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We live in a world where the lifetime of electronics is becoming shorter, now approaching an average of several months. This poses a growing ecological problem. This brief review will present some of the initial steps taken to address the issue of electronic waste with biodegradable organic electronic materials. Many organic materials have been shown to be biodegradable, safe, and nontoxic, including compounds of natural origin. Additionally, the unique features of such organic materials suggest they will be useful in biofunctional electronics; demonstrating functions that would be inaccessible for traditional inorganic compounds. Such materials may lead to fully biodegradable and even biocompatible/biometabolizable electronics for many low-cost applications. This review highlights recent progress in these classes of material, covering substrates and insulators, semiconductors, and finally conductors.
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ISSN:1369 7021 © Elsevier Ltd 2012
Sustainability is “the ability of a generation to ensure its needs
for the present without compromising the ability of the future
generations to meet their own needs”
. While it is difficult to provide
a thorough description for consumption, the definition provided in
reference 3 seems to incorporate the multitude of problems that
consumption poses to the sustainable development of modern
society: “Consumption is the human transformation of materials
and energy (along the production-consumption chain) that makes
the transformed materials or energy less available for future use,
or negatively impact biophysical systems in such a way to threaten
human health, welfare, or other things people value”.
Plastic consumption and waste are two of the major concerns in the
modern world. Polyethylene for example is currently the leading plastic
material, with a global consumption of about 83 million metric tons in
2010, mostly for use in plastic bag production (>1 trillion/year)
. Due
to the increased demand in countries with emerging economies, plastics
consumption is projected to increase approximately by a factor of three
during the current decade
. The outcome of the constant demand for
plastics is the buildup of non-biodegradable solid waste and plastic
litter (estimated at 25 million tons/year in the year 2000) with negative
consequences on our environment
7, 8
. Plastics normally biodegrade
very slowly, with full degradation occuring after 500 or 1000 years
We live in a world where the lifetime of electronics is becoming shorter,
now approaching an average of several months. This poses a growing
ecological problem. This brief review will present some of the initial steps
taken to address the issue of electronic waste with biodegradable organic
electronic materials. Many organic materials have been shown to be
biodegradable, safe, and nontoxic, including compounds of natural origin.
Additionally, the unique features of such organic materials suggest they
will be useful in biofunctional electronics; demonstrating functions that
would be inaccessible for traditional inorganic compounds. Such materials
may lead to fully biodegradable and even biocompatible/biometabolizable
electronics for many low-cost applications. This review highlights recent
progress in these classes of material, covering substrates and insulators,
semiconductors, and finally conductors.
Mihai Irimia-Vladu
, Eric. D. Głowacki
, Gundula Voss
, Siegfried Bauer
and Niyazi Serdar Sariciftci
Department of Soft Matter Physics, Johannes Kepler University, A-4040 Linz, Austria
Linz Institute for Organic Solar Cells (LIOS), Physical Chemistry, Johannes Kepler University, A-4040 Linz, Austria
Green and biodegradable
MATTOD_2012_2_Review_Sarif 340 23-08-12 13:58:27
Green and biodegradable electronics REVIEW
Plastic electronics on the other hand represent an emerging field of
science and technology that began in the realm of academic curiosity
thirty years ago, but which has now made some amazing advancements.
The industrial and commercial potential of organic molecules has been
demonstrated in recent years through mature OLED technology and the
recent surge of organic photovoltaics (OPV), with reports of efficiencies
of about 10 % or higher in 2012
. Samsung produced 45 million OLED
displays in 2011 and projects to build up to 600 million units by 2015
whereas printed flexible photovoltaics are currently commercialized at
a smaller scale for rooftop and small appliance applications. Although
compared to the global production and consumption of plastics in the
world, the organic electronics market contributes only a small amount
of waste, this number should not be neglected. A symbolic example of
the problems that e-waste poses to the environment is presented in
Fig. 1. In the race to match the performance of inorganic materials and
realize attractive new consumer products like OLED displays, the issues
of biodegradability and biocompatibility of the materials employed
in organic electronics are often not considered. Up to the present, a
large amount of research has focused on synthetic avenues for the
production of active layers in organic field effect transistors (OFETs),
organic light emitting diodes (OLEDs), and organic photovoltaics (OPVs)
– neglecting how these materials affect the environment, as well as
questions of toxicity to humans, animals, and plants. Nevertheless, the
true biocompatibility of such materials is of paramount importance, not
only for the development of biomedical devices and applications involving
interfacing with living tissue
but also for human friendly electronics in
. The development of neural prosthetics, neural implants, drug
delivery devices, and diagnostic electronics all require materials that are
the least invasive
. Research in this area is relatively well-developed
compared to ‘green’ technology for organic optoelectronics. This success
can be explained by considering the unique advantages of organic
conducting materials: (1) the combination of mechanical robustness with
flexibility; (2) nontoxicity and the property of not eliciting inflammatory
or immune responses; and (3) the ability to behave as ionic and electronic
conductors, and thus interface electronics with the protonic and ionic
currents present in biological systems. This brief review will present the
recent advancements in terms of biocompatible/biodegradable materials
as well as technologies and devices. In the following, organic materials will
be classified according to their functionality: substrates and insulators,
semiconductors, and conductors.
Substrates and insulators
Numerous materials with a bio-origin have been identified as suitable
substrates for the fabrication of organic electronics. Additionally, many
of these materials demonstrate excellent insulating properties, which
combined with the ease of their processability make them suitable as
gate electrode insulators for OFET applications. Such materials enable
several functionalities: low-cost, non-toxicity, biodegradability, and often
biocompatibility and bioresorbability for biomedical applications.
One of the oldest and most familiar ‘substrate’ materials of natural
origin is paper. Paper is made from plant-derived cellulose. Many varieties
of paper are known, and the science of mass-producing paper with
desirable mechanical and surface properties is mature. It is by far the
cheapest biodegradable substrate material and enables large-area printing
of ‘use-and-throw’ devices. Arrays of OFETs and OFET circuits have been
printed on paper, demonstrating flexible devices with performance on-par
with more traditional substrates (Fig 2a)
. Low-voltage active circuits
have been realized on banknotes, for anti-counterfeiting applications
. It
was shown that despite the surface roughness of banknote paper, OFETs
operating at less than 1 V with mobilities of 0.2 cm
/Vs could be fabricated
reliably. A photo of a circuit printed on a banknote is shown in Fig. 2b.
Electronic circuits on paper have been the subject of a recent extensive
. Paper substrates have been used for flexible electrowetting
, as well as thermochromic displays for disposable consumer
. Encouraging performance has been demonstrated for paper-
based organic photovoltaics. Recently, fully roll-to-roll solution printing
using gravure and flexographic printing techniques was utilized to make
printed-paper photovoltaics
. These devices use an inverted design, with
a printed ZnO/Zn back electrode and a conducting polymer transparent
top electrode. Such solar cells combine ultra low-cost materials, high-
throughput low-temperature roll-to-roll printing, and a flexible final
product. A photo and cross-sectional diagram of these paper solar
cells are shown in Fig. 2c. Another reported approach for paper-based
photovoltaics features thin semitransparent paper as a substrate, with
a conducting polymer transparent electrode, organic active layer, and
reflective back electrode all fabricated via low-temperature chemical
vapor deposition
. Arrays of these devices are shown in Fig. 2d, with
Fig. 2e illustrating folded devices. The cells can be folded repeatedly with
no degradation of the array performance.
Another natural material with a long history is silk. Silk is a polypeptide
polymer, consisting of two main proteins: fibroin and sericin. Fibroin is
made up primarily of repeating units of glycine, serine, and alanine that
afford interchain hydrogen bonding, providing the mechanical robustness
of silk fibers
. A molecular structure of fibroin is shown in Fig. 3a. This
material combines many advantages for biodegradable or biomedical
Fig. 1 Pictographic example of electronic waste. Electronic products bring pleasure
and comfort to consumers, but ultimately will reach the end of their life. Plastics
have a slow biodegradation route that lasts up to 1000 years, while the electronic
circuits contain many toxic and environmentally dangerous materials. Image
courtesy of Park Howell ( Reproduced with permission.
MATTOD_2012_2_Review_Sarif 341 23-08-12 13:58:29
ISSN:1369 7021 © Elsevier Ltd 2012
REVIEW Green and biodegradable electronics
applications. Silk is fully bioresorbable and elicits no immune response,
and thus can be safely implanted into the body. A recent study showed
that an ultrathin electronic sensor array can be fabricated on silk, which
can then be placed in vivo onto exposed brain tissue. The silk safely
dissolves and resorbs, resulting in conformal coating of folded brain
tissue with the sensor array (Fig. 3b)
. Silicon-based electronics can
also be fabricated onto silk, and the silk can be used as a bioresorbable
carrier to introduce the electronic element in vivo
. Silk can function
as an effective solution-processed gate insulator for OFETs (Fig. 3c),
supporting very high mobilities of ~23 cm
/Vs in pentacene combined
with low-voltage operation
. Recently, silk has been used as a substrate
for passive rf-ID circuits that can be integrated directly onto food, i.e.,
apples, eggs, etc., as sensors of food quality (Fig. 3d)
. Additionally, silk
is fully biodegradable and can be engineered to degrade under desired
conditions, enabling targeted drug storage and delivery, e.g,
Another protein-based material is gelatin, used commonly for
capsules for oral drug ingestion. It is also fully biocompatible and
biodegradable. Electronics built on hard gelatin may easily be ingested
for specific biomedical applications targeting short interrogation time.
Fabrication of OFET devices directly onto hard gelatin capsules has been
. The protein albumin, from chicken egg whites, has been
shown as a high performance cross-linkable solution processed material
for OFET dielectric
. Protein-based materials for sustainable applications
have been recently reviewed
Aside from protein-based polymers, polysaccharides can also be used
as biocompatible substrate materials. Polymers made from starches
and polylactic acid have recently been commercially mass-produced
as biodegradable plastics
. An example is Ecoflex (BASF), a foil plastic
produced from potato and corn starch and polylactic acid. Ecoflex degrades
in compost in six months without leaving any residue
. Caramelized
glucose was recently explored as an exotic substrate for electronics;
despite its sensitivity to moisture, the film forming characteristics of
Fig. 2 Organic electronic devices fabricated on paper substrates. (a) OFET array
deposited on a paper substrate. Reprinted with permission from
. Color image
courtesy of Hagen Klauk. Copyright 2004, American Institute of Physics. (b)
OFETs printed on a banknote. Reproduced with permission from
. Copyright
Wiley-VCH Verlag GmbH & Co. KGaA. (c) A fully solution-processed roll-to-roll
printed solar cell on paper. Reproduced with permission from
. Copyright Wiley-
VCH Verlag GmbH & Co. KGaA. (d) Solar cells fabricated using a monolithic
integrated chemical vapor deposition (CVD) process onto semitransparent
paper, (e) examples of CVD-prepared solar cells. Repeated folding of devices did
not result in degradation. Reproduced with permission from
. Copyright Wiley-
VCH Verlag GmbH & Co. KGaA.
Fig. 3 Silk in electronic devices. (a) Chemical structure of silk fibroin, a polypeptide.
(b) Example of a sensor array for neural recording fabricated on a silk substrate.
Once the device is applied onto the brain, silk is safely dissolved and resorbed,
resulting in a conformal coating of the sensor array onto the brain tissue.
Reprinted from
by permission from Macmillan Publishers Ltd, copyright 2010. (c)
Solution-processed silk fibroin as a gate dielectric for a flexible OFET. This device
was reported with a mobility of ~23 cm
/Vs. Reproduced with permission from
Copyright Wiley-VCH Verlag GmbH & Co. KGaA. (d) A silk-based passive rf-ID
tag, applied on an apple skin to function as a food quality monitor. Reproduced
with permission from
. Copyright Wiley-VCH Verlag GmbH & Co. KGaA.
MATTOD_2012_2_Review_Sarif 342 23-08-12 13:58:29
Green and biodegradable electronics REVIEW
glucose rivaled those of glass in terms of smoothness
. Another ‘historic’
natural polymeric material is shellac. This resin is naturally produced by
female lac beetles, and is harvested from trees in India and Thailand.
Chemically, it is a natural polyester copolymer of terpenic and aleuritic
acids, and can be processed from various polar organic solutions, such as
ethanol. Advantageously, shellac can also be synthetically fabricated in a
multitude of compositional grades and shades
. Both silk and shellac
have excellent surface smoothness (rms < 1 nm, rivaling glass) when
deposited as thin films. Shellac can easily be cast to produce substrate
foils (200 – 500 μm thickness), which have been used to make OFETs and
complementary-type circuits with the natural semiconductor indigo
Deoxyribonucleic acid, or DNA, is the building block of life on earth.
DNA is extracted in large amounts from waste products of the fishing
industry, and is thus a ‘natural feedstock’ material that can be produced
on an industrial scale. This fascinating molecule has inspired many
researchers to apply it in practical applications in photonics and organic
electronics. DNA can be processed from water solutions to produce films
with excellent optical transparency from 400 nm through the NIR region.
Recently, organic light emitting diodes (OLEDs), nonlinear optoelectronic
modulators, and photonic arrays based on DNA have been reported
DNA can also be applied as a gate insulator in organic field effect
transistors. Solution-processed and cross-linked DNA was successfully
implemented as a gate dielectric layer for low operating voltage OFETs
The individual nucleobases (guanine, adenine, thymine, and cytosine) are
extracted commercially for medical and cosmetic applications and have
been implemented as gate dielectrics for OFETs
. An example of a ‘green
OFET’ is shown in Fig. 4a. In this device the substrate is caramelized-
glucose, guanine, and adenine form the gate dielectric, and a nontoxic
textile dye, indanthrene yellow G is the organic semiconductor. A similar
device is shown in Fig. 4b, where guanine and adenine are used as the
gate dielectric for an OFET fabricated on a gelatin capsule. A thin film of
adenine used in combination with electrochemically-grown aluminum
oxide dielectric and C
fullerene can afford OFETs with a low operating
voltage (~0.5 V) and high semiconductor mobility (~5.5 cm
This summary of natural substrates and dielectrics shows that nature
offers an affluence of materials choice that could be integrated into
various organic electronic devices, offering alternatives for biocompatible,
biodegradable, and even bioimplantable and bioresorbable applications.
Nature is replete in π-conjugated molecules that can be used as
semiconductors. Additionally, the synthetic dye industry produces many
conjugated organic dyes that have been determined safe and nontoxic for
use as food or textile colorants and inks. Compared to the exploration of
biocompatible materials such as insulators and substrates, investigations
into biocompatible semiconductors remain sparse. The carotenoids,
such as β-carotene, are linear π-conjugated molecules that act as hole-
transporting semiconductors. Devices using β-carotene and natural gate
insulators such as glucose fabricated on biodegradable plastic substrates
are demonstrations of truly ‘natural’ OFETs
. Nevertheless with
mobilities in the range of 1 × 10
/Vs those devices are not efficient.
Attempts to utilize solution-processed β-carotene in solar cells showed
only modest performance
. The first reports of fully green’ OFET devices
featured biocompatible substrates and natural dielectrics and employed
nontoxic synthetic textile dyes such as anthraquinones and perylene
bisimides; these devices demonstrated mobilities in the 10
– 10
. Fully biomaterial-based OFETs with ambipolar charge transport
mobilities in the range 10
– 0.4 were demonstrated with indigo and its
. In such devices, as shown in Fig. 5, substrate, dielectric,
and semiconductor are all of natural-origin.
Indigo is the most mass-produced dyestuff worldwide, primarily used
for the coloring of blue jeans. Though today produced synthetically,
it originated from several species of plants and has been extracted and
used as a dye since ancient times. Indigo, and its brominated derivative,
6,6‘-dibromoindigo were arguably the subject of the world’s oldest
chemical industry, being produced from natural sources and prized as
commodities as valuable as gold
. Their structures are shown in Fig. 6.
Several indigo derivatives are present in nature, in both plants and animals,
and indigo itself has been reported to be biodegradable and non-toxic
while also having a biosynthetic route involving strains of bacteria
Indigo and its derivatives are thermally- and photochemically-
stable molecules due to intra- and intermolecular hydrogen bonding
between amine hydrogens and carbonyl groups. The excellent planarity
of the molecule and H-bonding result in tight π-stacking between
neighbors, with an interplanar spacing of ~3.4 Å
. Indigo ‘breaks the
rules’ of traditional molecular organic semiconductors as it has minimal
intramolecular conjugation, with carbonyl and amine groups seen to
interrupt conjugation in the resonance model. However, the excellent
charge transport properties of indigoid dyes are attributed to the strong
intermolecular interactions of π-stacking reinforced by hydrogen
bonding. Due to the directionality of π-stacking (typically along the
crystallographic b-axis for most indigoids) charge transport is highly
anisotropic. In order to achieve good OFET performance, molecules
Fig. 4 OFETs on natural substrates. (a) An entirely ‘bio-OFET’ utilizing a
caramelized-glucose substrate, Al gate electrode with an adenine/guanine gate
dielectric, and the nontoxic textile dye indanthrene yellow G functioning as the
semiconductor. (b) An OFET fabricated on a gelatin capsule, utilizing adenine/
guanine gate dielectric and a cosmetic perylene bisimide dye. Reproduced with
permission from
. Copyright Wiley-VCH Verlag GmbH & Co. KGaA.
MATTOD_2012_2_Review_Sarif 343 23-08-12 13:58:30
ISSN:1369 7021 © Elsevier Ltd 2012
REVIEW Green and biodegradable electronics
Fig. 5 An OFET device utilizing all natural materials. The substrate is shellac foil, produced by drop casting from ethanol solution. An electrochemically-grown Al
layer passivated with the natural oligoethylene tetratetracontane serves as the gate dielectric. The semiconductor material is indigo. The image on the right shows a
complete array of several such OFETs on a shellac substrate. These devices showed excellent stability and balanced electron and hole mobilities of 0.01 cm
/Vs. From E.
D. Glowacki, M. Irimia-Vladu, “Natural and Nature-inspired Materials in Organic Electronics”, SPIE Newsroom 2012, doi: 10.1117/2.1201201.004054. Reproduced with
Fig. 6 Chemical structures of indigo and its dibromo derivative. Indigo is derived from several species of plants, Tyrian purple originates from the glands of different
species of marine mollusks.
must adopt a ‘standing-up’ conformation, with π-stacking parallel to
the gate dielectric. To achieve this, aliphatic dielectric materials are used,
such as polyethylene or the natural oligoethylene tetratetracontane.
Indigo and Tyrian purple both show reversible two-electron
reduction and oxidation electrochemistry, and have small band gaps
(1.7 – 1.8 eV) and thus are suitable for ambipolar OFETs and voltage
inverter circuits
. An example of high performance ambipolar transport
in OFETs is displayed in Fig. 7a where Tyrian purple (6,6’-dibromoindigo)
is evaporated on a polyethylene-passivated aluminium oxide dielectric.
The device shows well-balanced electron and hole transport channels
with mobilities of 0.3 – 0.4 cm
. Fig. 7b shows complementary like
inverters fabricated with a Tyrian purple channel and Au source and
drain electrodes. The gain of ~250 – 290 is among the best reported
for a single semiconductor with a single type of contact electrode
These results show that cheap and nontoxic materials of natural origin
can compete with the best synthetic organic semiconductors. Research
into the biodegradation and biocompatibility of organic semiconductors
remains very limited.
Exploration of biocompatible conducting materials in recent years has
been a vibrant field. In addition to electronic conduction, many materials
with a bio-origin are ionic conductors. Both modes of conduction have
potential application in biodegradable electronic products as well as
biomedical devices. Historically, the earliest organic electronic ‘device’
, a
resistive switching element, was based on melanin, a biological polymeric
material responsible for brown-black pigmentations in animals, including
humans. Since the first reports on conductivity in melanin, it has been
employed in various sandwich diode type devices
. The conductivity
of melanin is heavily dependent on hydration of the material. Recently
it was found that it can be used in thin-film form as a conductor in
biomedical applications, showing both compatibility with living tissue
and bioresorbability
. Originally, an amorphous semiconductor model
was applied to understand the mechanism of charge transport in melanin,
however more recently concise evidence has been shown that in fact
proton conductivity is the mechanism responsible for charge transport
in melanin
. Proton-conducting materials, extensively researched for
MATTOD_2012_2_Review_Sarif 344 23-08-12 13:58:30
Green and biodegradable electronics REVIEW
fuel cell applications, have recently been recognized for great potential
in biocompatible electronics. The motivation is two-fold: firstly, natural-
origin proton conducting materials enable sustainable devices, and
secondly, a multitude of biological pathways involve protons, therefore
proton/electronic interfaces are of interest for creating biomedical
devices. Many conducting polymers are uniquely suited as bioelectronics
interface materials because they can conduct both ionic and electronic
currents. This is not accessible for traditional metallic conductors. A
recent demonstration of a proton-conducting polysaccharide thin-film
transistor device controlled by the electronic field effect of a gate is a
functional realization of the electronic/protonic interface
. A device
schematic is shown in the top portion of Fig. 8. This device utilizes the
polymer chitosan, obtained from the deacetylation of chitin, the structural
polymer composing the exoskeletons of crustaceans. Commercially
available chitosan is derived from shrimp. A recent report has shown that
transistors with solution-processed chitosan proton conductors could be
fabricated on paper substrates
While applied research with truly ‘natural’ conductors remains limited,
the field of synthetic conducting polymers is relatively mature. Conducting
polymers such as polyaniline, poly(pyrrole), and poly(thiophenes) have
demonstrate excellent biocompatibility in biological applications
Of the well-known electron-conducting polymers in organic electronics
the system poly(3,4-ethylenedioxythiophene) doped with the polyanion
poly(styrenesulfonate) (PEDOT:PSS) has been implemented in a variety
of biosensing applications, and even in vivo studies. Reports on cell growth
on PEDOT:PSS films suggest a lack of any toxicity. PEDOT:PSS can be
applied on living brain tissue as conformal polymer electrodes for in vivo
electrocorticography, showing a superior signal/noise ratio compared with
traditional measurements
. Interfacial conducting PEDOT nanotubes
have also been successfully used for neural recording
. Recent work
shows that PEDOT can even be electro polymerized in situ in a living brain,
accomplishing a therapeutic effect
. PEDOT has been shown to be an
effective conductor of anions as well, while PSS can serve as a conducting
medium for cations such as Ca
, Na
, K
and the neurotransmitter
. A PEDOT-based acetylcholine voltage-driven ‘pump’ is
shown in the bottom portion of Fig. 8. A sizeable body of work has been
published where PEDOT:PSS formulations function as a ionic/electronic
interface material to transduce ionic currents into electronic ones and vice
versa, and has been recently reviewed
. A number of other conducting
polymers have been found to be biocompatible, including polyaniline and
Fig. 7 Tyrian purple electronic devices. (a) Tyrian purple OFET transfer
characteristics. Reproduced from
Copyright 2010, with permission from
Elsevier. (b) Output characteristics of a Tyrian purple-based voltage inverter (a
complementary-like circuit) showing among the highest-reported gains for a
single organic ambipolar material. Adapted from
; used in accordance with the
Creative Commons Attribution 3.0 Unported License.
Fig. 8 (Top) A schematic of a bioprotonic transistor. Palladium functions as a
protode (proton-source and drain material) with an electronic gate modulating the
protonic current through chitosan, a biopolymer. Proton mobility was found to be
~4 × 10
/Vs. Reprinted from
by permission from Macmillan Publishers
Ltd, copyright 2011. (Bottom) An organic electronic ion pump, fabricated using
conventional microfabrication techniques. A single device has dimensions of 10 μm.
Reproduced with permission from
. Copyright Wiley-VCH Verlag GmbH & Co.
MATTOD_2012_2_Review_Sarif 345 23-08-12 13:58:30
ISSN:1369 7021 © Elsevier Ltd 2012
REVIEW Green and biodegradable electronics
. Both retain conductivity and favorable mechanical properties
like flexibility in biological systems while being nontoxic and not triggering
an immune response. Though these common conducting polymers have
been shown to be nontoxic and remarkably biocompatible, there is a lack of
reports in the literature concerning biodegradation of these materials.
Organic materials are uniquely suited to produce electronics that can not
only be sustainable and biodegradable, but can also have functionalities
inaccessible to standard crystalline semiconductors, such as the
functionalities required in many biomedical applications. Research in the
field of the biointegration of electronics is proceeding swiftly, primarily
because organic materials offer unique advantages. The consideration of
biodegradability and sustainability of organic-electronic based consumer
devices is still in its infancy at present. Recent demonstrations of high-
performance organic electronics based on biomaterials have shown that
truly ‘green’ electronics have potential and, hopefully, are poised to make
a positive impact in the future.
1. Kates, R. W. et al., Environment (2005) 10, 8.
2. Parris, T. M., et al., PNAS (2003) 100(14), 8068.
3. Kates, R. K., et al., PNAS (2003) 8(14), 8062.
4. Patel, R. M., et al., Polyethylene: an account of scientific discovery and industrial
innovations (Industrial and engineering chemistry, chapter 4), ACS Symp Series
(2008) 1000, 71.
5. Roach, J. “Are Plastic Grocery Bags Sacking the Environment?” National
Geographic 2 Sept (2003)
6. Rudnik, E., Compostable Polymer Materials, (Elsevier, Amsterdam, ed. 1), (2008) 3.
7. Orhan, Y., et al., Intern Biodeter Biodeg (2000) 45, 49.
8. Slack, R. J., et al., Critical Rev Environ Sci Tech (2004) 34, 419.
9. Shah, A. A. et al., Biotechnol Adv (2008) 26(3), 246.
10. Singh, B. et al., Polym Degrad Stability (2008) 93(3), 561.
11. Aramaki et al. Private communication (2011), Mitsubishi press release, certified
12. Heliatek Certified 9.8% efficient organic solar cell. 5.12.2012 press release.
13. Konarka Technologies 9% certified, 28.2.2012 press release
14. Kim, S.-S., Executive VP and CTO of Samsung Mobile Display, Keynote at Display
Week Conference of Society of Information Display (SID), Seattle USA, May 2010.
15. Angione, M. D, et al., Mater Today (2011) 14(9), 424.
16. Muskovich, M., et al., Adv Health Mater (2012) 1(3) 248, 266.
17. Serrano, M. C. et al., Adv Funct Mater (2010) 20, 192.
18. Sekitani, T., et al., Mater Today (2011) 14(9), 398.
19. Irimia-Vladu, M., et al., J Mater Chem (2011) 21, 1350.
20. Khodagholy, D. et al., Adv Mater (2011) 23, H 268.
21. Martin, D. C., Nature Mater (2007) 6, 626.
22. Torsi, L., et al., Nature Mater (2008) 7, 412.
23. Svennersten, et al., Biochim Biophys Acta (2011) 1810, 276.
24. Cui, X. et al., Sens Actuat B Chem, (2003) 89, (1992).
25. Eder, F. et al., Appl Phys Lett (2004) 84(14), 2673.
26. Bollstrom, R., et al Organ Electron (2009) 10, 1020.
27. Zschieschang, U., et al., Adv Mater (2011) 23, 654.
28. Töbjork, D., et al., Adv Mater (2011) 23, 1935.
29. Kim, D. Y., et al., Appl Mater Interf (2010) 2(11), 3318.
30. Siegel, A. C., et al., Lab on a Chip (2009) 9, 2775.
31. Hübler, A., et al., Adv Energ Mater (2011) 1, 1018.
32. Barr, M. C., et al., Adv Mater (2011) 23, 3500.
33. Marsh, R. E., et al., Biochem Biophys Acta (1955) 16, 1.
34. Kim, D.-H., et al., Nat Mat (2010) 9, 511.
35. Kim, D.-H., et al., Appl Phys Lett (2009) 95, 133701.
36. Wang, C.-H., et al., Adv Mater (2011) 23, 1630.
37. Tao, H., et al., Adv Mater (2012) 24, 1067.
38. Hofmann, S., et al., J Control Release (2006) 111, 219.
39. Benfenati, V., et al., Biomaterials (2010) 31, 7883.
40. Lu, Q., et al Biomacromolecules (2011) 12, 1080.
41. Irimia-Vladu, M., et al., Adv Funct Mater (2010) 20, 4069.
42 Chang, J.-W., et al., Adv Mater (2011) 23, 4077.
43. Hu, X., et al., Materials Today (2012) 15(5), 208.
44. Information about Ecoflex® is provided on the BASF internet page http://www2.
45. Irimia-Vladu, M., et al., Organ Electron (2010) 11, 1974.
46. Weinberger, H., et al., Ind Eng Chem (1938) 30, 454.
47. Altman, G. H., et al Biomaterials (2003) 24, 401.
48. Irimia-Vladu, M., et al., Adv Mater (2012) 24, 375.
49. Kwon, Y. W., et al., J Mater Chem (2009) 19, 1353.
50. Hagen, J. A. et al., Appl Phys Lett (2006) 88, 171109.
51. Dong, X et al., Adv Mater (2010) 22, 1649.
52. Lin, P. et al., Adv Mater (2011) 23, 4035.
53. Su, W., et al., Chem Europ J (2011) 17, 7982.
54. Palma, M., et al., Method Mol Biol (2011) 749, 169.
55. Singh, B., et al., J Appl Phys (2006) 100, 024514.
56. Stadler, P. et al., Organ Electron (2007) 8, 648.
57 Yumusak, C., et al., Appl Phys Lett (2009) 95, 263304
58 Schwabegger, G., et al., Synth Metal (2011) 161, 2058.
59. Głowacki, E. D., et al., Proc SPIE (2011) 8118, 81180M-1.
60. Głowacki, E. D., et al., AIP Advances (2011) 1, 042132.
61. Kanbur, Y., et al Organ Electron (2012) 13, 919.
62. Zollinger, H., Color Chemistry: Syntheses, properties, and applications of organic
dyes and pigments, 3
ed. (Wiley-VCH, Weinheim, 2003).
63. Cooksey, C. J., Molecules (2001) 6, 736.
64. Głowacki, E. D., et al., Isr J Chem., (2012) 52(6), 540.
65. Mark, K. K. et al., U.S. Patent No. 5457043 (1995).
66. Ensley B. D. Jr., U.S. Patent No. 4520103, (1985).
67. Pathak, H., et al., Appl Biochem Biotechnol, (2010) 160, 1616.
68. McGinness, J., et al., Science (1974) 183, 853.
69. Bothma, J. P., et al., Adv Mater (2008) 20, 3539.
70. Ambrico, M., et al., Adv Mater (2011) 23, 3332.
71. Bettinger, C. J., et al., Biomaterials (2009) 30, 3050.
72. Bernardus Mostert, A., et al., Appl Phys Lett (2012) 100, 093701.
73. Zhong, C., et al., Nature Comm (2011) 2, 476.
74. Dou, W., et al., arXiv:1205.1309v1 (2012).
75. Ravichandran, R., et al., J R Soc Interface (2010) 7, S559.
76 Abidian, M. R., et al., Adv Mater (2009) 21, 3764.
77. Richardson-Burns, S. M, et al., J Neural Eng (2007) 4, L6.
78. Isaksson, J., et al., Nature Mater (2007) 6, 673.
79. Bolin, M. H., et al., Sens Actuat B: Chem (2009) 142, 451.
80. Tybrandt, K., et al., Adv Mater (2009) 21, 4442.
81. Tybrandt, K., et al., PNAS (2010) 107, 9929.
82. George, P. M., et al., Biomaterials (2005) 26, 3511.
MATTOD_2012_2_Review_Sarif 346 23-08-12 13:58:32
... Because jellyfish are consumed by some natural predators, it is feasible that a predator could incidentally ingest a robotically stimulated jellyfish that has been deployed for more remote or autonomous ocean expeditions, where they remain unmonitored or where human monitors are unable to intervene. For ethical purposes, the environmental impacts can be minimized by using more natural or biodegradable materials for the electronic components, sensors, and housing elements (Irimia-Vladu et al., 2012). For scientific purposes, the use of multiple jellyfish as swarms to plan for data loss is key, although real-time data collection methods would be ideal to obtain sensor information before the jellyfish are offline. ...
... Even with ideal results regarding the longevity of individual jellyfish, because the ocean is a complex, unstructured environment for any robotic system, the longevity of individual robotically controlled jellyfish is unknown due to natural predation. To ensure that lost systems do not contribute to ocean pollution or cause harm to other organisms, advances in biodegradable electronics (Irimia-Vladu et al., 2012;Li et al., 2018), and either natural materials or biodegradable housing and structural elements, should be pursued to replace conventional hardware in these systems. Further ethical consideration should be given to addressing any concerns, from the individual level for jellyfish test subjects (e.g., by continuous monitoring of stress-related molecular biomarkers) to species-wide and ecological levels (e.g., by considering potential unintended consequences), so that this new approach is a model for both its technological innovation and its ethical pursuit of research. ...
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Substantial efforts have been made to expand our knowledge of the physics, biology, chemistry, and geography of the ocean using state-of-the-art measurement tools. With new global projects and technological advances, the collaborative efforts of the Ocean Decade (2021–2030) are well on the way to revolutionizing our knowledge of ocean sciences and sustainability. Yet even today, over three-quarters of the seafloor is still unmapped, more than 90% of marine life still awaits discovery and classification, and the number of ocean sensors required to study global phenomena at sufficient temporal and spatial resolutions is seemingly intractable. To address this challenge, new approaches such as bio-inspired robotics can expand our existing toolbox and bridge this knowledge gap. The concept of biology-inspired engineering has emerged as a powerful tool to complement traditional engineering approaches to technology development. For example, specific swimming features of jellyfish and fish have been applied to a variety of fields, from vehicular propulsion to wind energy to medical diagnostics. In particular, jellyfish are advantageous model organisms because of their energy efficiency, with the lowest known cost of transport compared to other animals, as well as their ubiquity and survivability in various ocean environments. In this article we highlight the evolution of research into jellyfish-inspired robotic constructs and their potential applications in ocean exploration. After initial projects using entirely engineered materials (i.e., jellyfish- inspired submarine propellers) and tissue engineering methods (i.e., rat cardiac cells seeded on flexible films), recent work to integrate microelectronic systems onto live jellyfish demonstrates that their swimming speeds can be increased (up to three times compared to their baselines) and their energy efficiency can be improved (up to four times compared to their baselines). This shows promise for the robotic control of jellyfish in real-world oceanic environments, where the animals are already distributed globally. Future work can improve the maneuverability of these bio-hybrid jellyfish robots, incorporate miniaturized sensors to profile regions of interest, and ultimately deploy swarms of these low-power, low-cost robots to obtain high-resolution data and improve ocean climate models. The synergy of bio-inspired technologies with existing ocean measurement tools holds promise to push the frontiers of ocean exploration and stewardship.
... The new paradigm of edible electronics, emerging in the more general framework of green electronics, 14,15 exploits the inherent electronic properties of food and food additives. 16,17 This makes it optimal for electronic devices in close contact with food as it aims to develop devices safe for human consumption. ...
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Improper freezing of food causes food waste and negatively impacts the environment. In this work, we propose a device that can detect defrosting events by coupling a temperature-activated galvanic cell with an ionochromic cell, which is activated by the release of ions during current flow. Both the components of the sensor are fabricated through simple and low-energy-consuming procedures from edible materials. The galvanic cell operates with an aqueous electrolyte solution, producing current only at temperatures above the freezing point of the solution. The ionochromic cell exploits the current generated during the defrosting to release tin ions, which form complexes with natural dyes, causing the color change. Therefore, this sensor provides information about defrosting events. The temperature at which the sensor reacts can be tuned between 0 and −50°C. The device can thus be flexibly used in the supply chain: as a sensor, it can measure the length of exposure to above-the-threshold temperatures, while as a detector, it can provide a signal that there was exposure to above-the-threshold temperatures. Such a device can ensure that frozen food is handled correctly and is safe for consumption. As a sensor, it could be used by the workers in the supply chain, while as a detector, it could be useful for end consumers, ensuring that the food was properly frozen during the whole supply chain.
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Smart wearable electronic textiles (e-textiles) that can detect and differentiate multiple stimuli, while also collecting and storing the diverse array of data signals using highly innovative, multifunctional, and intelligent garments, are of great value for personalized healthcare applications. However, material performance and sustainability, complicated and difficult e-textile fabrication methods, and their limited end-of-life processability are major challenges to wide adoption of e-textiles. In this review, we explore the potential for sustainable materials, manufacturing techniques, and their end-of-the-life processes for developing eco-friendly e-textiles. In addition, we survey the current state-of-the-art for sustainable fibers and electronic materials (i.e., conductors, semiconductors , and dielectrics) to serve as different components in wearable e-textiles and then provide an overview of environmentally friendly digital manufacturing techniques for such textiles which involve less or no water utilization, combined with a reduction in both material waste and energy consumption. Furthermore, standardized parameters for evaluating the sustainability of e-textiles are established, such as life cycle analysis, biodegradability, and recyclability. Finally, we discuss the current development trends, as well as the future research directions for wearable e-textiles which include an integrated product design approach based on the use of eco-friendly materials, the development of sustainable manufacturing processes, and an effective end-of-the-life strategy to manufacture next generation smart and sustainable wearable e-textiles that can be either recycled to value-added products or decomposed in the landfill without any negative environmental impacts.
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Lignin is an abundant biopolymer deriving from industrial pulping processes of lignocellulosic biomass. Despite the huge amount of yearly produced lignin waste, it finds scarce application as a fine material and is usually destined to be combusted in thermochemical plants to feed, with low efficiency, other industrial processes. So far, the use of lignin in materials science is limited by the scarce knowledge of its molecular structure and properties, depending also on its isolation method. However, lignin represents an intriguing feedstock of organic material. Here, the structural and chemical‐physical characteristics of two kraft lignins, L1 and L2, are analyzed. First, several molecular characterization techniques, such as attenuated total reflectance ‐ Fourier transform infrared spectroscopy, elemental analyses, gel permeation chromatography, evolved gas analysis‐mass spectrometry, UV–vis, 31P‐ and 13C‐ nuclear magnetic resonance spectroscopies are applied to get insights into their different structures and their degree of molecular degradation. Then, their efficient application as gate dielectric materials is demonstrated for organic field‐effect transistors, finding the increased capacity of L1 with respect to L2 in triggering functional and efficient devices with both p‐type and n‐type organic semiconductor molecules. Kraft lignin is applied as the gate dielectric layer in C60 or pentacene organic field‐effect transistors. The role of lignin molecular degradation and functional groups is elucidated.
Renewable source of energy and the materials involved in the design of new electronics are critical elements for sustainable development. Great improvement has been done to expand clean sources of energy; however, despite remarkable progress, clean energy combined to biodegradability is still a challenge. Since the first fully biodegradable silicon electronic system using thin‐film magnesium, magnesium oxide, and silicon nanomembranes in 2012, there has been an ongoing push to study a broader range of biodegradable electrocatalytic materials. To date, it is still challenging to produce biodegradable conductive nanomaterials. In this study, the oxygen reduction reaction (ORR) is demonstrated using a carbon-free fully biodegradable electrocatalyst designed with exfoliated 2D-hematene and ZnO nanostructure. This novel fully biodegradable system showed current density of ∼2.2⁻² (vs 1⁻² for ZnO) and good stability for 14 h. ZnO/Hematene electrocatalyst is a promising step towards biodegradable power source.
A polylactic acid (PLA)-graphene nanoplatelets (GnPs) based ink dispersion in an eco-friendly solvent (ethyl acetate/benzyl alcohol) was developed to prepare conductive coatings for porous and non-porous elastomeric surfaces. The ink dispersion remained highly stable over time and to induce conformability to the elastomeric substrates, the dispersion formulations were modified with glycerol triacetate and silicone (PDMS) for plasticisation and ductility. The ink was spray coated on nitrile rubber (non-porous) and a commercial stretchable fabric (porous). To help adhesion, the nitrile rubber surface was pre-treated with acetic acid and the fabric surface was coated with a thin polychloroprene primer adhesive layer. Coating conductivity was tested under several stretch-release cycles at 25 %. The coatings functioned and remained conductive during and after 150 cycles. Gradually, the coatings electrical resistance doubled over the cycles, but decreased to almost initial levels and stabilized before the full 150 cycles were completed. The mechanical behaviour of the uncoated and coated substrates was described by the Mullins effect (stress softening in rubbers). Upon termination of the cyclic stretch-release tests, the coating resistance recovered to its original value. This is attributed to the mechanical energy dissipated through the cycles within the coatings that triggered microstructural rearrangements establishing original physical connections among GnPs.
With a growing demand for packaging materials and witnessing many landfills and huge garbage islands floating in the Pacific oceans, the need for an alternative material such as bio-degradable plastics has risen. Cellulose-based materials are already in use in several packaging industries. Nanocellulose, a processed cellulose with a specific nanostructure, have several advantages such as high specific strength, modulus, high surface area and unique optical properties. By varying the crosslinking percentages, the kinetics of degradation can be tailored. In this work, extracted cellulose from sugarcane bagasse was hydrolyzed to obtain nanocellulose, which was used to fabricate packaging films (membrane) with PVA as matrix and nanocellulose. Variations of PVA and nanocellulose loadings, and crosslinking agent ratios. In the fabricated films were investigated for chemical, mechanical, optical, thermal, and topographical properties. Results from the degradation tests under appropriate physically simulated environments have suggested that the crosslinking has enhanced the mechanical properties, extent of degradation was dependent on percentages of crosslinking. A real-world device packaging application was demonstrated by encapsulation of perovskite solar cells with the fabricated nanocellulose film revealed that the lifetime of the devices improved which might be indicative of the film having lower permeability for oxygen and moisture.
Biodegradable polymers have been widely used in tissue engineering with the potential to be replaced by regenerative tissue. While conventional bionic interfaces are designed to be implanted in living tissue and organs permanently, biocompatible and biodegradable electronic materials are now progressing a paradigm shift towards transient and regenerative bionic engineering. For example, biodegradable bioelectronics can monitor physiologies in a body, transiently rehabilitate disease symptoms, and seamlessly form regenerative interfaces from synthetic electronic devices to tissues by reducing inflammatory foreign-body responses. Conventional electronic materials have not readily been considered biodegradable. However, several strategies have been adopted for designing electroactive and biodegradable materials systems: (1) conductive materials blended with biodegradable components, (2) molecularly engineered conjugated polymers with biodegradable moieties, (3) naturally derived conjugated biopolymers, and (4) aqueously dissolvable metals with encapsulating layers. In this review, we endeavor to present the technical bridges from electrically active and biodegradable material systems to edible and biodegradable electronics as well as transient bioelectronics with pre-clinical bio-instrumental applications, including biodegradable sensors, neural and tissue engineering, and intelligent drug delivery systems.
The generation of E‐waste is escalating both in developed and developing countries. The impact on the environment and human health is huge due to the toxic chemical components. E‐waste management needs more sophisticated technologies, where it could be carried out only by developed countries and due to the various associated challenges, developing countries could not. The usage of biodegradable material could act as a better replacement to address this issue. The possibilities of using the biodegradable material in the field of electronic industry and the advantages, challenges, limitations associated with it are discussed in this chapter.
Displays and indicators are an integral component of everyday electronics. However, the short lifecycle of most applications is currently contributing to the unsustainable growth of electronic waste. In this work, we utilize ecofriendly materials in combination with sustainable processing techniques to fabricate inkjet-printed, ecofriendly dual-mode displays (DMDs). These displays can be used in a reflective mode or an emissive mode by changing between DC and AC operation due to the combination of an electrochromic (EC) and electrochemiluminescent (ECL) layer in a single device. The EC polymer poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) serves as the reflective layer, while an ECL gel made of dimethylsulfoxide (DMSO), poly(lactic-co-glycolic acid) (PLGA), 1-butyl-3-methylimidazoliumbis(oxalato)borate (BMIMBOB), and tris(bipyridine)ruthenium(II) chloride (Ru2+(bpy)3Cl2) enables the emissive mode. The final dual-mode devices exhibited their maximum optical power output of 52 mcd/m2 at 4 V and 40 Hz and achieved an EC contrast of 45% and a coloration efficiency of 244 cm2/C at a wavelength of 690 nm. The fabricated devices showed clear readability in dark and light conditions when operated in reflective or emissive modes. This work demonstrates the applicability of ecofriendly and potentially biodegradable materials to reduce the amount of hazardous components in versatile display technologies.
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Since the term "sustainable development" was coined, a core set of guiding principles and values has evolved around it. However, its definition remains fluid, allowing institutions, programs of environment and development, and places from local to global to project their own aspirations onto the banner of sustainable development.
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We describe the history of indigo dye and its derivative Tyrian purple, from their roles in the ancient world to recent research showing the semiconducting properties of indigoids. Indigoids are natural dyes that have been produced for centuries, and indigo is currently the most produced dye worldwide. Herein we review the history of these materials, their chemistry and physical properties, and their semiconducting characteristics in the solid state. Due to hydrogen bonding and π-stacking, indigo and Tyrian purple form highly-ordered crystalline thin films. Such films have been used to fabricate high-performance organic field-effect transistors with ambipolar charge transport, as well as complementary-like circuits. Mobility values were found to be in the range of 10−2–0.4 cm2/Vs. With performance on par with the best available organic semiconductors, indigoids demonstrate the potential of sustainable electronics based on biodegradable and biocompatible materials.
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“Exotic” materials have become the focus of recent developments in organic electronics that envision biocompatibility, biodegradability, and sustainability for low-cost, large-volume electronic components. In this brief review, we discuss firstly the use of paper, leather, silk, hard gelatine, and bio-degradable plastics as substrates for electronic devices, and secondly smoothing agents, such as polydimethylsiloxane and aurin. Thirdly, we describe DNA and nucleobases as examples of exotic dielectrics with low dielectric losses and leakage currents as well as sufficiently high dielectric breakdown strength. Fourthly, natural, nature-inspired, and common-commodity semiconductors are presented that broaden the materials base for organic semiconductors and may inspire further work to identify semiconductors that are stable in the face of changing environmental conditions yet degradable at the end of their product lifetime. Sustainability in organic electronics, energy storage, and emerging concepts will also be reviewed briefly. Research on “exotic” organic materials may ultimately result in environmentally safe “green electronic” products.
The atomic-level structure of the crystalline region of B. mori silk fibroin was studied using solid state NMR and statistical mechanical calculations. Whereas lamellar structures were proposed for both (AG)15 and (AGSGAG)5 repetitive sequences of the crystalline region, the β-turn in (AGSGAG)5 was found to be more distorted compared to that in (AG)15. In view of this structural information, silk-like mineralized composite materials were designed and characterized by solid state NMR. Furthermore, recombinant proteins were designed and produced in E. coli and transgenic silkworm. The films of these recombinant proteins exhibit high calcium binding activity and rapid mineralization.
Introduction Polyethylene is the highest volume plastic available today. Global consumption of polyethylene was about 150 billion pounds (67.8 million metric tons) in 2006 and is forecast to grow to about 185 billion pounds (82.9 million metric tons) in 20101. Polyethylene demand, total capacity and percent operating rates from 1995 to 2010 are shown in Figure 11. Polyethylene is composed of mainly carbon and hydrogen (with some notable exceptions such as ethylene vinyl acetate copolymer, acid copolymers, etc.) which can be combined in number of ways. Various polyethylene molecular architectures have been commercialized over last 70 years to make different types of polyethylene. These various molecular architectures can be grouped into ten major types of polyethylene: • LDPE, low-density polyethylene • EVA, ethylene vinyl acetate copolymer • Acid copolymers such as ethylene acrylic acid (EAA) or ethylene methacrylic acid (EMA) copolymers • Ionomers • HDPE, high-density polyethylene • UHMWPE, ultra-high-molecular-weight
Household hazardous waste (HHW) includes waste containing hazardous substances originating from domestic sources. HHW has attracted attention recently because of the steadily increasing levels of municipal solid waste (MSW) of which HHW forms a proportion. A lack of detailed information exists on specific waste types composing HHW and the volumes of HHW produced. In addition, variations in the definition of HHW hamper the quantification process. The majority of MSW and associated HHW is disposed of to landfill. Understanding the flow of this waste from households to disposal facilities will assist in the evaluation of the potential of HHW to harm the environment or human health. This article provides a review of the hazardous components of household waste. It describes attempts to quantify this waste stream and provides an overview of the health and environmental risks posed by such substances. Results confirm a lack of information on sources and quantities of HHW, together with discrepancies in classification systems in different countries and conflicting conclusions concerning potential environmental and health risks.
An investigation based on new X-ray diffraction data, including quantitative spectrometric measurements of X-ray intensities, has led to the derivation of the fundamental structural features of silk fibroin. The structure consists of extended polypeptide chains bonded together by lateral N—H…O hydrogen bonds to form antiparallel-chain pleated sheets. The sequence-G-X-G-X-G-X-in which G represents glycyl and X alanyl or seryl residues predominates throughout the structure, so that adjacent sheets pack together at distances of about 3.5 and 5.7 A. Longer inter-sheet distances are explained by the presence in the structure of the larger amino-acid residues, such as tyrosine.
The skin pigment melanin is one of a few bio-macromolecules that display electrical and photo-conductivity in the solid-state. A model for melanin charge transport based on amorphous semiconductivity has been widely accepted for 40 years. In this letter, we show that a central pillar in support of this hypothesis, namely experimental agreement with a hydrated dielectric model, is an artefact related to measurement geometry and non-equilibrium behaviour. Our results cast significant doubt on the validity of the amorphous semiconductor model and are a reminder of the difficulties of electrical measurements on low conductivity, disordered organic materials.