsulation characteristics (c).
5Journal of Nanomaterials
retains water by forming hydrogen bonds. Above LCST, the
hydrogen bonds between water and the polymer chains are
disrupted rendering the polymer hydrophobic to precipitate
out. This phase change can be exploited for a controlled
destabilization of the polymeric micellar structure [33, 34].
(1) Thermosensitive Poly(N-isopropylacrylamide) (PNIPAm).
The most widely used thermoresponsive polymer is poly
(N-isopropylacrylamide) (PNIPAm). This polymer, which
has a LCST at 33
°
C, is then water-soluble below the LCST,
while it becomes hydrophobic at body temperature [35, 36].
Based on the thermosensitive property of PNIPAm, a wide
range of thermosensitive micelle nanocarriers can be devel-
oped, where the LCST of PNIPAm-based polymers can be
easily modied via copolymerization with hydrophilic or
hydrophobic monomers. The strategy to use thermosensitive
polymeric micelles aims at achieving drug delivery control by
changing the temperature of the environment slightly above
or below the LCST, thus resulting in destabilization of the
micelles structure and triggering a release of the encapsu-
lated drug [35, 36]. With this approach, the drug release
could be controlled by local heating (or cooling) during a
given time period.
Dedicated PNIPAM-based nanoplatforms can also
respond to further stimuli, including light and electric eld
stimuli [37]. Due to their distinct properties, responsive
microgels have been employed in various applications
including sensing, catalysis, drug delivery, optical devices,
cell attachment and culturing, radiotherapy, and optics
[37]. Rened control of thermoresponsive swelling/deswel-
ling and drug release properties of poly(N-isopropylacryla-
mide) hydrogels have been recently obtained by using
poly(ethylene glycol) (PEG) with varying chain lengths as
polymer crosslinkers [38]. Compared with PNIPAm hydro-
gels crosslinked with a conventional small molecular crosslin-
ker, N,N-methylenebisacrylamide, a greater degree and range
of thermoresponsive swelling/deswelling as well as tunable
LCST are demonstrated for PNIPAm-PEG hydrogels [38].
(2) Thermoresponsive (Pluronic) PEO-PPO-PEO Triblock
Copolymers. Thermoresponsive linear AB-type diblock and
ABA-type triblock copolymer architectures obtained by ver-
satile synthesis processes have attracted enormous interest
and have already found broad application in biomedicine
as tissue engineering and drug/protein delivery and stimu-
late the route for the rational design and engineering of
materials with desired properties [3941]. In thermorespon-
sive ABA triblock copolymers, the temperature can be used
as a trigger to form ower-type micelles or/and hydrogels
at the higher concentrations.
A special class of ABA triblock copolymers are repre-
sented by the commercially available Pluronic-type class of
amphiphilic poly(ethylene oxide)-poly(propyleneoxide)-po-
ly(ethylene oxide) PEO
m
-PPO
n
-PEO
m
triblock copolymers.
In those systems, the hydrophilic poly(ethyleneoxide)
(PEO) block assures the requested biocompatibility and the
desired stealthcharacteristic that minimize possible
unwanted interactions with cellular components. Moreover,
the possibility of molecular control by tuning the desired
polymer composition and architecture makes these sys-
tems a versatile tool to study, in a convenient way, the
rich and complex phenomenology in the eld of colloidal
science [4144]. A relevant number of studies involving
Pluronic block copolymers as drug delivery systems or
bioformulations for (pre)clinical use or trials are present
in literature [4446].
As recently evidenced by Pitto-Barry and Barry [44],
the encapsulation of the DOX anticancer drug in the
Pluronic micelles strongly inuences its biodistribution
and leads to a better accumulation of the micellar drug
in the tumors compared to the free drug. Moreover, it
exhibits a superior antitumor activity over DOX in a wide
range of doxorubicin-sensitive and -resistant human solid
(and hematopoietic) malignancies [44].
In conclusion, polymeric micelles have demonstrated
particular strength in solubilizing hydrophobic drugs in rele-
vant doses without the inclusion of toxic organic solvents or
surfactants, while the hydrophobic block can be tailored to
encapsulate drug molecules with a wide variety of structures.
Moreover, anticancer eciency can be obtained by modify-
ing the micelles surface with targeting ligands for specic
recognition of receptors (overexpressed on the surface of
tumor cells) [46, 47].
3.2.4. Polymeric Nanogels. Polymer-based micelles and vesi-
cles maintain their structure above the CMC. Below the
CMC, with the dissociation of their self-assembled nano-
structures into single polymer chains, they lose the function
as drug carriers. To overcome this problem, the employment
of chemically (or physically) crosslinked polymer networks
to obtain nanogels has become a common and eective
approach to obtain more stable nanocarriers in dierent bio-
logical conditions [48].
Polymer-based nanogels are three-dimensional networks
consisting of chemically (or physically) crosslinked polymer
containing both hydrophilic (or polar) and hydrophobic
monomers. They are generally dispersed in aqueous media
where they form semi-solid states (hydrogels) that may be
swollen by a large amount of water (hydrogels). The proper-
ties of hydrogels can be tuned to match the needs of specic
applications by the choice of a specic polymer (molecular
structure and segments length), the crosslinking mechanism,
and the eventual presence of acidic (or basic) polymer moi-
eties, whose state of protonation can be easily controlled
with pH or salt concentration. Stimulus-responsive (smart)
hydrogels can undergo structural transitions in response to
external stimuli or (internal) environment changes of the
physical properties of the system such as its temperature,
electric eld, and exposure to light [49, 50].
The choice of the hydrogel composition depends on the
specic biomedical application and may require specic
properties such as biocompatibility, transport/mechanical
properties, chemical stability and the ability to respond to
microenvironment changes [5153]. Another crucial factor
for hydrogel performance is the nature of the involved
(chemical or physical) crosslink interaction, as it inuences
many of the network properties, like swelling, elastic modulus,
and transport properties [54, 55]. In Figure 5(a) is reported the
6 Journal of Nanomaterials
chemical composition of some of the main hydrosoluble (i.e.,
PNIPAM, chitosan, and polyvinyl alcohol) hydrogels.
In chemical crosslinked hydrogels, a bifunctional (or
multifunctional) crosslinking agent is added to a dilute
solution of a hydrophilic polymer. Chemically crosslinked
hydrogels are developed by chain growth polymerization,
addition, and condensation polymerization and through
irradiation techniques (using high-energy ionizing radiation,
like electron beam, gamma, or X-ray). One common way to
create a covalently crosslinked network is to polymerize
end-functionalized polymers. The permanent linking (cova-
lent bonds) produced by chemical crosslinking will not
break, and this may limit the ability to control the hydro-
gel drug release characteristics. Among the numerous chem-
ical crosslinkers used, glutaraldehyde is one of the most
employed, as it can react with both proteins and carbohy-
drate functional groups and can provide substantial im-
provement of the hydrogel mechanical properties [5153].
Chitosan (gel) nanoparticles (<100 nm) crosslinked with glu-
taraldehyde evidenced an increased particle size with increas-
ing levels of crosslinking. However, an in vivo evaluation of
glutaraldehyde-crosslinked materials is necessary in order
to understand possible cytotoxicity eects and potential in
medical applications. Recent investigations have shown that
carboxylic acids (such as citric acid) are able to crosslink
the biopolymer in wet and dry conditions, thus improving
the mechanical properties and stability of biomaterials,
without the need for a potentially cytotoxic catalyst. More-
over, poly(carboxylic acids) can react with hydroxyl and/or
amine groups and therefore crosslink both proteins and
polysaccharides. Proteins crosslinked with carboxylic acids
have proved to be biocompatible and to provide the
desired improvements in properties for both protein- and
carbohydrate-based biomaterials [54].
Physically crosslinked hydrogels, on the other hand, can be
developed by hydrogen bond; ionic, van der Waals, and
hydrophobic interactions; stereocomplex formation; and
crystallization [53, 54]. The hydrogen bonding between poly-
mer chains of the hydrogels may be used to control drug
release through various factors including polymer concen-
tration (and molar ratio), type of solvent, solution temper-
ature, and degree of association of polymer functionalities.
Crosslinking by ionic interactions can be performed under
gentle conditions, at room temperature and physiological
pH [53, 54]. Anionic polymers crosslinked with the employ-
ment of metallic ions produce stronger hydrogels. Com-
plexation of polyanions with polycations has also been
exploited in several drug delivery applications [5153].
PNIPAM
Chitosan Poly(vinyl alcphol)
H3CCH3
NH2NH2NH2
NHO OOOO
O
nn
n
HO
HO
OH OH OH OH
HO HO
HO
(a)
Chemical hyrogel
Polymer A
Functional
groups
Crosslinker
Polymer B
Covalent
bond
(b)
Polymer A
Physical hydrogel
Polymer B
Non-covalent
(ionic) bond
(c)
Figure 5: Chemical composition of PNIPAM, chitosan, and poly(vinyl alcohol) hydrogels. Schematic representation of the main crosslinking
approaches employed for the construction of chemical (b) and physical hydrogels (c).
7Journal of Nanomaterials
Ionically crosslinked chitosan hydrogels are produced via
complex formation of chitosan and polyanions, like dextran
sulfate or polyphosphoric acid. A relevant number of investi-
gations on the self-assembling preparation of chitosan nano-
particles in drug delivery applications have been proposed
in recent years. In particular, the nanoparticle preparations
by polyelectrolyte complexation and by the self-assembly
of hydrophobically modied chitosans are able to encapsu-
late various typologies of dierent drugs (including doxo-
rubicin, paclitaxel, and amphotericin B) under dierent
conditions while preserving their stability and biocompati-
bility. Therefore, chitosan-based self-assembled nanoparti-
cles have great potential, as well as multiple applications
for the future in the design of novel drug delivery systems
[55]. In Figure 5, the schematic representation of the main
crosslinking approaches employed for the construction of
chemical (b) and physical hydrogels (c) is reported.
Alginate represents another important example of a
polymer that can be crosslinked by ionic interactions and
can be employed as nano-matrix for the encapsulation of
living cells and for protein release. It consists of a natural
polysaccharide having mannuronic and glucuronic acids
which remain complexed with calcium ions and which gen-
erate a crosslinked gel (at room temperature and physiologi-
cal pH) (Figure 6) [52].
The gels can be destabilized by extraction from the gel of
Ca ions (via chelating agent). Due to its biocompatible and
nonimmunogenic character, calcium-alginate hydrogels are
used in a variety of biomedical applications including scaf-
folding for cell cultures, drug release, and tissue engineering
(including wound dressing) [52]. The alginate backbone
can be modied with cell-interactive peptides binding integ-
rin receptors (such as RGD) or other cellular receptors (e.g.,
VEGF) in order to increase cell adhesion.
Finally, crystallization crosslinking is exploited in the for-
mation of poly(vinyl alcohol)- (PVA) based gels. In this case,
the gel formation is attributed to the arrangement crystallites
which acts like a physical crosslinking site in the network,
through the repeated freezing/thawing method [52].
Polymeric nanogels represent a new generation of
drug delivery systems due to their high drug encapsulation
capacity, tuneable size, ease of preparation, minimal toxicity,
stability in the presence of serum, and stimulus responsive-
ness. For those reasons, biomedical nanoplatforms based on
responsive hydrogels have found applications in biosensors,
drug delivery, tissue engineering, and biomimetic materials
development [4850].
3.3. Liposome Nanocarriers. Although the polymer-based
nanocarriers have many attractive properties for
in vitro/in vivo applications, lipid-based drug delivery sys-
tems are still prevalent in the market and still maintain
the supremacy in clinical applications. Vesicles composed
of natural or synthetic lipids (so called liposomes) represent
a versatile nanomaterial platform for the development of
enhanced drug delivery systems in a wide range of appli-
cations in the eld of biotechnology and nanomedicine
[56, 57]. Liposome nanocarriers oer many benets con-
nected with their ability for a versatile self-assembly [58
60] and governed by specic soft interactions that control
the colloidal stability of therapeutic drugs in a harsh bioen-
vironment of diseased tissues [6164].
Guluronic
Mannuronic
HO HO HO
HO
HO
HO
HO
HO
HO
HO
HO
HO
HO
HO
HO
HO
HO
HO
O
O
OO
O
O
O
O
O
OO
OO
OO
O
O
O
O
O
O
O
O
O
O
O
O
OO
O
O
OO
O
O
OO
OO
O
O
m
n
Egg-box-type
conguration
HO
HO
H
O
HO
HO
HO
H
O
H
O
H
O
H
O
H
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
m
n
Ca2+
Ca2+ Ca2+
Figure 6: Schematic representation of the calcium-alginate-based hydrogels. Ionic-type crosslinking of alginate is caused by chelation of
metal cation Ca
2+
by carboxylate groups of β-D-mannuronate and α-L-guluronate residues of alginate. The alginate chains are arranged
around a metal cation Ca
2+
in an egg-box(2 : 1) helical structure conguration.
8 Journal of Nanomaterials
Lipidbased systems are easy to manufacture than bio-
polymers, due to the large availability of the base (phospho-)-
lipid compounds. They also have better control over drug
release kinetics.
Synthetic or natural (phospho-)lipids consist of a hydro-
philic head and (one or more) hydrophobic tails. In water
solution, they self-assemble into a highly exible bilayer vesi-
cles (Figure 7(a)), with the hydrophilic heads facing the water,
and are able to undergo various conformational and dynamic
transitions which are essential for many biological functions.
From the structural point of view, the lipid bilayer vesicle
(liposomes) in aqueous solution strongly depends on the con-
ditions of preparation (i.e., stirring, sonication, extrusion,
microuidication, or electroformation), while their sizes
range mainly between 50 and 500 nm and may be composed
of small unilamellar vesicles (SUVs < 100 nm), large unilamel-
lar vesicles (LUVs 1001000 nm), or giant unilamellar vesicles
(GUVs > 1 μm). Finally, multilamellar vesicles (MLVs) are
composedof concentricbilayer surfacesin anonion-like struc-
ture (hydrated multilayers) [58, 59]. Finally, novel promising
lipid-based nanocarriers (especially for lipophilic drugs) are
givenby the solidlipid nanoparticles(SLN),consisting ofasolid
hydrophobic core that contains the drug (dissolved in a solid
high melting fat matrix), surrounded by a monolayer of
phospholipid coating that ensures the colloidal stability in
the aqueous environment [60].
Fluidity of a lipid bilayer, which depends on both its com-
position and temperature, has been shown to have a large
impact on uptake and release functions of cellular systems
[6567]. With increasing temperature, a bilayer made of phos-
pholipids passes from a highly ordered, rigid crystalline (or
gel) state to a more mobile uid state [68, 69]. An example is
given by structural changes in the dimyristoylphosphatidyl-
choline (DMPC) phospholipid bilayer in water at excess
during temperature-dependent phase transitions (Figure 7)
[68]. In Figure 7(b), the structural changes on multilamellar
vesicles (MLVs) of dimyristoyl phosphatidylcholine (DMPC)
lipids in H
2
O and D
2
O are investigated by dierential scan-
ning calorimetry (DSC) experiments as a function of temper-
ature. The rst endothermic peak at T=15
°C(pre-transition)
and the second endothermic peak at T=234°C(main phase
transition) identify the border between the three dierent
characteristic phases (passing from the gelLβto the ripplePβ
and nally to the liquid crystallineLαphases). It is worth notic-
ing how the substitution of the solvent from H
2
OtoD
2
O pro-
duces a sensitive shift of the main transition peaks [68].
The presence of inclusion of macromolecular com-
pounds (such as drugs) may strongly inuence the structure
Dimyristoylphosphatidylcholine
(DMPC)
Hydrophobic
drug
Hydrophilic
drug
10 15 20 25 30
0.05
0.10
0.15
0.20
DSC (a.u.)
T (°C)
D2O
Gel phase L𝛽′ Ripple phase P𝛽′ Liquid crystalline
phase L𝛼
(a)
(b)
Multilamellar
DMPC vesicles
Uni-lamellar DMPC
drug nanocarrier
Figure 7: Schematic representation of the encapsulation of hydrophobic/hydrophilic drugs into unilamellar dimyristoyl-phosphatidylcholine
DMPC liposome (a). Characteristic phases (gel Lβ,ripple Pβ, and liquid crystalline Lαphases) and main transitions of a multilamellar DMPC
lipid in H
2
O (and D
2
O) solution, obtained by DSC experiments (b).
9Journal of Nanomaterials
of the lipid bilayer nanocarriers while the nal morphology is
strongly determined by the size, charge, and composition
of the interacting components [7072]. Often, the inclusion
of macromolecular compounds may induce structural per-
turbations against the long-range cohesive tendency of the
lipid bilayer vesicles [7376]. The encapsulation of active
compounds into the bilayer of the liposomes, while facili-
tating drug solubilisation in aqueous media, also provides
additional protection and control against drug degradation.
These characteristics cause a sensitive amelioration in the
toxicity proles with a correlated improvement of thera-
peutic ecacy. While hydrophilic drugs are localized nearby
the hydrophilic head groups or in the aqueous core region,
the hydrophobic drugs are hosted within the liposome
acyl chain region. As many anticancer drugs are of interme-
diate solubility, they undergo then a partition between the
exterior (or interior) liposome aqueous phase and the hydro-
phobic interior of the bilayer.
Owing to a facile modulation of their size, hydrophobi-
c/hydrophilic character, low toxicity, and biocompatibility,
liposomal nanocarriers still represent the largest group of
clinically approved anticancer drug formulations [77, 78].
Liposome formulations are devoted mainly to cancer treat-
ment and are mainly administrated intravenously, due to
the high degradation of lipids in the gastrointestinal tract
[6567]. Anticancer drugs doxorubicin, daunorubicin, cis-
platin, paclitaxel, and vincristine are among the most exten-
sively investigated agents for the liposome-based drug
formulations, and several liposomal formulations of these
agents are currently in clinical use in cancer therapy [7779].
An important approach for the improvement of circu-
lation times of lipid nanocarriers consists in conjugation of
suitable polymers on their surface, such as natural (e.g.,
dextran, alginate, and chitosan) or synthetic (e.g., poly(eth-
ylene glycol) (PEG), poly(vinyl alcohol) (PVA), and poly
(vinyl pyrrolidone) (PVP)) hydrophilic polymers [80].
This approach allows overcoming the interception by the
immune system, the low blood circulation half-life, toxicity,
and biocompatibility issues. PEGylation of the liposome
surface, the most widely used polymer conjugation process,
creates a local surface concentration of highly hydrated poly-
mer brushes that sterically inhibit both hydrophobic and
electrostatic interactions with plasma proteins or cells, thus
reducing the liposomal uptake process by the RES [81]. As
a result, PEGylated liposomes are not opsonized and are able
to escape the capture by the cellsphagocytic systems by ren-
dering the nanocarriers invisible to macrophages (stealth
liposomes) [57]. The interaction between a nanomaterial
and biological tissue initiates, in fact, with the nonspecic
adsorption of proteins (such as albumin, globulin, and brin-
ogen) at the nanomaterial surface and could have a negative
impact on the availability of the nanocarriersactivity and
functionality. Moreover, the layer of adsorbed protein on
surfaces may favour cell attachment and subsequent bacterial
colonization which leads to the formation of bacterial lms.
The inhibition of protein adsorption, by surface functionali-
zation based on PEG polymers, represents therefore a crucial
step not only in order to prevent biomaterial failure but also
to inhibit biofouling (i.e., the contamination of surfaces by
microbes including bacteria, fungi, and viruses) [82]. Many
studies demonstrated that PEGylated liposomes were able
to improve the stability and blood-circulation time, together
with low plasma clearance and low volume of distribution
(with minimal interaction with nontumoral tissues) [57, 83,
84]. However, phase separation transition on liposome can
be induced due to liposome PEGylation, while excessive
PEGylation can also cause inhibition of cellular uptake,
which is undesired for cancer treatment [85]. Thus, a moder-
ated PEGylated multicomponent liposome may represent the
best compromise to hinder the protein adsorption but still
present a high cellular uptake in cancer cells [85]. In
Figure 8, we report a representation of the PEGylated phos-
pholipid 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-
[methoxy(polyethyleneglycol)-2000] (DSPE-PEG2000) ammo-
nium salt (a), together with a sketch of steric repulsion
between PEGylated liposomes (b).
Liposomes can also be used to target active drug mole-
cules to specic sites within the biological systems, such as
diseased tissues or tumors. The incorporation of dierent
ligands, such as peptides, monoclonal antibodies, aptamers,
and growth factors, improves the specicity of the liposome
interaction during the drug release process [86, 87]. Cationic
liposomes containing small interfering RNA (siRNA) were
developed to target EGFR (a surface receptors overexpressed
in many solid tumors) by conjugation of thiolated anti-
body with the maleimide (MAL) group at the distal end
of DSPE-PEG-MAL chains of preformed liposomes. The
liposomes showed an ecient transfer of siRNA to mouse
(transfection) compared to nontargeted liposomes, while
the suppression of lung cancer metastasis was observed
[88]. Recently, dual functional paclitaxel liposomes with pH
response and mitochondrial targeting were proposed as a
new approach for treating multidrug-resistant cancer. The
liposomes were eective in treating A549 (drug-resistant)
cancer cells [89]. Antibody molecules have groups (e.g., car-
boxyl, amine, and thiol groups) which can be easily modied
for active targeting. By using various surface engineering
techniques, antibodies or their fragments can be conjugated
to the liposomes surface to obtain immunoliposomes [90].
Recently, liposomes containing triptolide were functional-
ized with the anti-CA-IX antibody and showed higher e-
cacy in lung cancer therapy in mice bearing lung cancer
[91]. Recently, a number of studies have focused on mod-
ifying liposome drug-releasing mechanisms by using func-
tionalized stimulus-responsive liposomes. Drug release
processes from liposome nanocarriers can be triggered by
external stimuli, such as heat (hyperthermia) [92], light
[93], magnetic eld [94], ultrasound [95], or internal stimuli,
such as pH [96], enzymes [97], and redox [98]. Moreover,
liposomes have the ability to simultaneously conjugate
cancer-targeting molecules (active targeting) for therapy
treatment and diagnostic tasks (theranostics) [87]. Recently,
Li and coworkers reported the development of a thermosen-
sitive liposome formed by a mixture of 1,2-dipalmitoyl-
sn-glycero-3-phosphatidylcholine (DPPC), 1-myristoyl-2-
stearoyl-sn-glycero-3-phosphocholine (MSPC), and 1,2-dis-
tearoyl-sn-glycero-3-phosphoethanolamine- (DSPE-) PEG
and loaded with the MRI contrast agent Gd-DTPA as well
10 Journal of Nanomaterials
as doxorubicin (DOX) [99]. The simultaneous delivery of
both Gd-DTPA and DOX allows drug release to be simulta-
neously carried out and monitored, with triggerable release
in the environment of a tumor by localized heating. Thera-
nostic carboxymethyl dextran- (CMD-) coated magnetolipo-
somes (CMD-MLs) for controlled drug release under a
low-frequency alternating magnetic eld has been recently
developed [94]. This theranostic nanoplatform also acted as
an ecient T2-weighted contrast agent during in vitro MRI
measurements, evidencing the in vivo diagnostic/therapeutic
ecacy of DOX-loaded CMD-MLs for some cancers, such as
brain cancers [94].
Although the modication of the physicochemical
properties strongly inuences the structure and secondary
properties of functionalized liposome nanocarriers [76],
the lipid-based vesicle nanocarriers (liposomes) containing
therapeutic drugs produce fewer side eects than do non-
liposomal anticancer formulations and still represent the
best approach to eectively target the diseased tissues
(including tumors).
3.4. Dendrimers. Dendrimers are three-dimensional, hyper-
branched nanoparticles, consisting of polymeric branching
units covalently attached to a central core, organized in
concentric layers (named generations) and that terminate
with a number of external surface functional groups
[100, 101] (Figure 9(a)). Unlike the self-assembly systems
illustrated so far, they are obtained by specicsynthesis
methods, based on an iterative stepwise reaction sequence
that allows a precise control over molecular design parame-
ters (such as size, shape, and internal/surface chemistry)
which results in highly monodisperse nanostructures. One
of the most important applications of dendrimers consists
in the conjugation of suitable chemical species into their
surface. This approach stimulates the development of new
prototypes that can function as detecting anity ligands
and targeting components, or imaging agents, while drug
delivery applications indicated an ecient use of dendri-
mers for (in vitro) transfer of genetic material into cells
[102104]. The structure of dendrimers in solution can be
inuenced by many factors, such as the generation, spacer
length, surface modication, ionic strength, pH, and tem-
perature [105, 106]. On the other hand, the charge eects
and electrostatic forces seem to play the main role in drug
delivery processes.
Dendrimers have the ability to increase the solubility and
bioavailability of hydrophobic drugs that can be entrapped in
their intramolecular cavity or conjugated to their surface
functional groups (see Figures 9(b) and 9(c)). A quantitative
analysis of the physical interactions between dendrimers and
inclusion components is a crucial step for the development of
novel technology. In this respect, the small-angle scattering
techniques represent powerful approaches to study the
structure and interaction properties of dendrimers in a
solution environment [106108]. The modelling of the
inter-dendrimer interaction provides substantial insight
into the fundamental mechanisms of dendrimer-drug inter-
action in solution. Notably, the solution conditions (includ-
ing solvent pH, counterion distribution, and ionic strength)
have been shown to play a key role in the control of the
charge interaction and can be exploited in the rational design
of dendrimer properties for suitable applications in biotech-
nology [109112].
A new emerging eld of clinical application concerns
the combination of dendrimers and bioactive ligands.
Dendrimer conjugates containing saccharides or peptides
O
O
O
O
O
O
NH4
+
O
OO
(OCH2CH2)45OCH3
PN
H
H
O
O
O
O
NH
4
+
O
O
O
P
N
N
H
H
Steric
repulsion
PEG
B
(a)
(b)
PEGylated
liposome
(DSPE-PEG2000) ammonium salt
Figure 8: Schematic representation of the PEGylated nanocarrier composed of the phospholipid 1,2-distearoyl-sn-glycero-3-
phosphoethanolamine-N-[methoxy(polyethyleneglycol)-2000] (DSPE-PEG2000) ammonium salt (a). View of sterically stabilised lipid
bilayer nanocarriers (b).
11Journal of Nanomaterials
may exhibit therapeutic application for the development of
antimicrobial, antiprion, and antiviral agents. Moreover,
they oer additional advantages for their versatile capabil-
ities to enhance solubility and stability upon absorption of
various types of therapeutics. This approach has been used
for nucleic acid-based therapeutics and other charged ther-
apeutics [113]. Another relevant aspect of charge-mediated
self-assembly processes involving dendrimers regards the
study of the formation of dendrimer-surfactant (lipids)
complexes, as it has important implications for the under-
standing of the translocation mechanism of dendrimers
and biomacromolecules in living cells. In this respect, sev-
eral model systems that mimic the structure of biomem-
branes were developed during the last decades [109111].
Depending on the dendrimer chemical composition, size,
and surface charge, dierent mechanisms can be identied,
that depend on the main interactions between dendrimers
and lipid bilayers, including adsorption on membrane, hole
formation, and vesicle disruption. The dierent mechanisms
of interaction strongly depend on the force balance between
charged dendrimers and the zwitterionic lipids (that have a
net dipolar charge) and on the hydrophobic interaction
between the arms of the dendrimers and the lipid hydrocarbon
chains [111113]. The presence of functional groups in the
dendrimers exterior also permits the addition of other moie-
ties that can actively target certain diseases and improve the
drug delivery process, such as folate and antibodies, now
widely used as tumor targeting strategies [114].
Finally, dendrimers are promising nanocarriers of gene
therapy [114]. Nucleic acids usually form complexes with
the positively charged surface of most cationic dendrimers.
Under physiological conditions, polyamidoamine (PAMAM)
dendrimer-DNA complexes (called dendriplexes) maintain
a positive net charge and bind to negatively charged surface
molecules on the cell membranes. Dendrimers are taken up
into cells by nonspecic endocytosis and are then degraded
by lysosomes. The targeting genes are then released and enter
the nucleus to play a role in gene therapy [115, 116]. Trans-
fection eciency, mediated by PAMAM dendrimers, appears
to be dependent on dendrimer generation (with larger-size
dendrimers providing higher eciency) as well as on the
charge ratio of the complexes.
Core
Branching
points
Surface
end-groups
(a)
Physical encapsulation
(b)
Chemical conjugation
PEG
Fluorescent
dyes
siRNA
Antibodies
Peptides
Folic
acid Tumo ral
markers
PEG
PEG
(c)
Figure 9: Main structural features of dendrimers (a). Dendrimer nanocarriers. Active components (and drugs) are entrapped in the internal
cavity of the dendrimers (b) or conjugated to their surface functional groups (c).
12 Journal of Nanomaterials
In Figure 10, a possible route for the use of dendrimers as
gene delivery vectors is reported [94, 95]. As plasmid DNA
by itself is unable to penetrate the cell membrane, the rst
stage is then to form (in vitro) a complex between dendri-
mer and DNA (called dendriplex). The dendriplex is then
added to cells in vitro (or is introduced into animals
in vivo or ex vivo), where it will be transported to the spe-
cic cell via the blood system. The dendriplex will bind to
the cell membrane and wait for the cellular uptake (endo-
some uptake), thus allowing its internalization inside the
cytoplasm. When the pH changes from 7.4 (extracellular
value) to 5.5 (intracellular value), the deprotonation of den-
drimer surface groups causes the dendriplex destruction
and the release of nucleic acid. This causes the endosome
escape; otherwise, the dendriplex will be degraded after
the fusion of the endosome with lysosomes. Simultaneously,
the endosomes undergo lysis and the free nucleic acid
(DNA) is released into the cytoplasm. Finally, the DNA
travels through the cytoplasm to enter the nucleus for suc-
cessive gene expression [115, 116].
In conclusion, more studies are necessary to elucidate the
complex structurefunction relationship of liganddendri-
mer conjugates in drug delivery processes. Dendrimer nano-
carriers hold promise to facilitate targeted delivery and
improve drug ecacy for the smart application of modern
pharmaceutics and nanomedicine.
4. Inorganic Nanoparticles
The employment of inorganic nanostructured materials
has been recently used for the construction of ecient
nanocarriers for drug delivery application [117]. The inor-
ganic nanocarriers are generally composed of two regions:
acore containing the inorganic component (such as gold,
quantum dots, silica, or iron oxide) and a shell region com-
posed mainly of organic polymers (or metals) that provide
a suitable substrate for the conjugation of biomacromolecules
or protect the core region from unwanted physicochemical
interactions with the external biological microenvironment
[117, 118]. Due to the unique magnetic and plasmonic prop-
erties, inorganic nanomaterials may generate imaging con-
trast by magnetic resonance (MR), computed tomography
(CT), or positron emission tomography (PET) [117, 118].
This characteristic is employed and exploited for a
diagnostic imaging of the diseased region. However, in spite
of such advantages, inorganic nanoparticles have shown
only limited success in the treatment of disease tissues
due to the critical issues connected with limited amounts
of active drug carried and to the high degree of toxicity
of the nanoparticle [119]. In Figure 11, we report a list of
some of the most employed inorganic nanocarriers for drug
delivery applications.
4.1. Carbon Nanotubes. CNTs belong to the family of fuller-
enes (a third allotropic form of carbon) and are composed of
one or more graphene sheets rolled up into a cylindrical
tube-like single-walled carbon nanotube (SW-CNT) or mul-
tiwalled (MW-CNT) structure [120, 121]. CNTs possess
some distinctive physicochemical and biological characteris-
tics and high ability for surface modications that make them
a promising carrier for drug delivery. CNTs may assume the
shape of a hollow sphere, ellipsoid, and many other forms,
while the outer diameters are typically in the range of 0.4
2 nm for the SWCNTs (and 2100 nm for the MWCNTs).
They present peculiar structural properties characterised by
a high aspect ratio (length diameter > 200 1), and surface
area, ultralight weight, high mechanical strength, and electri-
cal and thermal conductivities [120, 121].
Endosome
Endosome
escape
Nucleus Gene
expression
erapeutic
protein
DNA
release
mRNA
Transcription
protein synthesis
+
DNA Dendrimer
Dendriplex
Figure 10: Schematic diagram for a possible route in the use of dendrimers as gene delivery vectors.
13Journal of Nanomaterials
The characteristic nano-needle shape is particularly inter-
esting, as it allows crossing the cell membrane via endocytosis
(so-called needle-like penetration ability), while CNTs with
sizes in the range from 50 to 100 nm are easy to be engulfed.
The (aggregation) structure, purity, and size distribution, as
well as area, surface charge, and chemistry, represent cru-
cial properties that regulate the reactivity of carbon nano-
tubes with biological systems [121, 122]. Owing to their
cell penetration abilities, unique physicochemical character-
istics, high drug payload, intrinsic stability, structural exi-
bility, and appropriate surface functionalization, CNTs
representone of the most investigated family of nanocarriers
for cancer therapy [122, 123]. Anticancer drugs can either be
encapsulated in the inner cavity or be attached (with covalent
or noncovalent functionalization) to the surface of CNTs
[119]. Furthermore, the attachment of dierent targeting
agents to the surface-functionalized CNTs allows targeted
delivery of anticancer including doxorubicin, methotrexate,
paclitaxel, and cisplatin [122, 123]. An approach was con-
ceived for the entrapment (via hydrophobichydrophobic
interactions) within the protective inner cavities of a multi-
walled carbon nanotube of an inert and strongly hydropho-
bic platinum(IV) complex (Figure 12). Upon chemical
reduction, the drug was converted to its cytotoxic and hydro-
philic form and released from the carrier [124],
CNTs have shown promise in carrying plasmid DNA,
small-interfering ribonucleic acid (siRNA), antisense oligo-
nucleotides, and aptamers [121]. In addition to gene delivery,
functionalized CNTs can be used as diagnostic tools for the
early detection of cancer, while the strong optical absorption
in the near-infrared region makes CNTs an interesting tool
for photothermal ablation of a cancer site (photothermal
therapy) [122]. The major problems with CNT nanocarriers
are their poor water solubility, their nonbiodegradable
nature, and their cytotoxicity. However, CNTs have the
ability to be surface-functionalized (either chemically or
physically), which render them water-soluble, biocompatible,
and non (or less) toxic. While PEGylation is employed to
increase solubility, to avoid the RES, and to lower the tox-
icity, their surface functionalization with the PNIPAM
polymer could be used to modify CNTs for (temperature)
stimulus-responsive nanocarriers. Due to those characteris-
tics, CNTs are considered a good candidate for the treat-
ment of cancer.
4.2. Gold Nanoparticles (Au NPs). Due to their special
electronic, optical, sensing, and biochemical properties,
gold nanoparticles (Au NPs) have been intensively investi-
gated for potential applications in medical imaging (early
detection and diagnosis) and treatment of diseases (includ-
ing tumor therapy) and drug delivery processes [125, 126].
Gold NPs are composed of a gold atom core surrounded
by negative reactive groups on the surface that can be eas-
ily functionalized by adding a monolayer of surface moie-
ties (ligands for active targeting). Although they can be
assembled by means of dierent chemical and physical
routes, Au NPs for biomedical applications are mainly
prepared using the colloidal synthesis method (utilizing a
Carbon
nanotube
(a)
Quantum
dot
Surfactant
coating
ZnS shell
CdSe core
(b)
Gold
nanoparticle
Gold
nanoparticle
Protective
layer
Surfactant
coating
Au
(c)
Mesoporous
silica
nanoparticle
(d)
Figure 11: Example of the most employed inorganic nanocarriers: carbon nanotube (a), quantum dot (b), gold nanoparticle (c), and
mesoporous silica nanoparticles (d).
O
O
O
O
Pt
Cl
Cl
NH3
NH3
O
O
O
O
O
O
Pt
Cl
Cl
NH
3
NH
3
Pt
Cl
Cl
NH3
NH3
Pt
Cl
Cl
NH3
NH3
Hydrophobic
prodrug
Hydrophilic
ciplatin
MW CNT
nanocarrier
Drug
entrapment
Drug
release
Figure 12: Entrapment of hydrophobic platinum(IV) prodrug within the cavities of multiwalled carbon nanotubes. Release from the CNT
carrier of hydrophilic anticancer drugs (cisplatin) upon chemical reduction and hydrophobicity reversal. Adapted from ref. [125].
14 Journal of Nanomaterials
metal precursor, a reductant, and a stabilizer). This approach
allows a precise control of the optical and electrical properties
that strongly depend on the shapes (as nanosphere, nano-
rod, nanocage, and nanoshell) and sizes (ranging from
1 nm to more than 100 nm) of the generated Au NP nano-
structures [125, 126]. A schematic representation of the vari-
ety of shapes for the gold nanoparticle-based nanocarriers is
reported in Figure 13.
Due to the presence of a negative charge on Au NPs, they
can be easily (bio)functionalized (via ionic or covalent bond-
ing or by physical absorption) by a wide range of dierent
biomolecules, including drug molecules, or large biomole-
cules, such as antibiotics, proteins, genes (DNA and RNA),
and a variety of targeting ligands, while recent investigations
evidenced their nontoxicity for some human cell lines and
their biocompatibility and biodegradability in vivo [125
128]. Au NPs are particularly attractive due to the presence
of the surface plasmon resonance (SPR) bands [129, 130],
which enable them to convert light to heat and scatter the
produced heat to kill the cancer cells. The interaction of light
with electrons on the Au NP surface at a given wavelength
(frequency) of light induces a collective oscillation of elec-
trons on the Au NP surface that causes the surface plasmon
resonance eect. This phenomenon generates a strong extinc-
tion of light (absorption and scattering) at a given wavelength
(or frequency) of light which strongly depends on AU NP
size, shape, surface, and aggregation state. By synthesizing
gold nanoparticles of dierent shapes, the surface plasmon
resonance can be easily tuned to give absorption maxima
from around 500 nm into the near-infrared part of the spec-
trum, thus allowing an ecient monitoring of the Au NPs
colloidal stability over time [128, 129].
Generally, Au NPs without surface modication present
a reduced colloidal stability in blood ow. To overcome these
limitations, the surface of Au NPs can be modied by using
polyethylene glycol (PEG) which ensures an increased colloi-
dal stability in the harsh physiological conditions of diseased
tissues [125]. Because of their ease in synthesis and surface
functionalization (with a large variety of organic and biolog-
ical molecules), the high biocompatibility and low toxicity
(which is related to their high physicochemical and colloidal
stability), and the variety of optical properties related to sur-
face plasmons, Au NPs have been employed in the synthesis
of various biomedical nanoplatforms and for a wide range
of applications, including biosensing, tumor imaging, and
targeting (multimodal) drug delivery systems [126, 127]. A
recent investigation of del Pino et al. [131] evidenced that
thinner, more hydrophilic coatings, combined with functio-
nalization with positively charged groups (such as quaternary
ammonium cations), result in a more ecient cellular uptake
[132], attributed to the favourable electrostatic interactions
with the negatively charged cellular membrane [131, 132].
Mosquera et al. evidenced that cellular uptake of gold nano-
particle functionalised with negatively charged pyranines can
be activated in situ through the addition of cationic covalent
cages that specically recognize the uorescent pyranine dyes
and counterbalance the negative charges. This highly selec-
tive and reversible host-guest recognition process is able to
activate the cellular uptake, even in protein-rich biological
media, as well as its regulation by rational addition of either
cage or pyranine [131]. Many investigated Au NP nanoplat-
forms incorporate cellular anity ligands into their surface
in aims for specic cellular targeting. Doxorubicin (DOX)
was attached to Au NPs, through a pH-sensitive linker, in
order to provide an ecient intracellular triggered DOX
release (inside acidic organelles), thereby enhancing thera-
peutic eects in drug-resistant tumor cells [133]. Based on
the conjugation of a uorescent DNAzyme onto Au NPs,
Wu et al. [134] developed the rst DNAzyme-based metal
sensors for intracellular metal ion detection. Finally, the Au
NPs have been successfully used in the development of novel
approaches for the control of the delivery and release of drugs
by means of external stimuli (such as light) or internal stimuli
(such as pH or glutathione) [127, 128].
4.3. Quantum Dots (QDs). Quantum dots are uorescent
semiconducting inorganic nanocarriers that have shown
potential use for many biomedical applications, such as
drug delivery and cellular imaging [135]. They are com-
posed of atoms of group II and group VI of the periodic
table (i.e., molecules such as CdS, CdTe, and ZnS). They
are synthesized either by means of a bottom-up approach
(by self-assembly processes in solution following chemical
reduction) or by a top-down method (by means of molec-
ular beam epitaxy, ion implantation, e-beam, or X-ray
lithography) [135, 136].
Most QDs consist of three parts: an extremely small core
(210 nm in diameter) of a semiconductor material (e.g.,
Nanosphere Nanorod
Nanocube Nanostar
Nanocages
Nanoshell
Nanocluster
Figure 13: Example of the morphology of gold nanoparticle synthesized nanostructures.
15Journal of Nanomaterials
CdSe) surrounded by another semiconductor (such as ZnS).
Finally, a cap made of dierent materials encapsulates
the double-layer structures of the QDs (Figure 14). QDs
with the inner semiconductor core of CdSe coated with
the outer shell of ZnS represent the most commonly inves-
tigated nanoplatform. Due to their size and quantum
eects, they show unique optical (photophysical) properties
that allow visualizing the tumor, in real-time monitoring,
during the drug-carrying and drug release processes at
the targeted site [135, 136]. Cells labeled with QDs were
intravenously injected into a living animal (mice) and
followed during tumor cell extravasation into the lung tis-
sue, by detecting the emission spectrum by scanning mul-
tiphoton microscopy [129].
Since most of the conventional organic label dyes do not
oer the near-infrared (>650 nm) emission possibility, QDs
with their tunable optical properties oer considerable
advantages over organic uorophores for this purpose. Due
to their uorescence properties (which is intense and stable
for a longer time), highly sensitive detection (high quantum
yield and resistance to photobleaching), and size-tunable
light emission, QDs are particularly suited for the develop-
ment of a new class of biosensors used for cancer imaging
and diagnosis [135, 136].
As reported for other nanocarriers, QDs also experience
nonspecic uptake by the RES. Moreover, some critical
issues connected with their biomedical employment regard
their toxicity, especially with the use of QD containing
heavy metal ions (such as Cd and Hg). However, their tox-
icity can be reduced by functionalizing the QD surface with
biocompatible molecules. In this respect, PEGylation allows
the QDs to accumulate in tumor sites by an enhanced per-
meability and retention (EPR) eect without the employ-
ment of a targeting ligand [130]. This eect has been
demonstrated recently by coating ITK705-amino QDs with
methoxy-terminated poly(ethylene glycol) (PEG) of dierent
chain lengths [130].
In order to passivate QDs for biological applications,
several advantages have been demonstrated by encapsulating
QDs in phospholipid micelles [137]. Finally, to actively target
a tumor site, various ligands, such as peptides, folate, and large
proteins (monoclonal antibodies), can be grafted on the QD
surface (Figure 12). This possibility stimulates an increasing
interest in the development of nanotheranostic platforms for
simultaneous sensing, imaging, and therapy [138].
4.4. Superparamagnetic Iron-Oxide Nanoparticles (SPIONs).
Superparamagnetic iron-oxide nanoparticles (SPIONs), such
as magnetite (Fe
3
O
4
) and maghemite (Fe
2
O
3
), have been suc-
cessfully proposed for target drug delivery by using a mag-
netic force [139141]. When magnetic particles are reduced
to 1020 nm, they show a super para-magnetism eect, con-
sisting of the magnetization of the nanoparticles up to their
saturation, but they show no residual magnetism upon
extinction of the magnetic eld. Functionalization of SPIONs
prevents the aggregation and protects their surfaces from
oxidation and also provides a surface to conjugate drugs
and targeting ligands, thus increasing the blood circulation
by avoiding the RES and reducing nonspecic targets [141].
Superparamagnetic nanohybrids may be concentrated at a
specic target site within the diseased tissues by an external,
high-gradient magnetic eld [140142]. Modication of the
SPION surface allows the bind with various proteins, anti-
bodies, peptides, and anticancer drugs which can bind specif-
ically to their target receptors that are expressed on cancer
cells [116]. Surface-modied SPIONs with the anticancer
drug methotrexate, which tags to the tumor cells expressing
folate receptors, showed an increased uptake of SPIONs in
tumor cells [142]. Despite their potential biomedical applica-
tion, the possible alteration in gene expression proles, dis-
turbance in iron homeostasis, oxidative stress, and altered
cellular responses are some of the main critical issues of
SPION-related nanocarriers that limit their application in
the clinic [141].
Surfactant
coating
ZnS shell
CdSe core
Engineering QD core
for photo-physical
detection and diagnosis
Design QD shell to
increase colloidal stability
and biocompatibility
Conjugation of surface QD
with ligands for specific
bio-functional tasks
Antibodies
Fluorescent
dyes
Peptides Tumoral
markers
Figure 14: Design strategies of QD nanocarriers for biomedical and nanomedicine applications.
16 Journal of Nanomaterials
4.5. Mesoporous Silica Nanoparticles. Silica (SiO
2
) materials
have increased biomedical and nanomedicine applications
owing to their simple synthesis procedures and their charac-
teristic porous architecture [143145]. Mesoporous silica
nanocarriers (MSNs) allow loading a large amount of (anti-
cancer) drugs, thus facilitating their accumulation in tumor
tissues via passive targeting. PEGylation processes promote
escape from the RES, thus prolonging the circulation time,
drug availability, and biodistribution of therapeutic drugs
[145148]. MSNs possess several attractive features such as
good biocompatibility, large specic surface area, highloading
(of hydrophilic/lipophilic drugs) capacity, controllable pore
diameters ranging from 2 to 50 nm (with narrow pore size dis-
tribution), and good thermal and chemical stability, which
make them promising nanoscale drug carriers. Moreover,
the convenient surface functionalization of MSNs (through
the chemical modication of the active silanol surface group)
with dierent site-specic targeting agents enables them to
target tumor tissues via an active targeting mechanism [147
150]. Many dierent anticancer drugs, including paclitaxel,
doxorubicin, and methotrexate, have been eectively delivered
via MSNs. A variety of stimuli responsive systems have been
developed to induce the controlled drug release triggered by
temperature, light, pH, magnetic, electric and mechanical
stimuli, as well as enzyme and chemical reactions [151, 152].
MSNs are particularly suited for diagnosis, targeted drug
delivery, biosensing, and cellular uptake, in the biomedical
application eld.
Recently, monodisperse spherical silica nanoparticles
(SNPs) with diameters of 20200 nm were employed to study
size, dose, and cell type-dependent cytotoxicity in A549 and
HepG2 epithelial cells and NIH/3T3 broblasts. The extent
and mechanism of SNP cytotoxicity (such as cell viability,
membrane disruption, oxidative stress, and cellular uptake)
were found to be not only size- and dose-dependent but also
highly cell type-dependent. Specically, the 60 nm SNPs were
preferentially endocytosed by cells and, at high doses, caused
a disproportionate decrease in cell viability [151]. Recently,
Hu et al. [152] synthetized a multifunctional theranostic nano-
platform (designated as MMTNP) for tumor imaging and
controlled drug release, consisting of MCM-41 mesoporous
silica nanoparticles (MSNs), functionalized with a diagnostic
probe of metalloprotease-2- (MMP-2-) activated uorescence
imaging peptides and an enzyme-responsive nanovalve block-
ing the pores, with the cRGD peptides further functionalized
on the surface of MSNs for tumor targeting. Endocytosis
experiments evidenced that MMTNP enhances the tumor
targeting in vitro through receptor-mediated endocytosis.
In addition, the antitumor drug CPT could be eciently
loaded in MMTNP and released rapidly in tumor cells, thus
leading to enhanced inhibition of tumor cell growth [152].
Although clinical translation of mesoporous silica nano-
particles still remains a challenge, their unique properties evi-
dence their ecient performances and a promising tool for
innovative biomedical application.
4.6. Organic/Inorganic Hybrid Nanocarriers. Organic/inor-
ganic hybrid nanocarriers combine the advantages of organic
and inorganic materials and can be obtained by specic
functionalization of the surface of inorganic nanocarriers
with organic materials or by employing an organic colloidal
macromolecular species as template for the controlled
growth of inorganic materials [153160]. For instance, sur-
face coating with polyethyleneimine of mesoporous silica
nanoparticles enhances the cellular uptake and allows safe
delivery of siRNA and DNA constructs [156]. Systems com-
posed of lipid bilayers supported on solid material have
attracted signicant interest owing to their biomaterial and
nanomedicine applications. In MSNs/lipid bilayer hybrid
nanocarriers, lipid bilayers are used to cap the MSN channels
to prevent premature release of loaded drug, circumvent
multidrug resistance, prolong retention of hydrophilic drug
cargo, and achieve stimulus-responsive drug release [154,
155]. Recently, lipid-coated mesoporous silica nanoparticles
(LC-MSNs) were employed to overcome the critical issues
connected with limited solubility and stability during deliv-
ery of antiviral molecule ML336 [161]. The large surface area
of the MSN core promotes hydrophobic drug loading while
the liposome coating retains the drug (ML336) and enables
enhanced circulation time and biocompatibility. In vivo
safety studies conducted in mice evidenced that LC-MSNs
were not toxic when dosed at 0.11 g LC-MSNs/kg/day (for
four days). ML336-loaded LC-MSNs showed signicant
reduction in brain viral titer in VEEV-infected mice com-
pared to PBS controls [161]. Choi et al. developed a model
PEGylated lipid bilayer-supported mesoporous silica nano-
particle (MSN) composite for synergistic codelivery of axi-
tinib (AXT) and celastrol (CST) in targeting angiogenesis
and mitochondrial-based apoptosis in cancer [162]. This
hybrid nanoplatform inhibited cell proliferation and induced
an apoptosis eect against cancer cells by blocking mito-
chondrial function, thus leading to enhanced antitumor
ecacy [162]. In Figure 15 we report a schematic descrip-
tion of the design strategies of PEGylated lipid bilayer
supported MSNs composite for dual drug synergistic
co-delivery.
5. Nanomedicine Formulations: Clinical
Development and Approved Materials
In recent years, both the broadening in nanocarrier typology
and the increase in the complexity of particles and materials
employed have inspired explorations for new nanodelivery
systems and brought about various products as well as
numerous clinical trials for biotechnology and nanomedicine
applications. However, before the premarket authorization,
nanocarriers are subject to a range of preclinical and clinical
validation by regulatory agencies, such as the European Med-
icines Agency (EMA) and the Food and Drug Administra-
tion (FDA) in the USA. Recent review articles provide a
summary of the range of approved therapeutic nanomedi-
cines and a description of novel nanoplatforms that are
emerging through the clinical trial pipeline [163165].
In Table 1, we report the main organic and inorganic
nanomedicines approved by FDA [163, 164]. Among the
organic nanomedicines approved for use on the market, it
is useful to distinguish between the two main categories of
polymer-based and lipid-based nanoparticles. Despite the
17Journal of Nanomaterials
intensive (preclinical) research involving block copolymer
micelle nanocarriers, only a few of them have reached
clinical evaluation or have been approved for the employ-
ment in the market [165]. The degradable hydrophobic
polymers PLA, PLGA, and PDLLA are the most promising
polymer systems for the development of nanoformulation, as
they slowly decompose into constituent monomeric units
over well-dened time courses. For example, Eligard for-
mulation, obtained from the incorporation of leuprolide (a
testosterone-inhibiting drug) into a polylactide-co-glycolic
acid (PLGA) nanoparticle, represents a long-established
polymer nanoparticle for the treatment of the prostate cancer
symptoms. As evidenced in previous sections, the employ-
ment of PEG ensures the shield action against the recognition
and degradation by the immune system and can amelio-
rate the biodistribution of the drug by increasing the circula-
tion half-life. This eect is exploited in the Adynovate
formulation (obtained by the PEGylation of the antihemo-
philic factor VIII), which is employed for the treatment of
hemophilia A, and in the Cimzia formulation (obtained by
the PEGylation of the antibody fragment Certolizumab)
which is employed for the treatment of Crohns disease,
rheumatoid/psoriatic arthritis, and ankylosing spondylitis
[163165]. Over the last decade, a number of promis-
ing block copolymer micelle formulations entered the clini-
cal development, with two receiving regulatory approval:
namely, the Cynviloq (paclitaxel-loaded PEG-PDLLA block
copolymers) and Nanoxel (docetaxel-loaded PEG-PDLLA
block copolymers) [165]. Finally, Genexol-PM is a poly(eth-
ylene glycol)-block-poly(D,L-lactide) (PEG-PDLLA) diblock
copolymer micelle loaded with paclitaxel (developed by
Samyang Corporation) which was rst introduced in the
Korean market and is actually approved by FDA in clinical
trials for the treatment of metastatic breast cancer.
Liposomal formulations represent the most success-
ful category of nanocarriers employed for drug delivery
purposes [163, 164]. Two largely employed PEGylated
liposome-based doxorubicin formulations for the treatment
of epithelial ovarian Kaposis sarcoma are Doxil, composed
of hydrogenated soy L-α-phosphatidylcholine (HSPC) : cho-
lesterol : PEG 2000-DSPE (56 : 39 : 5 molar ratio), and Lipo-
Dox, composed of 1,2-distearoyl-sn-glycero-3-phosphocholine
(DSPC) : cholesterol : PEG 2000-DSPE (56 : 39 : 5 molar ratio).
A non-PEGylated version of liposomal doxorubicin formula-
tion employed for breast cancer treatment is provided by
Myocet, composed of egg PC (EPC) : cholesterol (55 : 45
molar ratio). Other anticancer drugs formulated with lipo-
somes are approved for (dierent stages) clinical studies.
Finally, Thermodox (produced by Celsion Corporation),
composed of DPPC, mono steroyl PC (MSPC), and PEG
2000-DSPE, is a temperature-responsive version of the
PEGylated formulation of liposomal doxorubicin, actually
in phase III clinical trials, which is able to release its drug
content inside the tumor upon heat transfer, using a radiofre-
quency ablation (RFA) process.
In Figure 16, we report a sketch of the main characteristic
of PEGylated liposomal formulations employed for intrave-
nous administration of anticancer drugs (inset A), together
with the main components of Doxil (inset B), the rst
FDA-approved nanodrug formulation, developed by Bare-
nholz [166]. Anticancer drugs doxorubicin, daunorubicin,
paclitaxel, and vincristine (Figure 16(a)) are among the most
extensively investigated agents for the liposome-based drug
formulations, and several liposomal formulations of these
agents are currently in clinical use in cancer therapy.
Inorganic nanoparticles represent the category of nano-
carriers which received the approval of the minor number
of nanomedicines for their use in the market. A number
of metallic nanoparticles with applications in both therapeu-
tic and imaging processes (theranostics) are under the dif-
ferent phases of clinical trials. In particular, some of the
inorganic nanomaterials, such as gold nanoparticles and
DSPE-PEG2000
DMPC
Mesoporous silica
nanoparticles (MSNs)
Drug B loading
Coating with
PEGylated lipid bilayer
Drug A
loading
PEGylated lipid bilayer-supported MSN composite
for co-delivery of drug A and drug B
Figure 15: Design strategies of a PEGylated lipid bilayer-supported mesoporous silica nanoparticle (MSN) composite for dual drug
synergistic codelivery.
18 Journal of Nanomaterials
silica nanoparticles, have encountered obstacles in early-
stage clinical trials, due to toxicity and a lack of stability crit-
ical issues. To date, only three iron oxide nanoparticles have
completed FDA approval (Feraheme,Feridex, and Gastro-
MARK), two of which have been later withdrawn from the
market [163]. Although the iron oxide nanoparticles present
an increasing research interest as contrast enhancement
reagents for magnetic resonance imaging, the majority of
FDA-approved nanoplatforms (such as INFeD and Venofer)
are indicated as iron replacement therapies. NanoTherm is
the only one that has obtained approval for clinical use for
the treatment of glioblastoma, whereby tumors are thermally
ablated by magnetic hyperthermia induced by entrapped
superparamagnetic iron oxides.
Finally, signicant research is still required to understand
and predict how these materials will behave in biological sys-
tems, while further preclinical and clinical studies are
required in order to oer the best performance into the
(in vivo) environment where the actual release will take place.
6. Conclusions and Future Perspectives
We highlight recent advances of smart nanocarriers in the
development of novel platforms for the ecient transport
and controlled release of drug molecules. The main aim of
ecient nanostructured delivery systems is to reduce the
drug dose needed to achieve a specic therapeutic eect, thus
lowering the costs and reducing the side eects associated
Table 1: List of relevant (polymer-based and lipid-based) organic and inorganic (and metallic) nanomedicines approved by the FDA.
Clinical products Formulation Indication Company Year
Polymer-based nanoparticles
Renagel Poly(allylamine hydrochloride) Chronic kidney disease Sano2000
Eligard Leuprolide acetate and polymer PLGA
(poly (DL-lactide-co-glycolide)) Prostate cancer Tolmar 2002
Estrasorb Micellar estradiol Menopausal therapy Novavax 2003
Cimzia/certolizumab
pegol
PEGylated antibody fragment
(certolizumab)
Crohns disease
Rheumatoid/psoriatic arthritis
Ankylosing spondylitis
UCB 2008-2013
Genexol-PM mPEG-PLA micelle loaded with
paclitaxel Metastatic breast cancer Samyang Corporation
2007
South
Korea
Adynovate Polymer-protein conjugate
(PEGylated factor VIII) Hemophilia Baxalta 2015
Lipid-based nanoparticles
Doxil/Caelyx Liposomal doxorubicin
Ovarian, breast cancer,
Kaposis sarcoma, and
multiple myeloma
Janssen 1995-2008
DaunoXome Liposomal daunorubicin AIDS-related Kaposis
sarcoma Galen 1996
Myocet Liposomal doxorubicin
Combination therapy
with cyclophosphamide
in metastatic breast cancer
Elan Pharmaceuticals 2000
Marqibo Liposomal vincristine Acute lymphoblastic leukemia Talon Therapeutics Inc. 2012
AmBisome Liposomal amphotericin B Fungal/protozoal infections Gilead Sciences
Visudyne Liposomal verteporn
Choroidal neovascularisation,
macular degeneration,
wet age-related, myopia,
and ocular histoplasmosis
Bausch and Lomb 2000
Onivyde Liposomal irinotecan Pancreatic cancer Merrimack 2015
Inorganic and metallic nanoparticles
INFed Iron dextran (low MW) Iron deciency in chronic
kidney disease (CKD) SanoAventis 1957
Feridex/Endorem SPION coated with dextran Imaging agent AMAG Pharmaceuticals 1996-2008
Venofer Iron sucrose Iron deciency in chronic
kidney disease (CKD)
Luitpold
Pharmaceuticals 2000
GastroMARK;
umirem SPION coated with silicone Imaging agent AMAG Pharmaceuticals 2001-2009
NanoTherm Iron oxide Glioblastoma MagForce 2010
19Journal of Nanomaterials
with their use. The two main categories of organic and inor-
ganic nanostructured materials widely employed in drug
delivery processes present a variety of complementary and
synergistic properties that can be protably exploited. On
the one hand, the organic soft nanocarriers (such as amphi-
philic polymers and liposomes) present better properties to
match the physicochemical condition encountered in biolog-
ical (and pathological) tissues, thus furnishing the best exam-
ples of biocompatible nanostructures. On the other hand, the
hard nanoparticles composed of inorganic materials (such as
quantum dots and gold and mesoporous silica nanoparticles)
propose the complementary functions for the diagnosis and
detection of the pathological conditions within the diseased
tissues. As the microenvironment conditions within the dis-
eased tissues have a great impact on delivery eciency of
nanocarrier systems, the choice of nanocarrier properties
(such as the size, shape, material substrate, and surface chem-
istry) plays a crucial role in the design of ecient nanocar-
riers for specic functions.
Despite that a large variety of smart nanocarriers have
been developed in recent years, the intrinsic complexity of
biological environments strongly inuences the functionality
of the nanomaterial and often complicates their eective use
for therapeutic treatments. Although these nanomedicines
show good performance against specic diseases, their inher-
ent drawbacks, mainly connected with the limited absorption
and request of frequent injection for patients, cannot be
ignored [167]. Therefore, a deeper knowledge and under-
standing of the real interactions involved in the diseased tis-
sues is fundamental for the development of novel therapeutic
protocols based on the employment of smart nanocarriers.
The diculty to predict the behavior (and responses) of
nanocarriers during the drug delivery processes is connected
with the diculty to fully describe (and model) the complex
structural and dynamic processes involved in biological sys-
tems. In this respect, the investigation of a multiplicity of
simultaneous factors and biological functionality may be
replaced with the systematic study of the eect of a few
parameters at a time (such as surface charge density and/or
nanoparticle size/topology). The identication of the key fac-
tors for the design of ecient nanocarriers represents then
the fundamental (initial) step to decipher the complexity
involved in complex biological processes. Therefore, a dee-
per knowledge and understanding of the real interactions
involved in the diseased tissues is fundamental for the devel-
opment of novel therapeutic approaches and protocols based
on the employment of smart nanocarriers.
Conflicts of Interest
The authors declare that there is no conict of interest
regarding the publication of this paper.
Paclitaxel
Daunorubicin
Doxorubicin Vincristine
OO OOH
OOH
OH
OH
OH
NH2
H3C
HO
O
OOO
OH
OOH
OH
OH
NH2
CH3
CH3
O
OH OH
OH
O
O
H
HN
N
N
N
H
HH
NH
OO
O
OOH
OH
O
OOOO
OO
O
O
OO
O
O
O
(a)
PEG
Anticancer drugs
Intravenous
administration
Doxil
DSPE-PEG2000
HSPC
Cholesterol
Doxorubicin
(b)
Figure 16: Main characteristic of PEGylated liposomal formulations employed for (intravenous administration) of anticancer drugs (a). We
also reported the main components of the Doxil formulation, a PEGylated liposome-based doxorubicin formulation for the treatment of
epithelial ovarian Kaposis sarcoma.
20 Journal of Nanomaterials
Acknowledgments
D.L. acknowledges funding from Marie-Curie Actions under
European Commission FP7 Initial Training Network SNAL
608184. The work of M.A.K. was nanced by the Russian
Scientic Foundation (Project no. 14-12-00516).
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