Design of an artificial skin. II. Control of chemical composition

Vascularization of the dermal support enhances wound re-epithelialization by in situ delivery of epidermal keratinocytes

Despite significant advances in management of severe wounds such as burns and chronic ulcers, autologous split-thickness skin grafts are still the gold standard of care. The main problems with this approach include pain and discomfort associated with harvesting autologous tissue, limited availability of donor sites, and the need for multiple surgeries. Although tissue engineering has great potential to provide alternative approaches for tissue regeneration, several problems have hampered progress in translating technological advances to clinical reality. Specifically, engineering of skin substitutes requires long culture times and delayed vascularization after implantation compromises graft survival. To address these issues we developed a novel two-prong strategy for tissue regeneration in vivo: (1) vascularization of acellular dermal scaffolds by infiltration of angiogenic factors; and (2) generation of stratified epidermis by in situ delivery of epidermal keratinocytes onto the prevascularized dermal support. Using athymic mouse as a model system, we found that incorporation of angiogenic factors within acellular human dermis enhanced the density and diameter of infiltrating host blood vessels. Increased vascularization correlated with enhanced proliferation and stratification of the neoepidermis originating from the fibrin-keratinocyte cell suspension. This strategy promoted tissue regeneration in vivo with no need for engineering skin substitutes; therefore, it may be useful for treatment of major wounds when skin donor sites are scarce and rapid wound coverage is required.

An optical method to quantify the density of ligands for cell adhesion receptors in three-dimensional matrices

The three-dimensional matrix that surrounds cells is an important insoluble regulator of cell phenotypes. Examples of such insoluble surfaces are the extracellular matrix (ECM), ECM analogues and synthetic polymeric biomaterials. Cell-matrix interactions are mediated by cell adhesion receptors that bind to chemical entities (adhesion ligands) on the surface of the matrix. There are currently no established methods to obtain quantitative estimates of the density of adhesion ligands recognized by specific cell adhesion receptors. This article presents a new optical-based methodology for measuring ligands of adhesion receptors on three-dimensional matrices. The study also provides preliminary quantitative results for the density of adhesion ligands of integrins alpha(1)beta(1) and alpha(2)beta(1) on the surface of collagen-based scaffolds, similar to biomaterials that are used clinically to induce regeneration in injured skin and peripheral nerves. Preliminary estimates of the surface density of the ligands of these two major collagen-binding receptors are 5775 +/- 2064 ligands microm(-2) for ligands of alpha(1)beta(1) and 17 084 +/- 5353 ligands microm(-2) for ligands of alpha(2)beta(1). The proposed methodology can be used to quantify the surface chemistry of insoluble surfaces that possess biological activity, such as native tissue ECM and biomaterials, and therefore can be used in cell biology, biomaterials science and regenerative medical studies for quantitative description of a matrix and its effects on cells.

The healing of bony defects by cell-free collagen-based scaffolds compared to stem cell-seeded tissue engineered constructs

One of the key challenges in tissue engineering is to understand the host response to scaffolds and engineered constructs. We present a study in which two collagen-based scaffolds developed for bone repair: a collagen-glycosaminoglycan (CG) and biomimetic collagen-calcium phosphate (CCP) scaffold, are evaluated in rat cranial defects, both cell-free and when cultured with MSCs prior to implantation. The results demonstrate that both cell-free scaffolds showed excellent healing relative to the empty defect controls and somewhat surprisingly, to the tissue engineered (MSC-seeded) constructs. Immunological analysis of the healing response showed higher M1 macrophage activity in the cell-seeded scaffolds. However, when the M2 macrophage response was analysed, both groups (MSC-seeded and non-seeded scaffolds) showed significant activity of these cells which are associated with an immunomodulatory and tissue remodelling response. Interestingly, the location of this response was confined to the construct periphery, where a capsule had formed, in the MSC-seeded groups as opposed to areas of new bone formation in the non-seeded groups. This suggests that matrix deposited by MSCs during in vitro culture may adversely affect healing by acting as a barrier to macrophage-led remodelling when implanted in vivo. This study thus improves our understanding of host response in bone tissue engineering.

Concentrated collagen-chondroitin sulfate scaffolds for tissue engineering applications

Collagen-chondroitin sulfate biomaterial scaffolds have been used in a number of tissue-engineered products under development or in the clinics. In this article, we describe a new approach based on centrifugation for obtaining highly concentrated yet porous collagen scaffolds. Water uptake, chondroitin sulfate retention, morphology, mechanical properties, and tissue-engineering potential of the concentrated scaffolds were investigated. Our results show that the new approach can lead to scaffolds containing four times as much collagen as that in conventional unconcentrated scaffolds. Further, water uptake in the concentrated scaffolds was significantly greater while chondroitin sulfate retention in the concentrated scaffolds was unaffected. The value of mean pore diameter in the concentrated scaffolds was smaller than that in the unconcentrated scaffolds and the walls of the pores in the former comprised of a continuous sheet of collagen. The mechanical properties measured as moduli of elasticity in compression and tension were improved by as much as 30 times in the concentrated scaffolds. In addition, our tissue culture results with human mesenchymal stem cells and foreskin keratinocytes show that the new scaffolds can be used for cartilage and skin tissue-engineering applications.

Cross-linking affects cellular condensation and chondrogenesis in type II collagen-GAG scaffolds seeded with bone marrow-derived mesenchymal stem cells

The formation of cartilaginous tissue by chondroprogenitor cells, whether in vivo or in vitro, appears to require a critical initial stage of "condensation" in which intercellular space is reduced through an aggregation of cells, leading to development of cell-to-cell junctions followed by chondrocytic differentiation. The objective of this study was to investigate the association of aggregation (condensation) of mesenchymal stem cell (MSCs) and chondrogenesis in vitro. Previous work with chondrocytes indicated that the cross-link density and related cell-mediated contraction of collagen scaffolds significantly affects cartilaginous tissue formation within the cell-seeded construct. Based on this finding, we hypothesized that the cell-aggregating effect of the contraction of MSC-seeded collagen scaffolds of lower cross-link density favors chondrogenesis; scaffolds of higher cross-link density, which resist cell-mediated contraction, would demonstrate a lower cell number density (i.e., subcritical packing density) and less cartilage formation. Type II collagen-GAG scaffolds, chemically cross-linked to achieve a range of cross-link densities, were seeded with caprine MSCs and cultured for 4 weeks. Constructs with low cross-link densities experienced cell-mediated contraction, increased cell number densities, and a greater degree of chondrogenesis (indicated by the chondrocytic morphology of cells, and synthesis of GAG and type II collagen) compared to more highly cross-linked scaffolds that resisted cellular contraction. These results provide a foundation for further investigation of the mechanisms by which condensation of mesenchymal cells induces chondrogenesis in this in vitro model, and may inform cross-linking protocols for collagen scaffolds for use in cartilage tissue engineering.

Divalent cations modulate alpha2beta1 integrin-mediated malignancy in a novel 3-dimensional in vitro model of pancreatic cancer

OBJECTIVES

We previously showed that divalent cations regulate alpha2beta1 integrin-mediated pancreatic cancer cell interactions with type I collagen in 2 dimensions (2D), including cell adhesion, migration, and proliferation. Presently, we examined divalent cation-dependent alpha2beta1 integrin-mediated pancreatic cancer cell adhesion and proliferation on type I collagen in a novel 3D in vitro model.

METHODS

Cell attachment, proliferation, and antibody inhibition assays on type I collagen in both 2D and 3D, and microscopy and immunoblotting were used for these studies.

RESULTS

As in 2D, cell attachment on type I collagen in 3D is Mg-dependent and inhibited by Ca. Proliferation in 3D is also Mg-dependent, but maximal when Mg is present at concentrations that promote maximal cell adhesion and Ca is present at concentrations less than Mg. Immunoblotting studies demonstrate that the divalent cation-dependent changes in cell-cell adhesion observed on type I collagen in both 2D and 3D are associated with the changes in E-cadherin and beta-catenin expression. Antibody inhibition assays indicate further that the alpha2beta1 integrin specifically mediates proliferation on type I collagen in 3D under altered divalent cation conditions.

CONCLUSIONS

Divalent cation shifts could activate alpha2beta1 integrin-mediated malignancy in the type I collagen-rich 3D tumor microenvironment of pancreatic cancer.

Chondroitin-6-sulfate incorporation and mechanical stimulation increase MSC-collagen sponge construct stiffness

Using functional tissue engineering principles, our laboratory has produced tendon repair tissue which matches the normal patellar tendon force-displacement curve up to 32% of failure. This repair tissue will need to withstand more strenuous activities, which can reach or even exceed 40% of failure force. To improve the linear stiffness of our tissue engineered constructs (TECs) and tissue engineered repairs, our lab is incorporating the glycosaminoglycan chondroitin-6-sulfate (C6S) into a type I collagen scaffold. In this study, we examined the effect of C6S incorporation and mechanical stimulation cycle number on linear stiffness and mRNA expression (collagen types I and III, decorin and fibronectin) for mesenchymal stem cell (MSC)-collagen sponge TECs. The TECs were fabricated by inoculating MSCs at a density of 0.14 x 10(6) cells/construct onto pre-cut scaffolds. Primarily type I collagen scaffold materials, with or without C6S, were cultured using mechanical stimulation with three different cycle numbers (0, 100, or 3,000 cycles/day). After 2 weeks in culture, TECs were evaluated for linear stiffness and mRNA expression. C6S incorporation and cycle number each played an important role in gene expression, but only the interaction of C6S incorporation and cycle number produced a benefit for TEC linear stiffness.

Enhanced angiogenesis of gene-activated dermal equivalent for treatment of full thickness incisional wounds in a porcine model

Angiogenesis of dermal equivalent is one of the key issues for treatment of full thickness skin defects. To develop a gene-activated bilayer dermal equivalent (BDE), N,N,N-trimethyl chitosan chloride (TMC), a cationic gene delivery vector, was used to form complexes with the plasmid DNA encoding vascular endothelial growth factor-165 (VEGF-165), which was then incorporated into a collagen-chitosan/silicone membrane scaffold. To evaluate the angiogenesis property in vivo, full thickness skin defects were made on the back of pigs, into which the TMC/pDNA-VEGF complexes loaded BDE and other three control BDEs, i.e. the blank BDE, and the BDEs loaded with pDNA-VEGF and TMC/pDNA-eGFP complexes, respectively, were transplanted. Biopsy specimens were harvested at day 7, 10 and 14 after surgery for histology, immunohistochemistry, immunofluorescence, real-time quantitative PCR (RT-qPCR) and western blotting analyses. The results showed that the TMC/pDNA-VEGF group had the strongest VEGF expression in mRNA and protein levels, resulting in the highest densities of newly-formed and mature vessels. The ultra-thin skin graft was further transplanted onto the dermis regenerated by the TMC/pDNA-VEGF complexes loaded BDE at day 10 and well survived. At 112 days grafting, the healing skin had a similar structure and approximately 80% tensile strength of the normal skin.

Biologically active collagen-based scaffolds: advances in processing and characterization

A small number of type I collagen-glycosaminoglycan scaffolds (collagen-GAG scaffolds; CGSs) have unusual biological activity consisting primarily in inducing partial regeneration of organs in the adult mammal. Two of these are currently in use in a variety of clinical settings. CGSs appear to induce regeneration by blocking the adult healing response, following trauma, consisting of wound contraction and scar formation. Several structural determinants of biological activity have been identified, including ligands for binding of fibroblasts to the collagen surface, the mean pore size (which affects ligand density) and the degradation rate (which affects the duration of the wound contraction-blocking activity by the scaffold). Processing variables that affect these determinants include the kinetics of swelling of collagen fibres in acetic acid, freezing of the collagen-GAG suspension and cross-linking of the freeze-dried scaffold. Recent developments in the processing of CGSs include fabrication of scaffolds that are paucidisperse in pore size, scaffolds with gradients in physicochemical properties (and therefore biological activity) and scaffolds that incorporate a mineral component. Advances in the characterization of the pore structure of CGSs have been made using confocal and nonlinear optical microscopy (NLOM). The mechanical behaviour of CGSs, as well as the resistance to degradative enzymes, have been studied. Following seeding with cells (typically fibroblasts), contractile forces in the range 26-450 nN per cell are generated by the cells, leading to buckling of scaffold struts. Ongoing studies of cell-seeded CGSs with NLOM have shown an advantage over the use of confocal microscopy due to the ability of the former method to image the CGS surfaces without staining (which alters its surface ligands), reduced cell photodamage, reduced fluorophore photobleaching and the ability to image deeper inside the scaffold.

Design of a multiphase osteochondral scaffold. I. Control of chemical composition

This is the first in a series of articles that describe the design and development of a family of osteochondral scaffolds based on collagen-glycosaminoglycan (collagen-GAG) and calcium phosphate technologies, engineered for the regenerative repair of defects in articular cartilage. The osteochondral scaffolds consist of two layers: a mineralized type I collagen-GAG scaffold designed to regenerate the underlying subchondral bone and a nonmineralized type II collagen-GAG scaffold designed to regenerate cartilage. The subsequent articles in this series describe the fabrication and properties of a mineralized scaffold as well as a two-layer (one mineralized, the other not) osteochondral scaffold for regeneration of the underlying bone and cartilage, respectively. This article describes a technology through which the chemical composition-particularly the calcium phosphate mass fraction-of triple coprecipitated nanocomposites of collagen, glycosaminoglycan, and calcium phosphate can be accurately and reproducibly varied without the need for titrants or other additives. Here, we describe how the mineral:organic ratio can be altered over a range that includes that for articular cartilage (0 wt % mineral) and for bone (75 wt % mineral). This technology achieves the objective of mimicking the composition of two main tissue types found in articular joints, with particular emphasis on the osseous compartment of an osteochondral scaffold. Exclusion of titrants avoids the formation of potentially harmful contaminant phases during freeze-drying steps crucial for scaffold fabrication, ensuring that the potential for binding growth factors and drugs is maintained.

Emerging therapy for improving wound repair of severe radiation burns using local bone marrow-derived stem cell administrations

The therapeutic management of severe radiation burns remains a challenging issue today. Conventional surgical treatment including excision, skin autograft, or flap often fails to prevent unpredictable and uncontrolled extension of the radiation-induced necrotic process. In a recent very severe accidental radiation burn, we demonstrated the efficiency of a new therapeutic approach combining surgery and local cellular therapy using autologous mesenchymal stem cells (MSC), and we confirmed the crucial place of the dose assessment in this medical management. The patient presented a very significant radiation lesion located on the arm, which was first treated by several surgical procedures: iterative excisions, skin graft, latissimus muscle dorsi flap, and forearm radial flap. This conventional surgical therapy was unfortunately inefficient, leading to the use of an innovative cell therapy strategy. Autologous MSC were obtained from three bone marrow collections and were expanded according to a clinical-grade protocol using platelet-derived growth factors. A total of five local MSC administrations were performed in combination with skin autograft. After iterative local MSC administrations, the clinical evolution was favorable and no recurrence of radiation inflammatory waves occurred during the patient's 8-month follow-up. The benefit of this local cell therapy could be linked to the "drug cell" activity of MSC by modulating the radiation inflammatory processes, as suggested by the decrease in the C-reactive protein level observed after each MSC administration. The success of this combined treatment leads to new prospects in the medical management of severe radiation burns and more widely in the improvement of wound repair.

Combined effects of scaffold stiffening and mechanical preconditioning cycles on construct biomechanics, gene expression, and tendon repair biomechanics

Our group has previously reported that in vitro mechanical stimulation of tissue-engineered tendon constructs significantly increases both construct stiffness and the biomechanical properties of the repair tissue after surgery. When optimized using response surface methodology, our results indicate that a mechanical stimulus with three components (2.4% strain, 3000 cycles/day, and one cycle repetition) produced the highest in vitro linear stiffness. Such positive correlations between construct and repair stiffness after surgery suggest that enhancing structural stiffness before surgery could not only accelerate repair stiffness but also prevent premature failures in culture due to poor mechanical integrity. In this study, we examined the combined effects of scaffold crosslinking and subsequent mechanical stimulation on construct mechanics and biology. Autologous tissue-engineered constructs were created by seeding mesenchymal stem cells (MSCs) from 15 New Zealand white rabbits on type I collagen sponges that had undergone additional dehydrothermal crosslinking (termed ADHT in this manuscript). Both constructs from each rabbit were mechanically stimulated for 8h/day for 12 consecutive days with half receiving 100 cycles/day and the other half receiving 3000 cycles/day. These paired MSC-collagen autologous constructs were then implanted in bilateral full-thickness, full-length defects in the central third of rabbit patellar tendons. Increasing the number of in vitro cycles/day delivered to the ADHT constructs in culture produced no differences in stiffness or gene expression and no changes in biomechanical properties or histology 12 weeks after surgery. Compared to MSC-based repairs from a previous study that received no additional treatment in culture, ADHT crosslinking of the scaffolds actually lowered the 12-week repair stiffness. Thus, while ADHT crosslinking may initially stiffen a construct in culture, this specific treatment also appears to mask any benefits of stimulation among repairs postsurgery. Our findings emphasize the importance of properly preconditioning a scaffold to better control/modulate MSC differentiation in vitro and to further enhance repair outcome in vivo.

Electrospun silk biomaterial scaffolds for regenerative medicine

Electrospinning is a versatile technique that enables the development of nanofiber-based biomaterial scaffolds. Scaffolds can be generated that are useful for tissue engineering and regenerative medicine since they mimic the nanoscale properties of certain fibrous components of the native extracellular matrix in tissues. Silk is a natural protein with excellent biocompatibility, remarkable mechanical properties as well as tailorable degradability. Integrating these protein polymer advantages with electrospinning results in scaffolds with combined biochemical, topographical and mechanical cues with versatility for a range of biomaterial, cell and tissue studies and applications. This review covers research related to electrospinning of silk, including process parameters, post treatment of the spun fibers, functionalization of nanofibers, and the potential applications for these material systems in regenerative medicine. Research challenges and future trends are also discussed.

In vivo engineering of tissues: Biological considerations, challenges, strategies, and future directions

Moving forward materials-based regenerative medicine faces many challenges to ensure clinical success. Many of these challenges lie at the interface of molecular/structural biology and materials science. This review discusses this issue from a biological and material view-point, highlighting key biological processes and variables that can impact the repair processes. From a materials design stand point, developing materials that can promote healing over scarring is the key. All indicators suggest that polymeric materials are most well-suited for de novo engineering of tissues. In addition to biomolecular signals that are involved in controlling the fate of cells and neo-tissue morphogenesis at the site of implantation, this review also discusses recent advances in design of highly functional injectable biomaterials, that show promise in controlling local biological processes.

Evaluation of RGD Modification on Collagen Matrix

OBJECTIVE

To observe the changes during the process of modificating collagen with Arg-Gly-Asp (RGD) peptide, a series experiments were designed.

METHODS

The 3D porous collagen matrices modified with RGD peptide were constructed by lyophilization. The modified and unmodified matrices were characterized by Scanning Electron Microscopy (SEM) and Electron Spectroscopy for Chemical Analysis (ESCA). Fibroblasts were used to evaluate the cell compatibility of the matrices.

RESULTS

In terms of cell growth, the cells attached much better on the modified matrix than on the unmodified one. Compare to the unmodified matrices, the polar groups on the modified matrix increased.

CONCLUSIONS

The introducing of specific RGD receptor-mediated adhesion site on matrices obviously enhanced the cells adhesion on collagen matrices.

Study on Modification of Collagen with Chondroitin Sulfate on the Microcosmic Level

To observe the changes during the process of modificating collagen with chondroitin sulfate (CS), a series of experiments was designed. Collagen scaffold was constructed by lyophilization. CS was attached into the scaffold through crosslinking. Solid-liquid phase separation method was used to shape 3D porous structure. Both kinds of scaffolds were investigated by Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), and Electron Spectroscopy for Chemical Analysis (ESCA), Amino Acid Analyses (AAA), and so on for structures and characteristics. The results showed that CS could improve the properties of collagen both in biocompatibility and mechanical property. It increased the polar groups on the surface of scaffold and protected amino acid residues of collagen fiber. At the same time, the structure of scaffold became more regular because of the increase of polar groups. It was indicated that those were the real reasons for the improvement of modification.

The effect of dehydrothermal treatment on the mechanical and structural properties of collagen-GAG scaffolds

The mechanical properties of tissue engineering scaffolds are critical for preserving the structural integrity and functionality during both in vivo implantation and long-term performance. In addition, the mechanical and structural properties of the scaffold can direct cellular activity within a tissue-engineered construct. In this context, the aim of this study was to investigate the effects of dehydrothermal (DHT) treatment on the mechanical and structural properties of collagen-glycosaminoglycan (CG) scaffolds. Temperature (105-180 degrees C) and exposure period (24-120 h) of DHT treatment were varied to determine their effect on the mechanical properties, crosslinking density, and denaturation of CG scaffolds. As expected, increasing the temperature and duration of DHT treatment resulted in an increase in the mechanical properties. Compressive properties increased up to twofold, while tensile properties increased up to 3.8-fold. Crosslink density was found to increase with DHT temperature but not exposure period. Denaturation also increased with DHT temperature and exposure period, ranging from 25% to 60% denaturation. Crosslink density was found to be correlated with compressive modulus, whilst denaturation was found to correlate with tensile modulus. Taken together, these results indicate that DHT treatment is a viable technique for altering the mechanical properties of CG scaffolds. The enhanced mechanical properties of DHT-treated CG scaffolds improve their suitability for use both in vitro and in vivo. In addition, this work facilitates the investigation of the effects of mechanical properties and denaturation on cell activity in a 3D environment.

Topical negative pressure stimulates endothelial migration and proliferation: a suggested mechanism for improved integration of Integra

Topical negative pressure is an effective technique to promote wound healing and the integration of skin graft and synthetic dermal equivalents. We describe an in vitro model to investigate the effect of negative pressure on angiogenesis, a pivotal step. Dermal fibroblasts or human microvascular endothelial cells were cultured on Integra and subjected to intermittent or continuous negative pressure. At fixed intervals of over 120 hours, the Integra was fixed and assessed for cell migration (microscopy), cell viability (MTS assay), and cell proliferation (Ki67 immunostaining). Under control conditions, endothelial cells formed a monolayer and failed to ingress, whereas fibroblasts migrated throughout the Integra within 24 hours. Negative pressure switches endothelial cell to a migratory and proliferative phenotype. Ingress is greatest with intermittent rather than continuous negative pressure. It has no effect on dermal fibroblast function. This study identifies an important, potential pro-angiogenic mechanism by which topical negative pressure promotes wound healing.

Effect of low-level laser treatment of tissue-engineered skin substitutes: contraction of collagen lattices

Fibroblast-populated collagen lattices (FPCL) are widely used in tissue-engineered artificial skin substitutes, but their main drawback is that interaction of fibroblasts and matrix causes contraction of the lattice, reducing it to about 20% of its original area. The effect of low-level laser treatment (LLLT) on the behavior of 3T3 fibroblasts seeded in collagen lattices containing 20% chondroitin-6-sulphate was investigated to determine whether LLLT could control the contraction of FPCL. A He-Ne laser was used at 632.8 nm to deliver a 5-mW continuous wave with fluences from 1 to 4 J/cm(2). Laser treatment at 3 J/cm(2) increased contraction of collagen lattices in the absence of cells but decreased contraction of cell seeded lattices over a 7-day period. The effect was energy dependent and was not observed at 1, 2, or 4 J/cm(2). There was no alteration in fibroblast viability, morphology, or mitochondrial membrane potential after any laser treatments, but the distribution of actin fibers within the cells and collagen fibers in the matrices was disturbed at 3 J/cm(2). These effects contribute to the decrease in contraction observed. LLLT may offer a means to control contraction of FPCL used as artificial skin substitutes.

Density-property relationships in mineralized collagen-glycosaminoglycan scaffolds

Mineralized collagen-glycosminoglycan scaffolds have previously been fabricated by freeze-drying a slurry containing a co-precipitate of calcium phosphate, collagen and glycosaminoglycan. The mechanical properties of the scaffold are low (e.g. the dry Young's modulus for a 50 wt.% mineralized scaffold is roughly 780 kPa). Our previous attempt to increase the mechanical properties of the scaffold by increasing the mineralization (from 50 to 75 wt.%) was unsuccessful due to defects in the more mineralized scaffold. In this paper, we describe a new technique to improve the mechanical properties by increasing the relative density of the scaffolds. The volume fraction of solids in the slurry was increased by vacuum-filtration. The slurry was then freeze-dried in the conventional manner to produce scaffolds with relative densities between 0.045 and 0.187 and pore sizes of about 100-350 microm, values appropriate for bone growth. The uniaxial compressive stress-strain curves of the scaffolds indicated that the Young's modulus in the dry state increased from 780 to 6500 kPa and that the crushing strength increased from 39 to 275 kPa with increasing relative density. In the hydrated state, the Young's modulus increased from 6.44 to 34.8 kPa and the crushing strength increased from 0.55 to 2.12 kPa; the properties were further increased by cross-linking. The modulus and strength were well described by models for cellular solids.

Preparation of collagen/gelatin sponge scaffold for sustained release of bFGF

Artificial dermis (AD) has been used to regenerate dermis-like tissues in the treatment of full-thickness skin defects, but it takes 2 or 3 weeks to complete dermal regeneration. Our previous study demonstrated that injection of basic fibroblast growth factor (bFGF)-impregnated gelatin microspheres (MS) into the AD accelerates the regeneration of dermis-like tissue. However, injection of gelatin MS before clinical use is complicated and time consuming. This study investigated a new scaffold, in which collagen and gelatin are integrated, and which is capable of sustained bFGF release. We produced collagen/gelatin sponges with a gelatin concentration of 0wt%, 10wt%, 30wt%, and 50wt%. The mean pore size in each sponge decreased with the gelatin concentration. In an in vitro study, proliferation of fibroblasts in each sponge was not significantly different over 7 days of culture. As for in vivo sustained release of bFGF, a radioisotope study demonstrated that retention of bFGF in gelatin 10wt% and 30wt% sponges was significantly larger than that in gelatin 0wt% sponge. The collagen/gelatin sponges were grafted on full-thickness skin defects created on a rabbit ear, and we evaluated regeneration of dermis-like tissue by measuring the amount of hemoglobin and size of dermis-like tissue on histological sections. Seven days after implantation, the amount of hemoglobin in dermis-like tissue in gelatin 10wt% sponge was significantly larger than those in control and gelatin 50wt% sponge. Twenty-eight days after implantation, the area of dermis-like tissue in gelatin 10wt% sponge was significantly larger than those in the other specimens. We conclude that the collagen sponge integrated with 10wt% gelatin has the most potential for sustained release of bFGF and that the combination of collagen/gelatin 10wt% sponge and bFGF is a promising therapeutic modality for the treatment of full-thickness skin defects.

Negative pressure wound therapy with reticulated open cell foam-adjunctive treatment in the management of traumatic wounds of the leg: a review of the literature

Over the last decade, the application of and indications for negative pressure wound therapy with reticulated open cell foam (NPWT/ROCF) as delivered by V.A.C.(R) Therapy (KCI, San Antonio, TX) have grown tremendously. This is particularly true in orthopaedic trauma in the management of injuries to the leg, ankle, and foot. This article reviews the evidence-based medicine in terms of NPWT/ROCF, as a method of reducing bacterial counts in wounds, as a bridge until definitive bony coverage, for treating infections, and as an adjunct to wound bed preparation and for bolstering split-thickness skin grafts, dermal replacement grafts, and over muscle flaps. NPWT/ROCF has been shown to be an adjunct to the mainstays of wound management. No significant complications have been noted in the categories of NPWT/ROCF discussed in this review. In addition, evidence supports a decrease in complex soft tissue procedures in grade IIIB open fractures when NPWT/ROCF is employed. Although more research needs to be done, NPWT/ROCF appears to provide clinical benefit for the treatment of these complex lower extremity wounds.

Skin tissue engineering

The major applications of tissue-engineered skin substitutes are in promoting the healing of acute and chronic wounds. Several approaches have been taken by commercial companies to develop products to address these conditions. Skin substitutes include both acellular and cellular devices. While acellular skin substitutes act as a template for dermal formation, this discussion mainly covers cellular devices. In addressing therapeutic applications in tissue engineering generally, a valuable precursor is an understanding of the mechanism of the underlying pathology. While this is straightforward in many cases, it has not been available for wound healing. Investigation of the mode of action of the tissue-engineered skin substitutes has led to considerable insight into the mechanism of formation, maintenance and treatment of chronic wounds. Four aspects mediating healing are considered here for their mechanism of action: (i) colonization of the wound bed by live fibroblasts in the implant, (ii) the secretion of growth factors, (iii) provision of a suitable substrate for cell migration, particularly keratinocytes and immune cells, and (iv) modification of the immune system by secretion of neutrophil recruiting chemokines. An early event in acute wound healing is an influx of neutrophils that destroy planktonic bacteria. However, if the bacteria are able to form biofilm, they become resistant to neutrophil action and prevent reepithelialization. In this situation the wound becomes chronic. In chronic wounds, fibroblasts show a senescence-like phenotype with decreased secretion of neutrophil chemoattractants that make it more likely that biofilms become established. Treatment of the chronic wounds involves debridement to eliminate biofilm, and the use of antimicrobials. A role of skin substitutes is to provide non-senescent fibroblasts that attract and activate neutrophils to prevent biofilm re-establishment. The emphasis of the conclusion is the importance of preventing contaminating bacteria becoming established and forming biofilms.

The effects of glycosaminoglycan content on the compressive modulus of cartilage engineered in type II collagen scaffolds

OBJECTIVE

The current study determined the unconfined compressive modulus of tissue-engineered constructs with varying sulfated glycosaminoglycan (GAG) density produced by goat articular chondrocytes in type II collagen scaffolds prepared with a range of cross-link densities and various times in culture. The purpose of this work is to establish a basis for future studies employing constructs of selected maturity (e.g., 25%, 50%, or 75% normal GAG content) for cartilage repair in vivo.

METHODS

Porous scaffolds (8 mm diameter by 2 mm thick) were fabricated from porcine type II collagen by freeze-drying, followed by dehydrothermal treatment and carbodiimide cross-linking. In a pilot study, passage 3 adult caprine articular chondrocytes isolated from one goat were grown in scaffolds with six cross-link densities for 2, 3, 4, and 6 weeks (n=3). The goal was to select scaffold cross-link densities and times in culture that would produce constructs with approximately 25%, 50% and 75% the GAG density of native articular cartilage. Based on the results of the pilot study, chondrocytes from three goats were grown in scaffolds with two cross-link densities for three time periods: 3, 5, and 9 weeks (n=6; one of the cross-link groups was run in quadruplicate). The equilibrium modulus from unconfined compression testing of these samples was correlated with GAG content.

RESULTS

There was a notable increase in GAG density with decreasing cross-link density. Histological analysis verified a chondrogenic phenotype and revealed various amounts of GAG and type II collagen-containing cartilage. The correlation between modulus and GAG density had a linear coefficient of determination of 0.60. One group with a mean GAG density of 22 microg/mm(3), which was 140% the GAG density of normal caprine articular cartilage, averaged a compressive modulus of 31.5 kPa, which was 10% of caprine articular cartilage tested in this study.

CONCLUSIONS

The GAG density and modulus of tissue-engineered constructs can be controlled by the degree of cross-linking of type II collagen scaffolds and time in culture.

Topical advances in wound care

There are fundamental differences between acute wounds that proceed to uncomplicated healing and those that become chronic wounds. Non-healing or chronic wounds can result from a combination of overlapping factors that prevent healing, including local tissue ischemia, repetitive trauma and ischemia/reperfusion injury, presence of tissue necrosis, impaired cellular and systemic host response to stress, and critical bacterial contamination. The bacterial burden in the wound contributes to a sustained inflammatory state, which inhibits normal progression to the proliferative phase of healing, thereby preventing restoration of tissue integrity. Appropriate wound bed preparation removes local barriers to healing and optimizes the tissue environment to achieve wound healing. It is an essential element of wound management that advances endogenous healing as well as the efficacy of topical and other wound therapy. This article will summarize a systematic approach to wound bed preparation using the "TIME" principle, and will highlight important advances in topical wound care.

A microfabricated porous collagen-based scaffold as prototype for skin substitutes

An important element of artificial skin is a tissue scaffold that allows for fast host regeneration. We present a microfabrication strategy, based on gelling collagen-based components inside a microfluidic device, that produces well-controlled pore sizes inside the scaffold. This strategy can produce finely patterned tissue scaffolds of clinically relevant dimensions suitable for surgical handling. Compared to porous collagen-based sponges produced by lyophilization, microfabricated tissue scaffolds preserve the fibrous structure and ligand density of natural occurring collagen. A fibroblast migration assay revealed fast cellular migration through the pores, which is desired for rapid tissue ingrowth. Finally, we also demonstrate a strategy to use this microfabrication technique to build anatomically accurate, multi-component skin substitutes in a cost-effective manner.

Collagen scaffolds for tissue engineering

There are two major approaches to tissue engineering for regeneration of tissues and organs. One involves cell-free materials and/or factors and one involves delivering cells to contribute to the regeneraion process. Of the many scaffold materials being investigated, collagen type I, with selective removal of its telopeptides, has been shown to have many advantageous features for both of these approaches. Highly porous collagen lattice sponges have been used to support in vitro growth of many types of tissues. Use of bioreactors to control in vitro perfusion of medium and to apply hydrostatic fluid pressure has been shown to enhance histogenesis in collagen scaffolds. Collagen sponges have also been developed to contain differentiating-inducing materials like demineralized bone to stimulate differentiation of cartilage tissue both in vitro and in vivo.

[Porous matrix and primary-cell culture: a shared concept for skin and cornea tissue engineering]

Skin and cornea both feature an epithelium firmly anchored to its underlying connective compartment: dermis for skin and stroma for cornea. A breakthrough in tissue engineering occurred in 1975 when skin stem cells were successfully amplified in culture by Rheinwald and Green. Since 1981, they are used in the clinical arena as cultured epidermal autografts for the treatment of patients with extensive burns. A similar technique has been later adapted to the amplification of limbal-epithelial cells. The basal layer of the limbal epithelium is located in a transitional zone between the cornea and the conjunctiva and contains the stem cell population of the corneal epithelium called limbal-stem cells (LSC). These cells maintain the proper renewal of the corneal epithelium by generating transit-amplifying cells that migrate from the basal layer of the limbus towards the basal layer of the cornea. Tissue-engineering protocols enable the reconstruction of three-dimensional (3D) complex tissues comprising both an epithelium and its underlying connective tissue. Our in vitro reconstruction model is based on the combined use of cells and of a natural collagen-based biodegradable polymer to produce the connective-tissue compartment. This porous substrate acts as a scaffold for fibroblasts, thereby, producing a living dermal/stromal equivalent, which once epithelialized results into a reconstructed skin/hemicornea. This paper presents the reconstruction of surface epithelia for the treatment of pathological conditions of skin and cornea and the development of 3D tissue-engineered substitutes based on a collagen-GAG-chitosan matrix for the regeneration of skin and cornea.

Characterization of mineralized collagen-glycosaminoglycan scaffolds for bone regeneration

Mineralized collagen-glycosaminoglycan scaffolds designed for bone regeneration have been synthesized via triple co-precipitation in the absence of a titrant phase. Here, we characterize the microstructural and mechanical properties of these newly developed scaffolds with 50 and 75 wt.% mineral content. The 50 wt.% scaffold had an equiaxed pore structure with isotropic mechanical properties and a Ca-P-rich mineral phase comprised of brushite; the 75 wt.% scaffold had a bilayer structure with a pore size varying in the through-thickness direction and a mineral phase comprised of 67% brushite and 33 wt.% monetite. The compressive stress-strain response of the scaffolds was characteristic of low-density open-cell foams with distinct linear elastic, collapse plateau and densification regimes. The elastic modulus and strength of individual struts within the scaffolds were measured using an atomic force microscopy cantilevered beam-bending technique and compared with the composite response under indentation and unconfined compression. Cellular solids models, using the measured strut properties, overestimated the overall mechanical properties for the scaffolds; the discrepancy arises from defects such as disconnected pore walls within the scaffold. As the scaffold stiffness and strength decreased with increasing overall mineral content and were less than that of natural, mineralized collagen scaffolds, these microstructural/mechanical relations will be used to further improve scaffold design for bone regeneration applications.

Characterization of collagen II fibrils containing biglycan and their effect as a coating on osteoblast adhesion and proliferation

Collagen has been used as a coating material for titanium-based implants for bone contact and as a component of scaffolds for bone tissue engineering. In general collagen type I has been used, however very little attention has been focussed on collagen type II. Collagen-based coatings and scaffolds have been enhanced by the incorporation of the glycosaminoglycan chondroitin sulphate (CS), however the proteglycan biglycan, which is found in bone and contains glycosaminoglycan chains consisting of CS, has not been used as a biomaterial component. The study had the following aims: firstly, five different collagen II preparations were compared with regard to their ability to bind CS and biglycan and the changes in fibril morphology thereby induced. Secondly, the effects of biglycan on the adhesion of primary rat osteoblasts (rO) as well as the proliferation of rO, primary human osteoblasts (hO) and the osteoblast-like cell line 7F2 were studied by culturing the cells on surfaces coated with collagen II fibrils containing biglycan. Fibrils of the collagen II preparation which bound the most biglycan were used to coat titanium surfaces. Bare titanium, titanium coated with collagen II fibrils and titanium coated with collagen II fibrils containing biglycan were compared. It was found that different collagen II preparations showed different affinities for CS and biglycan. In four of the five preparations tested, biglycan reduced fibril diameter, however the ability of a preparation to bind more biglycan did not appear to lead to a greater reduction in fibril diameter. Fibrils containing biglycan promoted the formation of focal adhesions by rO and significantly enhanced the proliferation of hO but not of rO or 7F2 cells. These results should encourage further investigation of biglycan as a component of collagen-based scaffolds and/or coatings.

Culture of subconfluent human fibroblasts and keratinocytes using biodegradable transfer membranes

This study aims to assess the suitability of biodegradable membranes as transfer matrix materials for the culture of subconfluent fibroblasts and keratinocytes. The materials investigated were based on collagen, chitosan and enzyme-digestible cellulose. The proliferation and growth behaviour of human keratinocytes and dermal fibroblasts were analysed and morphology and distribution determined. Cultured fibroblasts exhibited no significant differences in proliferation for the different membrane types, whereas keratinocytes revealed significantly higher proliferation on collagen membranes compared with membranes based on cellulose and chitosan. Co-cultured fibroblasts and keratinocytes from the same donor on collagen membranes showed more homogenous cell distribution, but they segregated in heterologous co-cultures; this effect must be further investigated. Thus, collagen and collagen-coated chitosan membranes are suitable for the subconfluent transfer of human fibroblasts and keratinocytes.

Electrospun gelatin fiber mats containing a herbal-Centella asiatica-extract and release characteristic of asiaticoside

Ultra-fine gelatin (type A, porcine skin, ∼180 Bloom) fiber mats containing a methanolic crude extract of Centella asiatica (L.) Urban, a medicinal plant widely known for its traditional medical applications including its wound healing ability, were fabricated, for the first time, from the neat gelatin solution (22% w/v in 70 vol% acetic acid) containing the crude extract (mCA) in various amounts (i.e. 5-30 wt% based on the weight of gelatin powder) by electrospinning. Incorporation of mCA in the neat gelatin solution did not affect both the morphology and the size of the mCA-loaded gelatin fibers, as both of the neat and the mCA-loaded gelatin fibers were smooth and the average diameters of these fibers ranged between 226 and 232 nm. The cross-linked mCA-loaded e-spun gelatin fiber mat from the neat gelatin solution containing 30 wt% of mCA was further investigated for the release characteristic of asiaticoside, identified as the most active compound associated with the healing of wounds, in two different types of releasing medium, i.e. acetate buffer and the buffer containing 10 vol% of methanol, based on the thin-layer chromatography (TLC)-densitometry technique. Based on the unit weight of the actual amount of asiaticoside present in the specimens, the total amount of asiaticoside released from the fiber mat specimens was lower than that from the film counterparts while, based on the unit weight of the specimens, an opposite trend was observed.

Water absorbing and antibacterial properties ofN-isopropyl acrylamide grafted and collagen/chitosan immobilized polypropylene nonwoven fabric and its application on wound healing enhancement

A durable sandwich wound dressing system with high liquid absorbing, biocompatibility, and antibacterial properties was designed. Various solution weight ratios of collagen to chitosan were used to immobilize on the polypropylene nonwoven fabric, which were pregrafted with acrylic acid (AA) or N-isopropyl acrylamide (NIPAAm) to construct a durable sandwich wound dressing membrane with high water absorbing, easy removal, and antibacterial activity. Swelling properties and antibacterial activity of the membranes were measured, and wound healing enhancement by skin full-thickness excision on animal model was examined. The results indicated that NIPAAm-grafted and collagen/chitosan-immobilized polypropylene nonwoven fabric (PP-NIPAAm-collagen-chitosan) showed a better healing effect than AA-grafted and collagen/chitosan-immobilized polypropylene nonwoven fabric (PP-AA-collagen-chitosan). The wound treated with PP-NIPAAm-collagen-chitosan demonstrated the excellent remodeling effect in histological examination with respect to the construction of vein, epidermis, and dermis at 21 days after skin injury. The values of water uptake and water diffusion coefficient for PP-NIPAAm-collagen-chitosan were higher than that for PP-AA-collagen-chitosan under a given solution weight ratio of collagen/chitosan. Both PP-NIPAAm-collagen-chitosan and PP-AA-collagen-chitosan demonstrated antibacterial activity.

Nanofiber technology: designing the next generation of tissue engineering scaffolds

Tissue engineering is an interdisciplinary field that has attempted to utilize a variety of processing methods with synthetic and natural polymers to fabricate scaffolds for the regeneration of tissues and organs. The study of structure-function relationships in both normal and pathological tissues has been coupled with the development of biologically active substitutes or engineered materials. The fibrillar collagens, types I, II, and III, are the most abundant natural polymers in the body and are found throughout the interstitial spaces where they function to impart overall structural integrity and strength to tissues. The collagen structures, referred to as extracellular matrix (ECM), provide the cells with the appropriate biological environment for embryologic development, organogenesis, cell growth, and wound repair. In the native tissues, the structural ECM proteins range in diameter from 50 to 500 nm. In order to create scaffolds or ECM analogues, which are truly biomimicking at this scale, one must employ nanotechnology. Recent advances in nanotechnology have led to a variety of approaches for the development of engineered ECM analogues. To date, three processing techniques (self-assembly, phase separation, and electrospinning) have evolved to allow the fabrication of nanofibrous scaffolds. With these advances, the long-awaited and much anticipated construction of a truly "biomimicking" or "ideal" tissue engineered environment, or scaffold, for a variety of tissues is now highly feasible. This review will discuss the three primary technologies (with a focus on electrospinning) available to create tissue engineering scaffolds that are capable of mimicking native tissue, as well as explore the wide array of materials investigated for use in scaffolds.