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Role of Radicals in UV-Initiated Postplasma Grafting of Poly-epsilon-caprolactone: An Electron Paramagnetic Resonance Study
Abstract
Electron paramagnetic resonance (EPR) measurements were performed on poly-ε-caprolactone (PCL) films at different stages of the postplasma-grafting process. PCL films prepared by solvent casting (SC) or electrospinning (ESP) yield very similar EPR spectra after Ar-plasma treatment and subsequent exposure to air, but the EPR signal is much stronger in the PCL-ESP films. The free radicals appear to be mainly, and possibly exclusively, oxygen centered. The radicals generated by UV irradiation in PCL-ESP films were studied in situ with EPR, using a UV-LED (λ = (285 ± 5) nm). Their EPR spectrum is distinctly different from the plasma-induced signal, indicative of carbon-centered radicals, and appears to be independent of the plasma pretreatment. UV-induced homolytic splitting of (hydro)peroxide bonds was not observed. Both the plasma- and UV-induced radicals decay at room temperature (RT), even in an inert atmosphere. This study demonstrates the potential of electrospun films and UV-LEDs for the study of plasma- and UV-generated free radicals with EPR in polyesters, and raises questions with respect to the validity of some generally accepted molecular mechanisms underpinning the postplasma grafting technique for polyesters. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012
... This problem has been addressed by many researchers and led to the development of several surface modification strategies [64,65,66,67,68,69]. Among these techniques, the use of non-thermal plasma has proven to be of great potential, for 2D as well as 3D scaffolds [70,71]. Jacobs et al. describe the use of medium pressure non-thermal plasma treatment in dry air, helium and argon atmosphere to enhance the surface properties of PCL [72]. ...
Polyesters represent a class of polymers comprised of backbone ester linkages offering opportunities to tune the macromolecular properties according to the needs of specific applications. The polyesters developed to date have generated an enormous interest because of their applicability in the biomedical field. Although these are flexible materials in the sense that they can be chemically tuned to obtain the desired properties, one of the important parameters that has to be considered when designing the material for biomedical applications represents the biocompatibility.
The present book chapter aims to review the recent advances for the most commonly studied polyester biomaterials. The first section of this chapter will focus on the synthesis strategies of polyesters as well as possible modification strategies. The second section of this chapter will highlight several polymer processing methods used to obtain scaffolds with different architectures for tissue engineering applications.
This book addresses in an integrated manner all the critical aspects for building the next generation of biorecognition platforms - from biomolecular recognition to surface fabrication. The most recent strategies reported to create surface nano and micropatterns are thoroughly analyzed. This book contains descriptions of the types of molecules immobilized at surfaces that can be used for specific biorecognition, how to immobilize them, and how to control their arrangement and functionality at the surface. Small molecules, peptides, proteins and oligonucleotides are at the core of the biorecognition processes and will constitute a special part of this book. The authors include detailed information on biological processes, biomolecular screening, biosensing, diagnostic and detection devices, tissue engineering, development of biocompatible materials and biomedical devices. © Springer International Publishing Switzerland 2015. All rights reserved.
As introduced in Chap. 1 many biomolecules are involved in molecular recognition processes. These molecules include proteins, peptides, and nucleic acids. In the current chapter we introduce in detail the different recognition molecules found in nature. The chapter analyzes the recognition processes that they mediate and the key aspects of such recognition, including affinity and specificity. Finally, we hint about the tools that can be used in order to modify and expand the natural biorecognition diversity. The chapter aims to provide an overview of the biomolecular complexity and the array of biorecognition functions available in nature or by design, focusing mostly into the two major recognition moieties: proteins and peptides and nucleic acids. We cover some of the biorecognition pairs that are used in the applications described in the following book chapters.
The present chapter provides a collection of the protocols that have been involved in surface chemical derivatization of polymeric materials designed for immobilization of molecules involved in a biorecognition event. The introduction of primary reactive groups at the surface of initially non-reactive materials is discussed and split into two sections: wet chemical methods and physical “dry” methods. Further substrate functionalization is then described according to the type of primary reactive group present at the surface after initial treatment. Specific features such as reversibility or spatial control are highlighted.
This review surveys methods for the fabrication, by plasma surface treatments or plasma polymerization, of polymeric surfaces and thin plasma polymer coatings that contain reactive chemical groups useful for the subsequent covalent immobilization, by solution chemical reactions or vapor phase grafting, of molecules or polymers that can exert bio‐specific interfacial responses. Surfaces containing amine, carboxy, hydroxy, and aldehyde groups are the subject of this review. Aminated surfaces have been fabricated using various plasma vapors or mixtures and have found wide use for bio‐interface applications. However, in many cases the amine surfaces have a rather limited shelf life, with post‐plasma oxidation reactions and surface adaptation leading to the disappearance of amine groups from the surface. Aging is a widespread phenomenon that often has not been recognized, particularly in some of the earlier studies on the use of plasma‐fabricated surfaces for bio‐interfacial applications, and can markedly alter the surface chemistry. Plasma‐fabricated surfaces that contain carboxy groups have also been well documented. Fewer reports exist on hydroxy and aldehyde surfaces prepared by plasma methods. Hydroxy surfaces can be prepared by water plasma treatment or the plasma polymerization of alkyl alcohol vapors. Water plasma treatment on many polymer substrates suffers from aging, with surface adaptation leading to the movement of surface modification effects into the polymer. Both hydroxy and aldehyde surfaces have been used for the covalent immobilization of biologically active molecules. Aging effects are less well documented than for amine surfaces. This review also surveys studies using such surfaces for cell colonization assays. Generally, these surface chemistries show good ability to support cell colonization, though the effectiveness seems to depend on the process vapor and the plasma conditions. Carboxylate co‐polymer surfaces have shown excellent ability to support the colonization of some human cell lines of clinical interest. Immobilization of proteins onto plasma‐carboxylated surfaces is also well established.
XPS O/C ratios (0 ⁰ emission) as a function of storage time, of plasma‐polymerized methyl methacrylate deposited at power levels of 5 and 40 W.
magnified image XPS O/C ratios (0 ⁰ emission) as a function of storage time, of plasma‐polymerized methyl methacrylate deposited at power levels of 5 and 40 W.
One of the major applications of tissue-engineered skin substitutes for wound healing is to promote the healing of cutaneous wounds. In this respect, many important clinical milestones have been reached in the past decades. However, currently available skin substitutes for wound healing often suffer from a range of problems including wound contraction, scar formation, and poor integration with host tissue. Engineering skin substitutes by tissue engineering approach has relied upon the creation of three-dimensional scaffolds as extracellular matrix (ECM) analog to guide cell adhesion, growth, and differentiation to form skin-functional and structural tissue. The three-dimensional scaffolds can not only cover wound and give a physical barrier against external infection as wound dressing, but also can provide support both for dermal fibroblasts and the overlying keratinocytes for skin tissue engineering. A successful tissue scaffold should exhibit appropriate physical and mechanical characteristics and provide an appropriate surface chemistry and nano and microstructures to facilitate cellular attachment, proliferation, and differentiation. A variety of scaffolds have been fabricated based on materials ranging from naturally occurring ones to those manufactured synthetically. This review discusses a variety of commercial or laboratory-engineered skin substitutes for wound healing. Central to the discussion are the scaffolds/materials, fabrication techniques, and their characteristics associated with wound healing. One specifically highlighted emerging fabrication technique is electrospinning that allows the design and fabrication of biomimetic scaffolds that offer tremendous potential applications in wound healing of skin. WIREs Nanomed Nanobiotechnol 2010 2 510–525
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This review concerns the importance of length and time on physicochemical interactions between living tissue and biomaterials that occur on implantation. The review provides information on material host interactions, materials for medical applications and cell surface interactions, and then details the extent of knowledge concerning the role(s) that surface chemistry and topography play during the first stage of implant integration, namely protein adsorption. The key points are illustrated by data from model in vitro studies. Host implant interactions begin nanoseconds after first contact and from then on are in a state of flux due to protein adsorption, cell adhesion and physical and chemical alteration of the implanted material. The many questions concerning the conformational form and control of bound proteins and how this may impact on cell adhesion in the first instance and later on cell signalling and implant integration can be answered by systematic investigations using model materials. Only then we will be in a more informed position to design new materials for use in the body.
We have undertaken Ar plasma irradiation on ethylene–tetrafluoroethylene (1:1) alternating copolymer (ETFE) powder, and the radicals formed were studied by electron spin resonance (ESR). The room temperature ESR spectrum of plasma-irradiated ETFE shows a six-line spectrum (sextet) similar to that of polyethylene (PE), due to a small difference in hyperfine splitting constant between β-H and β-F atom. Thus, the major radical formed was assigned to the midchain alkyl radical generated by hydrogen elimination. The ESR spectral parameters for a representative sextet deduced from the simulated spectra are as follows: g=2.0029, Nα=2.19 mT (1H), Hβ=2.88 mT (2H), and Fβ=2.80 mT (2F). It was also found that the sextet spectrum gradually turned toward a broad single-line spectrum as the plasma duration increased indicating the successive formation of dangling bond sites (DBS) due to the occurrence of cross-linked surface reaction.
The applications of gas plasma and plasma modified materials in the emerging fields of medicine such as dentistry, drug delivery, and tissue engineering are reviewed. Plasma sterilization of both living and non-living objects is safe, fast and efficient; for example plasma sterilization of medical equipment quickly removes microorganisms with no damage to the tiny delicate parts of the equipment and in dentistry it offers a non-toxic, painless bacterial inactivation of tissues from a dental cavity. Devices that generate plasma inside the root canal of a tooth give better killing efficiency against bacteria without causing any harm to the surrounding tissues. Plasma modified materials fulfill the requirements for bioactivity in medicine; for example, the inclusion of antimicrobial agents (metal nano particles, antimicrobial peptides, enzymes, etc.) in plasma modified materials (polymeric, metallic, etc) alters them to produce superior antibacterial biomedical devices with a longer active life. Thin polymer films or coating on surfaces with different plasma processes improves the adherence, controlled loading and release of drug molecules. Surface functionalization by plasma treatment stimulates cell adhesion, cell growth and the spread of tissue development. Plasma applications are already contributing significantly to the changing face of medicine and future trends are discussed in this paper.
ESR measurements are reported in single crystals of urea--n-tetracosane complex. The crystals were irradiated by using a /sup 60/CO source. The structure of peroxy radicals is discussed.(AIP)
An irradiated polyethylene sample containing mostly dienyl free radicals was fabricated by using a combination of photo and thermal treatments. This allowed, for the first time, unambiguous observation of the dienyl radical’s EPR signal to be a singlet with eight hyperfine peaks 9 G apart. Accurate simulation parameters for the dienyl radical were determined. While inaccurate for the dienyl radical, the assumption that a smooth singlet can simulate polyenyl radicals holds true for higher-order polyenyl radicals. The line width of the polyenyl singlet is a function of the radical’s location, with the line width decreasing as the surrounding morphology becomes more organized and uniform, as is the case in the crystalline region. The decreasing line width of polyenyl spectra after exposure to air suggests that the rate of oxidatively induced crystallization is more rapid than oxygen diffusion within the polymer.
The nature and the frequencies of rotation of peroxy radicals in polyethylene and polytetrafluorethylene are determined over a wide temperature range theoretically calculated ESR spectra.
It is shown that various radicals exhibiting diverse ESR and ENDOR spectral characteristics are nonetheless a closely related family of alkoxy radicals. The relationship is established by correlating the g tensor with crystal structure and by relating dihedral angles inferred from proton hyperfine couplings to dihedral angles inferred from the g tensor and crystal structure. The analysis also serves to demonstrate that an ESR absorption observed in x-irradiated single crystals of uridine 5'-monophosphate is due to an alkoxy radical.
Economic and easy methods to tune surface properties of polymers as Poly(vinylidene fluoride) (PVDF) without altering bulk properties are of major interest for different applications as biotechnological devices, medical implant device… UV irradiation appears as one of the simplest, easy and safe method to modify surface properties. In the case of self-initiated grafting, it is generally assumed that the pre-treatment of the PVDF surface with UV irradiation can yield alkyl and per-oxy radicals originating from breaking bonds and capable of initiating the subsequent surface grafting polymerizations. Surprisingly, the present work shows that it is possible to obtain polymer grafting using low energetic UV-A irradiation (3.1–3.9eV) without breaking PVDF bonds. An EPR study has been performed in order to investigate the nature of involved species. The ability of the activated PVDF surface to graft different kinds of hydrophilic monomers using the initiated surface polymerization method has been tested and discussed on the basis of ATR FT-IR, XPS and NMR HRMAS results.
Argon (Ar) plasma pretreated high-density polyethylene (PHDPE) was grafted with acrylic acid (AAc-g-PHDPE) using benzoyl peroxide (BPO) as an initiator and alcohol as a solvent. Effects of plasma treatment time, plasma power, and storage time on CO/CO ratio of PHDPE were studied. Effects of initiator concentration and reaction time on percent grafting were also studied. In this study, the authors used electron spectroscopy for chemical analysis (XPS) to measure the maximum CO/CO ratio of PHDPE, except that the maximum peroxide concentration of PHDPE was measured by 1,1-diphenyl-2-picryhydrazyl measurement method. Furthermore, the PHDPE was susceptible to further surface modification by thermal-induced graft copolymerization with a vinyl monomer, such as AAc and used BPO as initiator. The grafting percentage of AAc-g-PHDPE was from 10% without BPO initiator to 25% with BPO 1×10−4 M. The plasma power and plasma treatment time affected not only the surface composition of treated film but also the grafting percentage. As AAc was grafting in alcohol solution, the carboxylic acid will be formed and an overlapped carboxylate ion.
Biointegration is the ideal outcome which is expected for an artificial implant. That means that the phenomena which seats at the interface between the implant and the host tissues does not induce neither any deleterious effect, such as chronic inflammatory response, nor the formation of unusual tissues. Thus it is of paramount importance to design biomaterials, used for the fabrication of implants, with the best appropriate surface properties. At the same time these biomaterials must feature bulk properties which meet other requirements, especially mechanical properties, deriving from the intended function of the implant in which they are involved. As it is quite impossible to design biomaterials which fulfil at the same time both types of requirements, it is commonly agreed that the solution to this issue goes through the selection or the design of biomaterials with adequate bulk properties, and a further treatment of the surface which would improve the properties of the latter. In this respect ionizing radiations and plasma based treatments, offer a wide panel of possibilities; as an example we describe here how the surface of expanded poly(tetrafluoroethylene) samples can be activated using cold plasma, in order to open a way to chemical modifications of such a surface. Subsequently, Radio Frequency Glow Discharge (RFGD) containing oligopeptides, known for their role in mediating the adhesion of cells to the extracellular matrix, were bound to the modified surface, and the affinity of endothelial cells for the latter was investigated.
Conversion of peroxy radicals into alkyl radicalshas been directly observed by the e.s.r. method in the case of polyethylene in a urea—polyethylene complex system. Abstraction of hydrogen atoms was found to be an intramolecular reaction in this case, of activation energy 24.6 kcal/mol. Evidence confirming this was provided by i.r. spectrometry.
The oxidation process in irradiated polyethylene was investigated by means of the e.s.r. method, and diffusion of oxygen molecules into the crystalline region of polyethylene was studied in detail. Computer simulation was carried out in order to determine various parameters of the process including diffusion constant. In the course of the simulation, the crystallite was assumed to have a plate-shaped form and the diffusion constant was considered to be larger at the region near the surface of the crystallite, Df, and to be smaller at the inner region of the crystallite, Ds. Making these assumptions in the simulation gave a much better result than the assumption based on a sphere-shaped crystallite. was found to be about 2 for thicker crystallite and it was 4 ∼ 5 for thinner crystallite. Diffusion constants at various temperatures were determined, and the order of magnitude of diffusion constants in the crystalline region was found to be 10−16 cm2/sec at about 320K both for thicker and thinner crystallites. The activation energy of diffusion of oxygen in the crystallites was found to be 32 kcal/mol.
An SCF-MO calculation is performed to obtain principal components for the g-tensor of HOO and FOO. Results are then generalized to all peroxy radicals ROO through an equation which relates 〈g〉 to Taft's inductive and resonance substituent parameters. The theory can be used to aid analysis of ESR spectra for chemical identification of peroxy radicals.
Temperature dependent e.s.r. spectra of peroxy radicals in urea—polyethylene complex materials (UPEC) were discussed. Peroxy radicals in UPEC were much more stable than in normal polyethylene. Change of anisotropic g-values of the radicals with temperature were investigated and it was found that the g1-axis, which was assigned to be parallel to the chain axis, seemed to be almost unchanged throughout the temperature range in the present study, but the g2- and g3-axes changed drastically. Using these results, it was concluded that the main motion of the radical sites was rotation around the chain axis and this motion was found to be much faster than in normal polyethylene, which we have already studied and which we reported recently. This is a reflection of the properties of the materials studied, i.e. interaction between molecular chains in UPEC must be much weaker than in normal polyethylene because of the presence of the wall of urea molecules in the former.
Oxygen-17 labeled peroxy radicals were produced in polypropylene by electron-beam irradiation in the presence of enriched oxygen gas. Subsequent exposure to normal air did not reduce the extent of labeling. This finding suggest that the chain reactions ROâ + RH ..-->.. ROOH + R and R + Oâ ..-->.. ROâ are not important in this system.
The electron paramagnetic resonance (EPR) spectra of the radicals formed during the γ irradiation of polytetrafluoroethylene (PTFE) are examined and assigned. It is shown that both chain radicals (e.g., RCF2CFCF2R) and propagating radicals (e.g., RCF2CF2⋅) are formed and stabilized when PTFE is irradiated in vacuum and at room temperature; the yield of the chain radical is 10 times that of the propagating radical. When PTFE is irradiated in air, the peroxide radicals RCF2CFOO⋅CF2R and RCF2CF2OO⋅ are stabilized. The difference in yields of the two peroxide radicals is a function of total dosage, with the relative contribution of RCF2CF2OO⋅ increasing with increasing dose. Both of the peroxide radicals are converted to the RCF2CF2⋅ radical when they are irradiated with ultraviolet light (λ=280±20 mμ) in vacuum. Some chain radical is produced by the photolysis of its peroxide, but the yield of the propagating radical is 50 times that of the chain radical. The EPR spectrum of the RCF2CF2⋅ radical consists of three sets of lines. The central set is a well‐resolved triplet with a splitting between adjacent lines of 16±0.5 G. The wing lines are broadened by anisotropic nuclear hyperfine interactions, and the expected triplet structure is not resolved. From the room‐temperature spectrum, it is estimated that the isotropic component of the α‐fluorine interaction is ∼85 G. Also, from the 77°K spectrum, it is estimated that the maximum anisotropic component of the hyperfine interaction tensor is ∼140 G. The g value for this radical is 2.0038±0.0001.
Oxidation processes in irradiated polyethylene were studied by e.s.r. By utilization of the characteristic feature of power saturation of the e.s.r. spectrum of peroxide radicals, quantitative measurement of the process of the reaction of oxygen with allylic radicals trapped in the amorphous part of the polyethylene were made and the data were analysed based on the diffusion-controlled process theory. Diffusion equations were solved by computer simulation method. Diffusion of the oxygen into the amorphous part of the polyethylene is discussed and diffusion constants at low temperature are estimated. Related values such as the activation energy of the diffusion process and the solubility constant, are also estimated.
The nature of peroxy radical formation from plasma-induced surface radicals of polyethylene (PE), both low-density polyethylene (LDPE) and high-density polyethylene (HDPE), was studied by electron spin resonance with the aid of systematic computer simulations. It was found that peroxy radical formation varies with the structure of component radicals of plasma-irradiated PE, both LDPE and HDPE: Among three plasma-induced radicals of PE, dangling bond sites (DBS) undergo an instant conversion into the corresponding peroxy radicals in contact with oxygen, while the midchain alkyl radical is of very low reactivity with oxygen in both LDPE and HDPE. Computer simulations disclosed that ESR spectra of peroxy radicals are similar to each other in LDPE and HDPE, both being composed of two types of spectra, a partial g-averaging anisotropic spectrum and a nearly isotropic single line spectrum due to different molecular motional freedom at the trapping sites of peroxy radicals.
An approach is presented for the graft copolymerization of acrylamide (AAm) onto the surface of polyethylene films treated with an oxidative plasma or inert gas plasmas, followed by exposure to air. In both cases, peroxides formed by the plasma treatment are likely to be the species responsible for initiating the graft copolymerization. It is shown that the amount of peroxides (∼10-10 mol·cm-2) and grafted PAAm (∼10 μg·cm-2) can be determined with good accuracy by the 1,1-diphenyl-2-picrylhydrazyl and the ninhydrin method, respectively. A striking finding is that the amounts of peroxides and the grafted PAAm do not monotonously increase with the plasma exposure time but decrease after passing a maximum. A mechanism is proposed to explain this peculiar dependence of the grafted amount on the exposure time. Optical microscopy on the cross-section of the grafted film reveals the graft copolymerization to be limited to a very thin surface region. Both the merely plasma-treated film and the subsequently grafted film are hydrophilic, but only the grafted film has an invariably low contact angle and a slippery surface when hydrated.
Plasma-induced polyethylene (PE) radicals were studied in detail by electron spin resonance (ESR). The room temperature ESR spectrum of plasma-irradiated PE exhibits an apparent sextet spectrum, which is virtually identical with that of gamma-irradiated PE at 77 K. It was found with the aid of systematic computer simulations, however, that the room temperature ESR spectrum consists of three kinds of spectral components: a sextet spectrum (I) as a major spectrum, a septet spectrum (II), and a smeared-out broad line (III). The sextet and septet spectra were assigned to the midchain alkyl radical, -CH2CHCH2-, and the allylic radical, -CH2CHCH=CHCH2-, respectively, as in the case of those in gamma-irradiated PE. The smeared-out broad line, thought to be an intermediate level of conversion to a broad single line, was assigned to an immobilized dangling-bond site (DBS) at the surface cross-linked region, not to a polyenyl radical, -CH2CH-(CH=CH)nCH2-, as in most studies of gamma-irradiated PE. This indicates that studies of temperature-dependent ESR spectra of peroxy radical probe as a direct evidence of the molecular motion of a linear PE chain should be carefully discussed. The observation of a well-defined sextet spectrum of plasma-irradiated PE at room temperature can be ascribed to the fact that the radical formation of PE has been achieved with a brief plasma irradiation using the sample which is completely unsaturated bond-free. Thus, the nature of radical formation of PE was found to be reflected by the presence or absence of unsaturated bonds in the virgin sample in a very sensitive manner.
The mechanism of decay of alkyl radicals in the crystalline region of polyethylene has been investigated. The decay of alkyl radicals is observed with various kinds of polyethylene by electron spin resonance. A close relation is found between the decay rate and the sizes of crystallites. It is suggested that the alkyl radicals trapped in the crystalline region migrate to the surfaces of crystallites through hydrogen abstraction and decay by reaction with oxygen at the surfaces. The kinetics of the radical decay is well explained by the diffusion theory assuming that the decay rate is controlled by the rate of radical migration in the crystallites. The diffusion constant of alkyl radicals is estimated to be 3.0 × 10-18 cm2/sec at 20°. It is shown that the migration of alkyl radicals occur intermolecularly rather than intramolecularly and the activation energy of migration is 18 kcal/mol.
The mid-chain peroxy radical -CF2CF(OO•)CF2- in a copolymer of tetrafluoroethylene-hexafluoropropylene (FEP) has been obtained by γ-irradiation in vacuum, annealing to 100°C, and exposure to oxygen. Reversible changes in the ESR spectra from peroxy radicals have been studied from 77 to 297 K and assigned to g-tensor averaging due to motion. Principal values of the g tensor at 77 K are 2.0376, 2.0085, and 2.0033. Spectra are simulated by using the modified Bloch equations in order to deduce the specific dynamics of the peroxy label in the copolymer. Best agreement with experimental results is obtained by assuming a chain axis rotation with 180° jumps, with the rotation axis at 42° to the perpendicular to the COO plane. This specific motion has not been detected in previous ESR studies of peroxy labels in polymeric systems. The chain rotation motion with a jump angle at 180° can be interpreted as a rapid interchange between two helical forms of the twisted zigzag conformation of the polymer chain.
Polyacrylic acid (PAA) was grafted onto the surface of silicone rubber membrane (SR) by plasma-induced graft copolymerization (PIP). Ar-plasma was used to activate the surface of SR. Also, a determination was made of the influences of plasma treatment power, pressure, time, grafted copolymerization reaction time, and monomer concentration on polymerization yield. The surface properties of SR were measured by ATR-FTIR, ESCA, and SIMS. In those analyses, the elemental composition and different carbon bindings on the surface of SR were examined by ESCA with the amount of O1s/C1s being approximately 0.7 at 60 s by Ar-plasma treatment (60 W, 200 mtorr). The peroxide group introduced on SR was measured via 1,1-diphenyl-2-picryhydrazyl (DPPH). The optimum amount of peroxide groups was 6.85 × 10−8 mol/cm2 at 60 s of Ar-plasma treatment. The peroxide group (33D peak) was introduced onto the surface of SR by negative spectra of SIMS analysis after SR treatment by Ar-plasma. An increase was obtained in grafted polymerization yield with a region of 5 to 50% (v/v) of acrylic acid aqueous solution. Both sites of polyacrylic acid were detected after staining by toluidine blue O. That is, a homobifunctional membrane was developed via this surface modification method. © 1996 John Wiley & Sons, Inc.
Plasma induced graft polymerization of acrylic acid onto polypropylene (PP) monofilament was carried to introduce carboxyl functionality on its surface. The monofilament was treated with oxygen plasma to create hydroperoxide groups and subsequent graft polymerization was initiated on this exposed monofilament. It was observed that in the absence of an added inhibitor, the grafting did not proceed because of the extensive homopolymerization which left behind hardly any monomer for the grafting reaction. The addition of ferrous sulfate to the grafting medium led to the homopolymer free grafting reaction. The addition of organics, such as methanol, butanone, and acetone led to complete inhibition of the homopolymerization at 60% content. However, the addition of butanone led to much lower degree of grafting than methanol and acetone. The contact angle of the monofilament showed drastic reduction by plasma treatment and by the subsequent grafting of acrylic acid. The grafting in ferrous sulfate medium showed higher contact angles as compared to the grafting in organic medium. The surface morphology was significantly influenced by the nature of the additive in the grafting medium. © 2007 Wiley Periodicals, Inc. JAppl Polym Sci, 2008
Besides their use as packaging materials, biodegradable polymers can play a major role in tissue engineering as three-dimensional porous structures (scaffolds). The success of these biodegradable scaffolds is, however, determined by the response it elicits from the surrounding biological environment and this response is governed, to a large extent, by the surface characteristics of the scaffold. Multiple approaches have been developed to obtain the desired surface properties, however, in the past decade, the use of non-thermal plasmas for selective surface modification has been a rapidly growing research field. This paper therefore presents a critical overview on recent advances in plasma-assisted surface modification of biodegradable polymers.
Generation and recombination processes of plasma‐induced radicals in plasma polymers are investigated using ESR, which provides valuable information about radicals in thin polymer films. The polymer films are prepared by means of r.f. plasma polymerization of c ‐C 4 F 8 . Radical numbers and densities of fluorocarbon plasma polymer coatings of different thicknesses are measured by extrapolating mixed‐order recombination processes. The kinetics of radical generation in plasma polymer films is modeled using the results of the time‐dependent radical decay studies. The life times, line‐broadening effects, activation energies, and the temperature behavior of the radicals are discussed.
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The graft polymerization of acrylic acid was carried out onto poly(ethylene terephthalate) films that had been pretreated with argon plasma and subsequently exposed to oxygen to create peroxides. The influence of synthesis conditions, such as plasma treatment time, plasma power, monomer concentration, temperature, and the presence of Mohr's salt, on the degree of grafting was investigated. The observed initial increase in grafting with monomer concentration accelerated at about 20% monomer. The grafting reached a maximum at 40% monomer and subsequently decreased with further increases in monomer concentration. The reaction temperature had a pronounced effect on the degree of grafting. The initial rate of grafting increased with increasing temperature, but the degree of grafting showed a maximum at 50°C. The activation energy of the grafting obtained from an Arrhenius plot was 29.1 kJ/mol. The addition of Mohr's salt to the reaction medium not only led to a homopolymer-free grafting reaction but also diminished the degree of grafting. The degree of grafting increased with increasing plasma power and plasma treatment time. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 2993–3001, 2001
Bioresorbable vascular grafts can be used for direct implantation. Over time, the grafts will degrade and be replaced by natural tissue. In this study, the potential application of biaxially drawn poly(ε-caprolactone) (PCL) films for the design of vascular grafts was examined. PCL films were first modified to enhance cell physiological response to the surface. Two methods of surface modification were studied: surface hydrolysis by immersion in sodium hydroxide, and immobilization of collagen onto PCL film surface. Tensile tests indicate that immersion in sodium hydroxide results in a significant drop in ultimate tensile strength, whereas collagen-immobilized films remained uncompromised. Human coronary artery smooth muscle cells were cultured on the different surfaces, and it was demonstrated that collagen-immobilized films elicited the most favorable response from the cultured cells. This indicates the potential for collagen-immobilized PCL films for vascular tissue engineering applications.
Poly(tetrafluoroethylene) films were surface modified by argon plasma treatment followed by graft polymerization. Peroxidе groups were introduced on the surface of poly(tetrafluoroethylene) films after plasma treatment and the consequent contact with air when the films were taken out of the reactor. Grafting polymerization initiated by the surface peroxide (hydroxide) groups was performed on the poly(tetrafluoroethylene) film surface by using acrylic acid, 4-vinylpyridine and 1-vinylimidazole as monomers. Copolymers were obtained with grafting yield from 0.436 to 0.457 mg/cm2 for poly(acrylic acid), from 0.299 to 0.390 mg/cm2 for poly(4-vinylpyridine) and from 0.212 to 0.256 mg/cm2 for poly(1-vinylimidazole), respectively. The free surface energies of the copolymers were determined. The chemical structures and the copolymer surfaces were characterized by IR, XPS and SEM analyses. High energy resolution X-ray photoelectron spectroscopy (XPS) confirmed the grafting of acrylic acid, 4-vinylpyridine and 1-vinylimidazole. The surface hydrophilicities of modified polytetrafluoroethylene films were significantly enhanced after plasma treatment and grafting modification.
It is worth emphasizing that in this work acrylic acid, 4-vinylpyridine and 1-vinylimidazole were used as the reactive monomers for grafting on the poly(tetrafluoroethylene) film by plasma treatment. We believe that this vinyl monomers may be employable as functional groups, permitting a potentially wide range of applications: as ionomers, membranes, carriers for immobilization of biomolecules, for complex formation with heavy metals as catalysts.
A novel antibacterial material was developed by surface modification of electrospun polyurethane (PU) fibrous membranes, using a process which involved plasma pretreatment, UV-induced graft copolymerization of 4-vinylpyridine (4VP), and quaternization of the grafted pyridine groups with hexylbromide. The success of modification with poly(4-vinyl-N-hexyl pyridinium bromide) groups on these was ascertained by X-ray photoelectron spectroscopy (XPS). The morphologies and mechanical properties were investigated by scanning electron microscopy (SEM) and tensile test, respectively. The results showed that the morphologies of PU fibrous membranes changed slightly during the modification process and the fiber structures were maintained. The tensile strength of PU fibrous membranes decreased after surface modification, with the smallest decrease (<20%) observed in the membrane with largest diameter. The antibacterial activities of the modified PU fibrous membranes were assessed against Gram-positive Staphylococcus aureus (S. aureus) and Gram-negative Escherichia coli (E. coli). The modified PU fibrous membranes possessed highly effective antibacterial activities and may have a wide variety of potential applications in high-performance filters, protective textiles, and biomedical devices.
This paper reviews recent advances in the covalent attachment of bioactive compounds to functionalized polymer surfaces including relevant techniques in polymer surface modification such as wet chemical, organosilanization, ionized gas treatments, and UV irradiation. Methods of analysis of biofunctionalized polymer surfaces, including spectral methods (X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, atomic force microscopy, and others) as well as non-spectral methods (contact angle, dye assays, biological assays, and zeta potential) are also considered. State-of-the-art techniques in covalent conjugation of bioactive compounds to the modified surfaces, such as usage of hydrophilic, bifunctional, and/or branched spacer molecules, are presented. Relevant bioconjugation reagents and chemistries are described and tabulated. Recently reported applications in areas such as biomedicine, biosensors, enzyme reactors, and textiles, all of which utilize a common set of surface bioconjugation techniques to address these diverse needs, are discussed. Finally, challenges to this emerging field of research are critically evaluated.
Porous hydrophobic polypropylene (PP) membranes were subjected to the surface modification by the γ-ray induced graft copolymerization with hydrophilic 2-hydroxyethyl methacrylate (HEMA). The structural changes and surface morphologies of the modified PP membranes were characterized by a Fourier transform infrared spectroscopy (FT-IR), elemental analysis (EA) and field emission scanning electron microscopy (FE-SEM). Peroxides produced from γ-ray irradiation were determined by a 1,1-diphenyl-2-picryl hydrazyl (DPPH) method and the surface hydrophilicities of membranes were measured by a static contact angle measurement. The contact angle of the modified membranes reduced with the degree of grafting (DG) of HEMA onto the membrane surface, and it decreased up to about half of that before modification. The permeation behaviors of all membranes were investigated by a bovine serum albumin (BSA) filtration experiment. As a result, the DG of the modified membrane increased with the reaction time. However, in the case of irradiation dosage it showed the maximum value at 20 kGy. Also, the modified membrane showed a higher solution flux, lower BSA adsorption, and the better flux recovery after cleaning than that of the unmodified membrane. Particularly, 40.6% grafted membrane showed a two-fold increase in a BSA solution flux, 62% reduction in total fouling and three-fold increase in flux recovery after chemical cleaning.
Electron paramagnetic resonance (EPR) technique has been employed to detect and characterise a series of different radical species generated in ultra-high molecular weight polyethylene (UHMWPE) via electron beam irradiation. Three different radical species have been found and assigned on the basis of their EPR spectra and of the related computer simulations. A secondary alkyl species, the prevalent one, is present immediately after irradiation, an allyl species appears only 24 h after irradiation when the alkyl species disappears.The third species, clearly visible at high microwave power only, has been observed for the first time and assigned to a tertiary alkyl carbon radical, whose formation is strictly connected with a Y-shape crosslink and a migration of the unpaired electron on a carbon atom localised in an adjacent position. Copyright © 2011 John Wiley & Sons, Ltd.
In the last decade, substantial research in the field of post-plasma grafting surface modification has focussed on the introduction of carboxylic acids on surfaces by grafting acrylic acid (AAc). In the present work, we report on an alternative approach for biomaterial surface functionalisation. Thin poly-ε-caprolactone (PCL) films were subjected to a dielectric barrier discharge Ar-plasma followed by the grafting of 2-aminoethyl methacrylate (AEMA) under UV-irradiation. X-ray photoelectron spectroscopy (XPS) confirmed the presence of nitrogen. The ninhydrin assay demonstrated, both quantitatively and qualitatively, the presence of free amines on the surface. Confocal fluorescence microscopy (CFM), atomic force microscopy (AFM) and scanning electron microscopy (SEM) were used to visualise the grafted surfaces, indicating the presence of pAEMA. Static contact angle (SCA) measurements indicated a permanent increase in hydrophilicity. Furthermore, the AEMA grafted surfaces were applied for comparing the physisorption and covalent immobilisation of gelatin. CFM demonstrated that only the covalent immobilisation lead to a complete coverage of the surface. Those gelatin-coated surfaces obtained were further coated using fibronectin. Osteosarcoma cells demonstrated better cell-adhesion and cell-viability on the modified surfaces, compared to the pure PCL films.
In modern technology, there is a constant need to solve very complex problems and to fine-tune existing solutions. This is definitely the case in modern medicine with emerging fields such as regenerative medicine and tissue engineering. The problems, which are studied in these fields, set very high demands on the applied materials. In most cases, it is impossible to find a single material that meets all demands such as biocompatibility, mechanical strength, biodegradability (if required), and promotion of cell-adhesion, proliferation, and differentiation. A common strategy to circumvent this problem is the application of composite materials, which combine the properties of the different constituents. Another possible strategy is to selectively modify the surface of a material using different modification techniques. In the past decade, the use of nonthermal plasmas for selective surface modification has been a rapidly growing research field. This will be the highlight of this review. In a first part of this paper, a general introduction in the field of surface engineering will be given. Thereafter, we will focus on plasma-based strategies for surface modification. The purpose of the present review is twofold. First, we wish to provide a tutorial-type review that allows a fast introduction for researchers into the field. Second, we aim to give a comprehensive overview of recent work on surface modification of polymeric biomaterials, with a focus on plasma-based strategies. Some recent trends will be exemplified. On the basis of this literature study, we will conclude with some future trends for research.
After surface modification with collagen immobilization through covalent bonding, porous polyethylene pieces with an average pore size of 400 microns were implanted subcutaneously into the back of rats for 1 yr. It was found that connective tissues with abundant blood vessels were formed clearly, filling more than 90% of the pore volume and bound firmly to the pore walls. A tumour was found in only one of 24 implanted pieces (4.2%). On the other hand, the virgin porous polyethylene pieces without collagen immobilization exhibited inflammatory reactions within the pores and the connective tissues produced filled only 15% of the pore volume. Formation of a malignant histiocytoma was observed in 11 of the 24 pieces which had been implanted (45.8%). Thus, immobilization of collagen on the surface of an artificial material through covalent bonding proved to be very effective not only for firm bonding with soft connective tissues but also for a reduction of tumour formation.
Collagen-immobilized porous polyethylene, in which the immobilization was through covalent bonding, and virgin porous polyethylene were implanted subcutaneously in rats from 1 to 20 wk. The results were the ingrowth of the connective tissue into collagen-immobilized porous polyethylene was rich and contained a low level of inflammatory cellular infiltration compared with that of virgin porous polyethylene. The material-tissue interface showed that the living body-originated collagen fibres were firmly anchored into the immobilized collagen layer. These results suggested that covalent immobilization of collagen on to the biomaterial surface is useful in promoting the ingrowth of soft tissue and the tissue adhesion.
A method for producing various surfaces of silicone rubber membrane (SR) was developed in this study by grafting various amounts of poly(2-hydroxy ethyl methacrylate) (pHEMA) onto SR by plasma-induced grafted polymerization (PIP) as a homobifunctional membrane. The elemental composition and different carbon bindings on the surface of SR were examined by electron spectroscopy for chemical analysis with the amount of O1s/C1s being approximately 0.7 at 1 min, 60 W, 200 mTorr of Ar-plasma treatment. The peroxide group introduced on SR was measured via 1,1-diphenyl-2-picrylhydrazyl (DPPH) and the amount of 6.85 x 10(-8) mol cm-2 reached optimum value at 1 min of Ar-plasma treatment. After Ar-plasma treated SR, the peroxide group (33D peak) was introduced on the surface of SR by negative spectra of secondary ion mass spectroscopy analysis, whereas ester groups (72D peak) were observed for pHEMA-grafted SR. For the in vitro test, the influence of various surfaces of SR on attachment and growth of rabbit corneal epithelial cells (CEC) was studied by cell culture assay. These results indicated that 56-150 micrograms cm-2 of pHEMA grafted onto SR were suitable values for attachment and growth of CEC. On the contrary, the large grafted amounts (500-1650 micrograms cm-2) of pHEMA on SR were insufficient for attachment and growth of CEC. For the in vivo test, the migration of CEC from host cornea to implant was investigated by slit lamp microscopy. The experimental results indicated that SRs grafted with pHEMA were completely covered with CEC 3 weeks after implantation of the membranes into the host cornea. These results provide a valuable reference for developing an artificial cornea.
Polyacrylic acid (pAA) was introduced onto Ar-plasma treatment silicone rubber (SR) membrane surfaces by plasma-induced grafted polymerization. Collagen (type III) was also linked with the carboxylic group of pAA grafted onto the SR surface via a carbodiimine agent to obtain a secondary structure of SR. The SR surface properties were characterized by ATR-FTIR, ESCA, contact angle, and SEM. The biocompatibility of the SR surface was evaluated by a culture of cornea epithelial (CE) cells. Subsequently, 75-450 micrograms cm-2 of pAA were obtained on the SR surfaces under different reactive conditions; 3-12 micrograms cm-2 of collagen were linked on modified surfaces of SR. Moreover, ATR-FTIR and ESCA were utilized to confirm the proceedings of these reactions. The hydrophility of the modified SR was measured by a contact angle meter. The values of contact angle for SR grafted with pAA were approximately 45-50 degrees; a 50-55 degrees contact angle on pAA-g-SR to be further linked with collagen was subsequently obtained. Moreover, the influence of surface properties toward migration, growth and attachment of CE cells on the modified surfaces was also examined. Here, untreated SR was used as a control. Experimental results indicated that the number of CE cells attached onto the controlled SR was negligible. The attachment of cells onto pAA-grafted surfaces was clearly observed and peusopoda occurred; however, cell growth was depressed. This depression may have been caused by the acid environment of the pAA-grafted membrane. Nevertheless, both cell attachment and growth onto collagen-linked surfaces were significant. In addition, the morphology of the cells attached onto this surface was considered normal for primary cells. Collagen introduced on the SR surface was not denatured, i.e the natural properties of collagen were maintained. The results obtained in this study will hopefully lead to the successful development of modified SR for clinical applications.
Fast-neutron irradiation of natural topaz crystals produces a single paramagnetic radiation damage center in high concentration. ESR of this center shows a holelike spectrum with {ital S}=1/2 and a strongly anisotropic {ital g} tensor: {ital g}{sub {ital x}{ital x}}=2.0027, {ital g}{sub {ital y}{ital y}}=2.0055, and {ital g}{sub {ital z}{ital z}}=2.0407. We identify this defect as an intrinsic O{sub 2}{sup {minus}} center in the form of a peroxy radical. The orientation of the {ital g} tensor helps confirm this assignment, as does the extraordinary thermal stability; annealing temperatures near 800 {degree}C are required for complete removal. Two uv absorption bands are associated with the peroxy radical, each with oscillator strength near 0.09. Pumping in the higher energy band leads to a polarization-sensitive 2.5-eV luminescence; the other uv band apparently relaxes nonradiatively.
Poly (epsilon-caprolactone) (PCL) has been used as a bioresorbable polymer in numerous medical devices as well as for tissue engineering applications. Its main advantage is its biocompatibility and slow degradation rate. PCL surface, however, is hydrophobic and cell-biomaterial interaction is not the best. We attempt for the first time to modify an ultra thin PCL surface with collagen. The PCL film was prepared using solvent casting and biaxial stretching technique developed in our laboratory. This biaxial stretching produced an ultra thin PCL 3-7 microm thick, ideal for membrane tissue engineering applications. The PCL film was pretreated using Argon plasma, and then UV polymerized with acrylic acid (AAc). Collagen immobilization was then carried out. The modified film surface was characterized by Fourier Transform Infrared (FT-IR) and X-ray Photoelectron Spectroscopy (XPS). Water contact angles were also measured to evaluate the hydrophilicity of the modified surface. Results showed that the hydrophilicity of the surface has improved significantly after surface modification. The water contact angle dropped from 66 degrees to 32 degrees. Atomic Force Microscopy (AFM) showed an increase in roughness of the film. A change from 46 to 60 nm in the surface morphology was also observed. The effect of cells attachment on the PCL film was studied. Human dermal fibroblasts and myoblasts attachment and proliferation were improved remarkably on the modified surface. The films showed excellent cell attachment and proliferation rate.
EasySpin, a computational package for spectral simulation and analysis in EPR, is described. It is based on Matlab, a commercial technical computation software. EasySpin provides extensive EPR-related functionality, ranging from elementary spin physics to data analysis. In addition, it provides routines for the simulation of liquid- and solid-state EPR and ENDOR spectra. These simulation functions are built on a series of novel algorithms that enhance scope, speed and accuracy of spectral simulations. Spin systems with an arbitrary number of electron and nuclear spins are supported. The structure of the toolbox as well as the theoretical background underlying its simulation functionality are presented, and some illustrative examples are given.
Gamma radiation of poly (lactide-co-glycolide) raw polymers and processed microspheres under vacuum and at 77 K results in the formation of a series of free radicals. The resulting powder electron paramagnetic resonance (EPR) spectrum contains a distribution of several different radicals, depending on the annealing temperature, and is therefore difficult to interpret. By utilising the selectivity of the electron nuclear DOuble resonance (ENDOR) and associated ENDOR induced EPR (EIE) techniques, a more direct approach for the deconvolution of the EPR spectrum can be achieved. Using this approach, the radiolytically induced CH3 *CHC(O)R- chain scission radical was identified at 120 K by simulation of the EIE spectrum. At elevated temperatures (250 K), this radical decays considerably and the more stable radicals -O*CHC(O)-, CH3 *C(OR)C(O)- and CH3 *C(OH)C(O)- predominate. This work demonstrates the utility of the EIE approach to supplement and aid the interpretation of powder EPR spectra of radicals in a polymer matrix.
Using a high-throughput surface discovery approach, we have generated a 1600-member library of metal-containing surfaces and screened them for antibody binding potential. The surface library assembly involved graft modification of argon plasma-treated polyvinylidenedifluoride (PVDF) membranes with alternating maleic anhydride-styrene copolymer followed by anhydride ring opening with a range of secondary amines and microarray contact printing of transition metal complexes. The microarrays of metal-containing surfaces were then tested for their antibody binding capacity by incubation with a biotinylated mouse antibody in a chemiluminescence assay. A total of 11 leads were identified from the first screen, constituting a "hit" rate of 0.7%. A smaller 135-member surface library was then synthesized and screened to optimize existing hits and generate additional leads. To demonstrate the applicability of these surfaces to other formats, high-binding surface leads were then transferred onto Luminex beads for use in a bead flow cytometric immunoassay. The novel one-step antibody coupling process increased assay sensitivity of a Luminex tumor necrosis factor immunoassay. These high-binding surfaces do not require prior incorporation of polyhistidine tags or posttreatments such as oxidation to achieve essentially irreversible binding of immunoglobulin G.
Surface graft polymerization of 1-vinyl-2-pyrrolidone onto a silicon surface was accomplished by atmospheric pressure (AP) hydrogen plasma surface activation followed by graft polymerization in both N-methyl-2-pyrrolidone (NMP) and in an NMP/water solvent mixture. The formation of initiation sites was controlled by the plasma exposure period, radio frequency (rf) power, and adsorbed surface water. The surface number density of active sites was critically dependent on the presence of adsorbed surface water with a maximum observed at approximately a monolayer surface water coverage. The surface topology and morphology of the grafted polymer layer depended on the solvent mixture composition, initial monomer concentration, reaction temperature, and reaction time. Grafted polymer surfaces prepared in pure NMP resulted in a polymer feature spacing of as low as 5-10 nm (average feature diameter of about 17 nm), an rms surface roughness range of 0.18-0.72 nm, and a maximum grafted polymer layer thickness of 5.5 nm. Graft polymerization in an NMP/water solvent mixture, however, resulted in polymer feature sizes that increased up to a maximum average feature diameter of about 90 nm at [NMP] = 60% (v/v) with polymer feature spacing in the range of 10-50 nm. The surface topology of the polymer-modified silicon surfaces grafted in an NMP/water solvent mixture exhibited a bimodal feature height distribution. In constrast, graft polymerization in pure NMP resulted in a narrow feature height distribution of smaller-diameter surface features with smaller surface spacing. The results demonstrated that, with the present approach, the topology of the grafted polymer surface was tunable by adjusting the NMP/water ratio. The present surface graft polymerization method, which is carried out under AP conditions, is particularly advantageous for polymer surface structuring via radical polymerization and can, in principle, be scaled to large surfaces.
Tissue engineering and regenerative medicine is an exciting research area that aims at regenerative alternatives to harvested tissues for transplantation. Biomaterials play a pivotal role as scaffolds to provide three-dimensional templates and synthetic extracellular matrix environments for tissue regeneration. It is often beneficial for the scaffolds to mimic certain advantageous characteristics of the natural extracellular matrix, or developmental or wound healing programs. This article reviews current biomimetic materials approaches in tissue engineering. These include synthesis to achieve certain compositions or properties similar to those of the extracellular matrix, novel processing technologies to achieve structural features mimicking the extracellular matrix on various levels, approaches to emulate cell-extracellular matrix interactions, and biologic delivery strategies to recapitulate a signaling cascade or developmental/wound healing program. The article also provides examples of enhanced cellular/tissue functions and regenerative outcomes, demonstrating the excitement and significance of the biomimetic materials for tissue engineering and regeneration.