Comparison of proliferation and growth of human keratinocytes on plasma copolymers of acrylic acid/1,7-octadiene and self-assembled monolayers

Functional Skin Grafts: Where Biomaterials Meet Stem Cells

Skin tissue engineering has attained several clinical milestones making remarkable progress over the past decades. Skin is inhabited by a plethora of cells spatiotemporally arranged in a 3-dimensional (3D) matrix, creating a complex microenvironment of cell-matrix interactions. This complexity makes it difficult to mimic the native skin structure using conventional tissue engineering approaches. With the advent of newer fabrication strategies, the field is evolving rapidly. However, there is still a long way before an artificial skin substitute can fully mimic the functions and anatomical hierarchy of native human skin. The current focus of skin tissue engineers is primarily to develop a 3D construct that maintains the functionality of cultured cells in a guided manner over a period of time. While several natural and synthetic biopolymers have been translated, only partial clinical success is attained so far. Key challenges include the hierarchical complexity of skin anatomy; compositional mismatch in terms of material properties (stiffness, roughness, wettability) and degradation rate; biological complications like varied cell numbers, cell types, matrix gradients in each layer, varied immune responses, and varied methods of fabrication. In addition, with newer biomaterials being adopted for fabricating patient-specific skin substitutes, issues related to escalating processing costs, scalability, and stability of the constructs under in vivo conditions have raised some concerns. This review provides an overview of the field of skin regenerative medicine, existing clinical therapies, and limitations of the current techniques. We have further elaborated on the upcoming tissue engineering strategies that may serve as promising alternatives for generating functional skin substitutes, the pros and cons associated with each technique, and scope of their translational potential in the treatment of chronic skin ailments.

Plasma treatments of dressings for wound healing: a review

This review covers the use of plasma technology relevant to the preparation of dressings for wound healing. The current state of knowledge of plasma treatments that have potential to provide enhanced functional surfaces for rapid and effective healing is summarized. Dressings that are specialized to the needs of individual cases of chronic wounds such as diabetic ulcers are a special focus. A summary of the biology of wound healing and a discussion of the various types of plasmas that are suitable for the customizing of wound dressings are given. Plasma treatment allows the surface energy and air permeability of the dressing to be controlled, to ensure optimum interaction with the wound. Plasmas also provide control over the surface chemistry and in cases where the plasma creates energetic ion bombardment, activation with long-lived radicals that can bind therapeutic molecules covalently to the surface of the dressing. Therapeutic innovations enabled by plasma treatment include the attachment of microRNA or antimicrobial peptides. Bioactive molecules that promote subsequent cell adhesion and proliferation can also be bound, leading to the recruitment of cells to the dressing that may be stem cells or patient-derived cells. The presence of a communicating cell population expressing factors promotes healing.

Hydrophilic surface modification of acrylate-based biomaterials

Acrylic polymers have proved to be excellent with regard to cell adhesion, colonization and survival, in vitro and in vivo. Highly ordered and regular pore structures thereof can be produced with the help of polyamide templates, which are removed with nitric acid. This treatment converts a fraction of the ethyl acrylate side groups into acrylic acid, turning poly(ethyl acrylate) scaffolds into a more hydrophilic and pH-sensitive substrate, while its good biological performance remains intact. To quantify the extent of such a modification, and be able to characterize the degree of hydrophilicity of poly(ethyl acrylate), poly(ethyl acrylate) was treated with acid for different times (four, nine and 17 days), and compared with poly(acrylic acid) and a 90/10%wt. EA/AAc copolymer (P(EA-co-AAc)). The biological performance was also assessed for samples immersed in acid up to four days and the copolymer, and it was found that the incorporation of acidic units on the material surface was not prejudicial for cells. This surface modification of 3D porous hydrophobic scaffolds makes easier the wetting with culture medium and aqueous solutions in general, and thus represents an advantage in the manageability of the scaffolds.

Photocrosslinkable Gelatin Hydrogel for Epidermal Tissue Engineering

Natural hydrogels are promising scaffolds to engineer epidermis. Currently, natural hydrogels used to support epidermal regeneration are mainly collagen- or gelatin-based, which mimic the natural dermal extracellular matrix but often suffer from insufficient and uncontrollable mechanical and degradation properties. In this study, a photocrosslinkable gelatin (i.e., gelatin methacrylamide (GelMA)) with tunable mechanical, degradation, and biological properties is used to engineer the epidermis for skin tissue engineering applications. The results reveal that the mechanical and degradation properties of the developed hydrogels can be readily modified by varying the hydrogel concentration, with elastic and compressive moduli tuned from a few kPa to a few hundred kPa, and the degradation times varied from a few days to several months. Additionally, hydrogels of all concentrations displayed excellent cell viability (>90%) with increasing cell adhesion and proliferation corresponding to increases in hydrogel concentrations. Furthermore, the hydrogels are found to support keratinocyte growth, differentiation, and stratification into a reconstructed multilayered epidermis with adequate barrier functions. The robust and tunable properties of GelMA hydrogels suggest that the keratinocyte laden hydrogels can be used as epidermal substitutes, wound dressings, or substrates to construct various in vitro skin models.

Dual Precursor Atmospheric Plasma Deposition of Transparent Bilayer Protective Coatings on Plastics

We demonstrate a dual organic and inorganic precursor method to deposit transparent organosilicate protective bilayer coatings on poly methyl methacrylate (PMMA) substrates with atmospheric plasma deposition in ambient air. The bottom layer was a hybrid organosilicate adhesive layer deposited with dual organic 1,5-cyclooctadiene (CYC) and widely used inorganic tetraethoxysiline (TEOS) precursors. The selection of the organic CYC precursor allowed incorporation of a carbon chain in the organosilicate adhesive layer, which resulted in improved adhesion. The top layer was a dense silica coating with high Young's modulus and hardness deposited with TEOS. The deposited bilayer structure showed ∼100% transparency in the visible light wavelength region, twice the adhesion energy, and five times the Young's modulus of commercial polysiloxane sol-gel coatings.

The development of a 3D immunocompetent model of human skin

As the first line of defence, skin is regularly exposed to a variety of biological, physical and chemical insults. Therefore, determining the skin sensitization potential of new chemicals is of paramount importance from the safety assessment and regulatory point of view. Given the questionable biological relevance of animal models to human as well as ethical and regulatory pressure to limit or stop the use of animal models for safety testing, there is a need for developing simple yet physiologically relevant models of human skin. Herein, we describe the construction of a novel immunocompetent 3D human skin model comprising of dendritic cells co-cultured with keratinocytes and fibroblasts. This model culture system is simple to assemble with readily-available components and importantly, can be separated into its constitutive individual layers to allow further insight into cell-cell interactions and detailed studies of the mechanisms of skin sensitization. In this study, using non-degradable microfibre scaffolds and a cell-laden gel, we have engineered a multilayer 3D immunocompetent model comprised of keratinocytes and fibroblasts that are interspersed with dendritic cells. We have characterized this model using a combination of confocal microscopy, immuno-histochemistry and scanning electron microscopy and have shown differentiation of the epidermal layer and formation of an epidermal barrier. Crucially the immune cells in the model are able to migrate and remain responsive to stimulation with skin sensitizers even at low concentrations. We therefore suggest this new biologically relevant skin model will prove valuable in investigating the mechanisms of allergic contact dermatitis and other skin pathologies in human. Once fully optimized, this model can also be used as a platform for testing the allergenic potential of new chemicals and drug leads.

The effect of adipose tissue derived MSCs delivered by a chemically defined carrier on full-thickness cutaneous wound healing

Mesenchymal stem cells (MSCs) have properties which make them promising for the treatment of chronic non-healing wounds. A major so far unmet challenge is the efficient, safe and painless delivery of MSCs to skin wounds. Recently, a surface carrier of medical-grade silicone coated by plasma polymerisation with a thin layer of acrylic acid (ppAAc) was developed, and shown to successfully deliver MSCs to deepithelialised human dermis in vitro. Here we studied the potential of the ppAAc carrier to deliver human adipose tissue derived MSCs (AT-MSCs) to murine full-thickness excisional skin wounds in vivo. Further we investigate the mechanism of action of MSCs in accelerating wound healing in these wounds. AT-MSCs cultured on ppAAc carriers for 4 days or longer fully retained their cell surface marker expression profile, colony-forming-, differentiation- and immunosuppressive potential. Importantly, AT-MSCs delivered to murine wounds by ppAAc carriers significantly accelerated wound healing, similar to AT-MSCs delivered by intradermal injection. More than 80% of AT-MSCs were transferred from carriers to wounds in 3 days. AT-MSCs were detectable in wounds for at least 5 days after wounding. Carrier delivered AT-MSCs were demonstrated to have the capacity to down-modulate TNF-α-dependent inflammation, increase anti-inflammatory M2 macrophage numbers, and induce TGF-β(1)-dependent angiogenesis, myofibroblast differentiation and granulation tissue formation, thereby enhancing overall tissue repair.

Plasma polymer coatings to aid retinal pigment epithelial growth for transplantation in the treatment of age related macular degeneration

Subretinal transplantation of functioning retinal pigment epithelial (RPE) cells grown on a synthetic substrate is a potential treatment for age-related macular degeneration (AMD), a common cause of irreversible vision loss in developed countries. Plasma polymers give the opportunity to tailor the surface chemistry of the artificial substrate whilst maintaining the bulk properties. In this study, plasma polymers with different functionalities were investigated in terms of their effect on RPE attachment and growth. Plasma polymers of acrylic acid (AC), allyl amine (AM) and allyl alcohol (AL) were fabricated and characterised using X-ray photoelectron spectroscopy (XPS) and water contact angle measurements. Octadiene (OD) hydrocarbon films and tissue culture polystyrene were used as controls. Wettability varied from hydrophobic OD to relatively hydrophilic AC. XPS demonstrated four very different surfaces with the expected functionalities. Attachment, proliferation and morphological examination of an RPE cell line and primary RPE cells were investigated. Both cell types grew on all surfaces, with the exception of OD, although the proliferation rate of primary cells was low. Good epithelial morphology was also demonstrated. Plasma polymerised films show potential as cell carrier surfaces for RPE cells in the treatment of AMD.

Chemically well-defined self-assembled monolayers for cell culture: toward mimicking the natural ECM

The extracellular matrix (ECM) is a network of biological macromolecules that surrounds cells within tissues. In addition to serving as a physical support, the ECM actively influences cell behavior by providing sites for cell adhesion, establishing soluble factor gradients, and forming interfaces between different cell types within a tissue. Thus, elucidating the influence of ECM-derived biomolecules on cell behavior is an important aspect of cell biology. Self-assembled monolayers (SAMs) have emerged as promising tools to mimic the ECM as they provide chemically well-defined substrates that can be precisely tailored for specific cell culture applications, and their application in this regard is the focus of this review. In particular, this review will describe various approaches to prepare SAM-based culture substrates via non-specific adsorption, covalent immobilization, or non-covalent sequestering of ECM-derived biomolecules. Additionally, this review will highlight SAMs that present ECM-derived biomolecules to cells to probe the role of these molecules in cell-ECM interactions, including cell attachment, spreading and 'outside-in' signaling via focal adhesion complex formation. Finally, this review will introduce SAMs that can present or sequester soluble signaling molecules, such as growth factors, to study the influence of localized soluble factor activity on cell behavior. Together, these examples demonstrate that the chemical specificity and variability afforded by SAMs can provide robust, well-defined substrates for cell culture that can simplify experimental design and analysis by eliminating many of the confounding factors associated with traditional culture substrates.

Nanoscale tissue engineering: spatial control over cell-materials interactions

Cells interact with the surrounding environment by making tens to hundreds of thousands of nanoscale interactions with extracellular signals and features. The goal of nanoscale tissue engineering is to harness these interactions through nanoscale biomaterials engineering in order to study and direct cellular behavior. Here, we review two- and three-dimensional (2- and 3D) nanoscale tissue engineering technologies, and provide a holistic overview of the field. Techniques that can control the average spacing and clustering of cell adhesion ligands are well established and have been highly successful in describing cell adhesion and migration in 2D. Extension of these engineering tools to 3D biomaterials has created many new hydrogel and nanofiber scaffold technologies that are being used to design in vitro experiments with more physiologically relevant conditions. Researchers are beginning to study complex cell functions in 3D. However, there is a need for biomaterials systems that provide fine control over the nanoscale presentation of bioactive ligands in 3D. Additionally, there is a need for 2- and 3D techniques that can control the nanoscale presentation of multiple bioactive ligands and that can control the temporal changes in the cellular microenvironment.

Surface chemistry and polymer film thickness effects on endothelial cell adhesion and proliferation

Adherence and growth rates of human aortic endothelial cells (HAEC) on plasma polymerized poly(vinylacetic acid) films were measured as functions of the surface density of --COOH groups and plasma deposited film thickness. Pulsed plasma polymerization was employed to produce films containing 3.6 to 9% --COOH groups, expressed as a percent of total carbon content. Endothelial cells exhibited increased cell adherence and proliferation with increasing --COOH surface densities. Additionally, and unexpectedly, cell growth was also dependent on the film thicknesses, which ranged from 25 to 200 nm. The results indicate that optimization of the functional group surface density and film thickness could produce significant enhancements in initial adhesion and subsequent growth of the HAEC cells.

Clinical experience using cultured epithelial autografts leads to an alternative methodology for transferring skin cells from the laboratory to the patient

We report a 10-year audit using cultured epithelial autografts (CEAs) for patients with extensive burns. Clinical take using CEAs averaged only 45% (as has been reported by others) but over half of all cells cultured for these patients had to be discarded owing to difficulties of timing the production of CEA sheets to the needs of the patients. CEAs could not be used until they had reached confluence and formed an integrated sheet, which took, on average, 12 days. However, once formed, they needed to be used within 2-3 days or they lost the ability to attach to wound beds. In response to this we developed a simpler carrier dressing methodology for transferring cultured subconfluent keratinocytes from the laboratory to the wound bed. This methodology offers an increase in speed of delivery but its major contribution is the greater flexibility in timing the transfer of cells from the laboratory to the changing needs of the patients.

Fabrication of Cell-Based Arrays Using Micropatterned Alkanethiol Monolayers for the Parallel Silencing of Specific Genes by Small Interfering RNA

The emergence of small interfering RNA (siRNA) opened a new opportunity to study gene functions in a genome. However, large-scale loss-of-function analyses further require cell-based high-throughput methods that allow simultaneous silencing of the huge number of genes by siRNA. In this study, we aim at fabricating the cell-based siRNA arrays that facilitate parallel introduction of multiple siRNAs into cultured mammalian cells. The siRNA arrays were prepared using surface chemical processes including the micropatterning of a self-assembled monolayer and the layer-by-layer assembly of siRNA and cationic lipid. We examined the feasibility of the siRNA array for the sequence-specific gene silencing in an array format. Furthermore, the effects of siRNA loading and culture period after transfection were studied to optimize cell-based assays on the siRNA arrays. The results obtained in this study demonstrated that our method provides the siRNA arrays with spatial specificity in gene silencing, which will serve to obtain a quantitative data set from the cell-based screens on siRNA arrays.

Macrophage Serum-Based Adhesion to Plasma-Processed Surface Chemistry is Distinct from That Exhibited by Fibroblasts

Plasma-polymerized films deposited from AlAm, HxAm, NVP, NVFA, AA and FC were compared to TCPS and PS surfaces in supporting cellular attachment, viability, and proliferation in serum-based culture in vitro for extended periods of time (>7 d). Surface patterns were created using multi-step depositions with physical masks. Cell adhesion in the presence of serum was compared for (monocyte-) macrophage and fibroblast cell lines. Cellular response was tracked over time, reporting adhesive behavior, proliferative rates, and morphological changes as a function of surface chemistry. Micropatterned surfaces containing different surface chemistries and functional groups (e.g. -NH(2), -COOH, -CF(3)) produced differential cell adhesive patterns for NIH 3T3 fibroblasts compared to J774A.1, RAW 264.7 or IC-21 (monocyte-) macrophage cell types. Significantly, macrophage adhesion is substantial on surfaces where fibroblasts do not adhere under identical culture conditions.

A chemically defined surface for the co-culture of melanocytes and keratinocytes

Patients with stable vitiligo can be helped surgically using transplantation of autologous cultured melanocytes, but there is a need for a culture methodology that is free from xenobiotic agents and for a simple way of delivering cultured melanocytes to the patient to achieve pigmentation with good wound healing. The aim of this study was to develop a chemically defined surface, suitable for the co-culture of melanocytes and keratinocytes which could be used in the future for the treatment vitiligo patients to achieve both restoration of pigmentation and good wound healing. Two keratinocyte growth media and two melanocyte growth media were compared; two of these were serum free. Cells were seeded on a range of chemically defined substrates (produced by plasma polymerisation of acrylic acid, allylamine or a mixture of these monomers) either as mono- or co-cultures. Melanocytes and keratinocytes attached and proliferated on both acid and amine substrates (without significant preferences), and co-cultures of cells proliferated more successfully than individual cultures. One media, M2, which is serum free, supported expansion of melanocytes and to a lesser extent keratinocytes on several plasma polymer substrates. In conclusion, these data indicate that a combination of a chemically defined substrate with M2 media allows serum-free co-culture of melanocytes and keratinocytes.

Effect of biomaterial surface properties on fibronectin-alpha5beta1 integrin interaction and cellular attachment

The ability of fibronectin (Fn) to mediate cell adhesion through binding to alpha(5)beta(1) integrins is dependent on the conditions of its adsorption to the surface. Using a model system of alkylsilane SAMs with different functional groups (X=OH, COOH, NH(2) and CH(3)) and an erythroleukemia cell line expressing a single integrin (alpha(5)beta(1)), the effect of surface properties on the cellular adhesion with adsorbed Fn layers was investigated. (125)I-labeled Fn, a modified biochemical cross-linking/extraction technique and a spinning disc apparatus were combined to quantify the Fn adsorption, integrin binding and adhesion strength, respectively. This methodology allows for a binding equilibrium analysis that more closely reflects cellular adhesion found in stable tissue constructs in vivo. Differences in detachment strength and integrin binding were explained in terms of changes in the adhesion constant (psi, related to affinity) and binding efficiency of the adsorbed Fn for the alpha(5)beta(1) integrins (CH(3) approximately NH(2)<COOH approximately OH) and the resulting average bond strength. Fn interacted more strongly with alpha(5)beta(1) integrins when adsorbed on COOH vs. OH surfaces suggesting that negative charge may be a critical component of inducing efficient cellular adhesion. As evident by the low psi values, Fn adsorbed on NH(2) and CH(3) surfaces interacted inefficiently with alpha(5)beta(1) integrins and also possessed significant non-specific components to adhesion. Lastly, comparison of cellular adhesion to Fn adsorbed onto smooth and rough surfaces showed that nano-scale roughness altered cellular adhesion by increasing the surface density of adsorbed Fn.

Substrate-mediated delivery from self-assembled monolayers: effect of surface ionization, hydrophilicity, and patterning

Gene transfer has many potential applications in basic and applied sciences. In vitro, DNA delivery can be enhanced by increasing the concentration of DNA in the cellular microenvironment through immobilization of DNA to a substrate that supports cell adhesion. Substrate-mediated delivery describes the immobilization of DNA, complexed with cationic lipids or polymers, to a biomaterial or substrate. As surface properties are critical to the efficiency of the surface delivery approach, self-assembled monolayers (SAMs) of alkanethiols on gold were used to correlate surface chemistry of the substrate to binding, release, and transfection of non-specifically immobilized complexes. Surface hydrophobicity and ionization were found to mediate both DNA complex immobilization and transfection, but had no effect on complex release. Additionally, SAMs were used in conjunction with soft lithographic techniques to imprint substrates with specific patterns, resulting in patterned DNA complex deposition and transfection, with transfection efficiencies in the patterns nearing 40%. Controlling the interactions between complexes and substrates, with the potential for patterned delivery, can be used to locally enhance or regulate gene transfer, with applications to tissue engineering scaffolds and transfected cell arrays.

Cellular alignment by grafted adhesion peptide surface density gradients

The extracellular matrix and extracellular matrix-associated proteins play a major role in growth and differentiation of tissues and organs. To date, few methods have been developed that allow researchers to examine the affect of surface density gradients of adhesion molecules in a controlled manner. Fibroblasts cultured on surfaces with a surface density gradient of RGD peptide aligned parallel to the gradient while fibroblasts on constant density RGD surfaces spread but did not align as has been shown in numerous earlier studies. Not only did fibroblasts align on the gradient surfaces, but they also showed significantly greater elongation than on constant density peptide surfaces or on control surfaces. This type of method is easy to replicate and can be used by laboratories interested in investigating alignment of various cell types without mechanical force or other stimulation, and without cell-cell interaction or for investigation of affects of surface density gradients of molecules on cellular biochemistry and biophysics. This method also has potential applications for developing scaffolds for tissue engineering applications where cellular alignment is necessary.

Developments in xenobiotic-free culture of human keratinocytes for clinical use

We have recently reported that irradiated human fibroblasts can be used as a feeder layer, to expand keratinocytes under serum-free conditions, on a chemically defined plasma polymer surface developed for the culture and transfer of keratinocytes for clinical use. While this is a significant advance in developing a serum-free keratinocyte culture approach, the need to irradiate fibroblasts to growth-arrest them and prevent them from overgrowing the keratinocytes introduces another small, but potential, risk for the patient. The aim of the present study was to develop conditions for the coculture of normal human keratinocytes with nonirradiated normal human fibroblasts under serum-free conditions. We examined the fibroblast/keratinocyte relationship on three separate surfaces: tissue culture plastic, non-tissue culture plastic, and a plasma polymer surface designed for clinical use. We report that it is possible to achieve rapid and successful expansion of human keratinocytes under serum-free conditions on all three surfaces providing one uses a keratinocyte-friendly media, a minimum seeding density of keratinocytes, and a ratio of fibroblasts to keratinocytes that does not exceed 1:1. These results provide us with a rapid laboratory expansion of proliferative human keratinocytes, under completely defined culture conditions, without any xenobiotic cells (mouse fibroblasts) or material (bovine serum), for the treatment of patients with extensive skin loss.

A review of keratinocyte delivery to the wound bed

Over the last 20 years, confluent sheets of cultured epithelial autograft have been used for patients with major burns. Problems with the lack of "take" and long-term durability, as well as the time delay to produce such grafts, have led to the development of delivery systems to transfer keratinocytes to the wound bed. This review article describes the problems of using cultured epithelial autograft and the advantages of using preconfluent keratinocytes. Despite the numerous delivery systems that have been reported, most studies are limited to animal wound bed models. There are a few small clinical studies that have demonstrated enhanced healing using mainly subjective methods. There is a need for controlled, randomized clinical trials to prove the efficacy of keratinocyte delivery systems. Proposals for the use of this technology are made.

Surface modification of ultra thin poly (ε-caprolactone) films using acrylic acid and collagen

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.

In vivo CH3(CH2)11SAu SAM electrodes in the beating heart: in situ analytical studies relevant to pacemakers and interstitial biosensors

To study in vivo modification of the SAM equivalent circuit when a highly ordered SAM is used as a bioelectrode, dodecanethiolate SAM-Au intramuscular electrodes were studied in living rat heart in a challenging in situ perfused rat model by impedance spectroscopy, cyclic voltammetry, and neutron activation analysis (NAA). The SAM layer experienced disintegration in vivo biological system, as NAA detected the presence of Au atoms that had leached into the surrounding living tissue. Therefore, the underlying Au surface became exposed during biological implant. Study by impedance spectroscopy, however, revealed perfect capacitive behavior for the SAM, similar to in vitro behavior. Electrodes showed a pure capacitive Nyquist plot with 86.1-89.4 degrees near-vertical line segments as the equivalent circuit locus, as for a parallel plate capacitor. Impedance magnitude varied linearly with 1/omega excluding diffusionally limited ionic charge transport. There was no diffusional conductive element Z(W infinity ) or spatially confined Warburg impedance Z(D). The effect of in vivo exposure of a highly ordered SAM is a 'sealing over' effect of new defects by the binding of proteinaceous or lipid species in the biological milieu, a fact of significance for SAM electrodes used either as pacemaker electrodes or as a platform for in vivo biosensors.

Plasma-polymerized surfaces for culture of human keratinocytes and transfer of cells to an in vitro wound-bed model

The aim of this study was to develop plasma-polymerized surfaces suitable for the attachment and culture of human keratinocytes and that would allow their subsequent transfer to a wound-bed model. Keratinocyte attachment has been assessed on a carrier polymer, either untreated or treated with a hydrocarbon plasma polymer, collagen I, or carboxylic-acid-containing plasma copolymers. Cell attachment was poor on the "bare" carrier polymer and hydrocarbon plasma polymer (PP) surfaces. Cell attachment was good and comparable on collagen I-coated carrier polymer and carrier polymer plasma coated with carboxylic acid functionalities. After 24 h of cell culture, surfaces were inverted so that cells were adjacent to a de-epidermalized dermis (DED) for 4 days. After 4 days in contact with DED, the surfaces were removed and the level of residual cells and cells transferred to DED were assessed using a cell viability assay. Cell transfer from the collagen I-coated surface was on the order of 90%. Transfer from the carrier polymer surface and the hydrocarbon-coated surface was poor while cells cultured on acid-containing surfaces showed high levels of transfer. Cell transfer was greatest from those surfaces containing the highest level of acid functionality (ca. 21%). Cell transfer was not significantly affected by the choice of carrier polymer material although some sample-to-sample variation was seen. To determine that plasma-polymerized surfaces could be used clinically, selected samples were sterilized with ethylene oxide. Subsequent analysis and cell culture indicated that the surface chemistry and cell-transfer capability of these plasma-polymerized surfaces were unaffected by the sterilization procedure. Plasma-polymerized carboxylic-acid-containing surfaces show great promise in the field of wound healing, encouraging keratinocyte attachment and permitting keratinocyte transfer to a wound bed.