Characterization of a hyaluronic acid-Arg-Gly-Asp peptide cell attachment matrix

Self-Forming Norbornene-Tetrazine Hydrogels with Independently Tunable Properties

Although photopolymerization reactions are commonly used to form hydrogels, these strategies rely on light and may not be suitable for delivering therapeutics in a minimally invasive manner. Here, hyaluronic acid (HA) macromers are modified with norbornene (Nor) or tetrazine (Tet) and upon mixing click into covalently crosslinked Nor-Tet hydrogels via a Diels-Alder reaction. By incorporating a high degree of Nor and Tet substitution, Nor-Tet hydrogels with a broad range in elastic moduli (5 to 30 kPa) and fast gelation times (1 to 5 min) are achieved. By pre-coupling methacrylated HANor macromers with thiolated peptides via a Michael addition reaction, Nor-Tet hydrogels are peptide-functionalized without affecting their physical properties. Mesenchymal stem cells (MSCs) on RGD-functionalized Nor-Tet hydrogels adhere and exhibit stiffness-dependent differences in matrix mechanosensing. Fluid properties of Nor-Tet hydrogel solutions allow for injections through narrow syringe needles and can locally deliver viable cells and peptides. Substituting HA with enzymatically degradable gelatin also results in cell-responsive Nor-Tet hydrogels, and MSCs encapsulated in Nor-Tet hydrogels preferentially differentiate into adipocytes or osteoblasts, based on 3D cellular spreading regulated by stable (HA) and degradable (gelatin) macromers.

A Polydopamine-Coated Platinum Nanoplatform for Tumor-Targeted Photothermal Ablation and Migration Inhibition

In this work, Arg-Gly-Asp (RGD) peptide-coupled polydopamine-modified mesoporous platinum nanoparticles (mPt@PDA-RGD NPs) were developed for targeted photothermal therapy (PTT) and migration inhibition of SKOV-3 cells. mPt@PDA-RGD NPs with obvious core/shell structure demonstrated high photothermal performance under 808-nm near-infrared (NIR) laser irradiation. mPt@PDA-RGD NPs with favorable biocompatibility exhibited remarkable SKOV-3 inhibition ability under NIR laser irradiation. Moreover, compared to mPt@PDA NPs, the RGD-functionalized NPs achieved more tumor uptake and PTT performance, which was attributed to the specific interaction between RGD of NPs and α_vβ₃ integrin overexpressed by SKOV-3. Importantly, cell scratch experiments indicated that the photothermal effect of mPt@PDA-RGD NPs can effectively inhibit the migration of surviving SKOV-3 cells, which was assigned to disturbance of the actin cytoskeleton of SKOV-3. Thus, mPt@PDA-RGD NPs presented great potential for targeted tumor photothermal ablation and migration inhibition.

Water-based carbodiimide mediated synthesis of polysaccharide-amino acid conjugates: Deprotection, charge and structural analysis

We report here a one-step aqueous method for the synthesis of isolated and purified polysaccharide-amino acid conjugates. Two different types of amino acid esters: glycine methyl ester and L-tryptophan methyl ester, as model compounds for peptides, were conjugated to the polysaccharide carboxymethylcellulose (CMC) in water using carbodiimide at ambient conditions. Detailed and systematic pH-dependent charge titration and spectroscopy (infrared, nuclear magnetic resonance: ¹H, ¹³C- DEPT 135, ¹H- ¹³C HMBC/HSQC correlation), UV-vis, elemental and ninhydrin analysis provided solid and direct evidence for the successful conjugation of the amino acid esters to the CMC backbone via an amide bond. As the concentration of amino acid esters increased, a conjugation efficiency of 20-80% was achieved. Activated charcoal aided base-catalyzed deprotection of the methyl esters improved the solubility of the conjugates in water. The approach proposed in this work should have the potential to tailor the backbone of polysaccharides containing di- or tri-peptides.

High-throughput hyaluronic acid hydrogel arrays for cell selective adhesion screening

As a component of extracellular matrix (ECM), hyaluronic acid (HA) has plenty of applications in the biomedical field such as tissue engineering. Due to its non-adhesive nature, HA requires further grafting of functional molecules for cell related study. RGD and YIGSR are two kinds of cell adhesion peptides. YIGSR enhances endothelial cell (EC) adhesion, which is important for endothelialization after implantation of stents to prevent in-stent restenosis. However, the effect of combined densities of these peptides for EC and smooth muscle cell (SMC) adhesion has not been explored in a quantitative and high-throughput manner. In this work, single or orthogonal gradient densities of RGD and YIGSR were grafted onto the HA hydrogel array surfaces using thiol-norbornene click chemistry. Optimized peptide combinations for EC preponderant adhesion were found in hydrogel arrays and confirmed by scaling samples.

Polymer microcapsules and microbeads as cell carriers for in vivo biomedical applications

Polymer microcarriers are being extensively explored as cell delivery vehicles in cell-based therapies and hybrid tissue and organ engineering. Spherical microcarriers are of particular interest due to easy fabrication and injectability. They include microbeads, composed of a porous matrix, and microcapsules, where matrix core is additionally covered with a semipermeable membrane. Microcarriers provide cell containment at implantation site and protect the cells from host immunoresponse, degradation and shear stress. Immobilized cells may be genetically altered to release a specific therapeutic product directly at the target site, eliminating side effects of systemic therapies. Cell microcarriers need to fulfil a number of extremely high standards regarding their biocompatibility, cytocompatibility, immunoisolating capacity, transport, mechanical and chemical properties. To obtain cell microcarriers of specified parameters, a wide variety of polymers, both natural and synthetic, and immobilization methods can be applied. Yet so far, only a few approaches based on cell-laden microcarriers have reached clinical trials. The main issue that still impedes progress of these systems towards clinical application is limited cell survival in vivo. Herein, we review polymer biomaterials and methods used for fabrication of cell microcarriers for in vivo biomedical applications. We describe their key limitations and modifications aiming at improvement of microcarrier in vivo performance. We also present the main applications of polymer cell microcarriers in regenerative medicine, pancreatic islet and hepatocyte transplantation and in the treatment of cancer. Lastly, we outline the main challenges in cell microimmobilization for biomedical purposes, the strategies to overcome these issues and potential future improvements in this area.

Self-regenerating giant hyaluronan polymer brushes

Tailoring interfaces with polymer brushes is a commonly used strategy to create functional materials for numerous applications. Existing methods are limited in brush thickness, the ability to generate high-density brushes of biopolymers, and the potential for regeneration. Here we introduce a scheme to synthesize ultra-thick regenerating hyaluronan polymer brushes using hyaluronan synthase. The platform provides a dynamic interface with tunable brush heights that extend up to 20 microns - two orders of magnitude thicker than standard brushes. The brushes are easily sculpted into micropatterned landscapes by photo-deactivation of the enzyme. Further, they provide a continuous source of megadalton hyaluronan or they can be covalently-stabilized to the surface. Stabilized brushes exhibit superb resistance to biofilms, yet are locally digested by fibroblasts. This brush technology provides opportunities in a range of arenas including regenerating tailorable biointerfaces for implants, wound healing or lubrication as well as fundamental studies of the glycocalyx and polymer physics.

Achieving Controlled Biomolecule-Biomaterial Conjugation

The conjugation of biomolecules can impart materials with the bioactivity necessary to modulate specific cell behaviors. While the biological roles of particular polypeptide, oligonucleotide, and glycan structures have been extensively reviewed, along with the influence of attachment on material structure and function, the key role played by the conjugation strategy in determining activity is often overlooked. In this review, we focus on the chemistry of biomolecule conjugation and provide a comprehensive overview of the key strategies for achieving controlled biomaterial functionalization. No universal method exists to provide optimal attachment, and here we will discuss both the relative advantages and disadvantages of each technique. In doing so, we highlight the importance of carefully considering the impact and suitability of a particular technique during biomaterial design.

Hyaluronic acid and Arg-Gly-Asp peptide modified Graphene oxide with dual receptor-targeting function for cancer therapy

Graphene oxide (GO) modified with hyaluronic acid (HA) and Arg-gly-asp peptide (RGD) was designed as a dual-receptor targeting drug delivery system to enhance the specificity and efficiency of anticancer drug delivery. Firstly, GO-HA-RGD conjugate was characterized to reveal its structure and morphology. Whereafter, doxorubicin (Dox) as a model drug was loaded on GO-HA-RGD carrier, which displayed a high loading rate (72.9%, GO:Dox (w/w) = 1:1), pH-response and sustained drug release behavior. Cytotoxicity experiments showed that GO-HA-RGD possessed excellent biocompatibility towards SKOV-3 and HOSEpiC cells. Additionally, the GO-HA-RGD/Dox had a stronger cytotoxicity for SKOV-3 cells than either GO-HA/Dox (single receptor) or GO/Dox (no receptor). Moreover, celluar uptake studies illustrated that GO-HA-RGD conjugate could be effectively taken up by SKOV-3 cells via a synergic effect of CD44-HA and integrin-RGD mediated endocytosis. Hence, GO-HA-RGD nanocarrier is able to be a promising platform for targeted cancer therapeutic.

Bacterial cellulose and hyaluronic acid hybrid membranes: Production and characterization

In this study, the effect of the addition of hyaluronic acid (HA) on bacterial cellulose (BC) production, under static conditions was evaluated in terms of the properties of the resulting BC hybrid membranes. HA was added to the fermentation process in three distinct time points: first day (BC-T0), third day (BC-T3) and sixth day (BC-T6). Analyses of FT-IR and CP/MAS (13)C NMR confirmed the presence of HA in bacterial cellulose membranes. The crystal structure, crystallinity index (Ic) surface roughness, thermal stability and hybrophobic/hydrophilic character changed. Membranes with higher roughness were produced with HA added on the first and third day of fermentation process. The surface energy of BC/HA membranes was calculated and more hydrophilic membranes were produced by the addition of HA on the third and sixth day, also resulting in more thermally stable materials. The results demonstrate that bacterial cellulose/hyaluronic acid hybrid membranes can be produced in situ and suggest that HA interacts with the sub-elementary bacterial cellulose fibrils, changing the properties of the membranes. The study and understanding of the factors that affect those properties are of utmost importance for the safe and efficient use of BC as biomaterials in numerous applications, specifically in the biological field.

Molecular engineering of glycosaminoglycan chemistry for biomolecule delivery

Glycosaminoglycans (GAGs) are linear, negatively charged polysaccharides that interact with a variety of positively charged growth factors. In this review article the effects of engineering GAG chemistry for molecular delivery applications in regenerative medicine are presented. Three major areas of focus at the structure-function-property interface are discussed: (1) macromolecular properties of GAGs; (2) effects of chemical modifications on protein binding; (3) degradation mechanisms of GAGs. GAG-protein interactions can be based on: (1) GAG sulfation pattern; (2) GAG carbohydrate conformation; (3) GAG polyelectrolyte behavior. Chemical modifications of GAGs, which are commonly performed to engineer molecular delivery systems, affect protein binding and are highly dependent on the site of modification on the GAG molecules. The rate and mode of degradation can determine the release of molecules as well as the length of GAG fragments to which the cargo is electrostatically coupled and eventually released from the delivery system. Overall, GAG-based polymers are a versatile biomaterial platform offering novel means to engineer molecular delivery systems with a high degree of control in order to better treat a range of degenerated or injured tissues.

Incorporation of functionalized gold nanoparticles into nanofibers for enhanced attachment and differentiation of mammalian cells

Background

Electrospun nanofibers have been widely used as substrata for mammalian cell culture owing to their structural similarity to natural extracellular matrices. Structurally consistent electrospun nanofibers can be produced with synthetic polymers but require chemical modification to graft cell-adhesive molecules to make the nanofibers functional. Development of a facile method of grafting functional molecules on the nanofibers will contribute to the production of diverse cell type-specific nanofiber substrata.

Results

Small molecules, peptides, and functionalized gold nanoparticles were successfully incorporated with polymethylglutarimide (PMGI) nanofibers through electrospinning. The PMGI nanofibers functionalized by the grafted AuNPs, which were labeled with cell-adhesive peptides, enhanced HeLa cell attachment and potentiated cardiomyocyte differentiation of human pluripotent stem cells.

Conclusions

PMGI nanofibers can be functionalized simply by co-electrospinning with the grafting materials. In addition, grafting functionalized AuNPs enable high-density localization of the cell-adhesive peptides on the nanofiber. The results of the present study suggest that more cell type-specific synthetic substrata can be fabricated with molecule-doped nanofibers, in which diverse functional molecules are grafted alone or in combination with other molecules at different concentrations.

Difficulties in the translation of functionalized biomaterials into regenerative medicine clinical products

There are many ways to influence cell activities, and biomaterials with functional groups attached is an attractive method that clearly has the ability to modulate cell behavior. The evidence is clear that biomaterials, with or without growth factors and cells, have resulted in numerous products for the regenerative medicine field. In contrast the functionalized biomaterial products remain in the development phase.

Characterization of esterified hyaluronan-gelatin polymer composites suitable for chondrogenic differentiation of mesenchymal stem cells

Composite scaffolds of homogeneously mixed esterified hyaluronan (HY) and gelatin (G) were manufactured with variable component compositions (HY100%; HY95%/G5%; HY70%/G30%). The goals of this study were to analyze the produced composite scaffolds using physical and chemical methods, for example, scanning electron microscopy, IR-spectroscopy, water contact angle, protein assay, and tensile testing as well as to assess the effects of adding gelatin to the composite scaffolds on attachment, proliferation, and chondrogenic differentiation of human mesenchymal stem cells. Numbers of attached cells were significantly higher on the composite material compared to pure hyaluronan at different time points of two-dimensional or three-dimensional cell culture (p< 0.02). In composite scaffolds, a significantly greater amount of cartilage-specific extracellular matrix components was deposited after 28 days in culture (glycosaminoglycan: p < 0.001; collagen: p < 0.001) as compared with 100% hyaluronan scaffolds. Additionally, gelatin-containing composite scaffolds displayed stronger promotion of collagen type II expression than pure hyaluronan scaffolds. The mechanism, based on which gelatin influences cell adhesion, was examined. The effect was inhibited by collagenase treatment of the composites or by addition of alpha5beta1-integrin blocking antibodies to the cell suspension. In summary, the results describe the establishment of a class of composite polymer scaffolds, consisting of esterified hyaluronan and gelatin, which are potentially useful for cell-based tissue engineering approaches using mesenchymal stem cells for chondrogenic differentiation.

Role of electrospun fibre diameter and corresponding specific surface area (SSA) on cell attachment

In order to develop scaffolds for tissue regeneration applications, it is important to develop an understanding of the kinetics of cell attachment as a function of scaffold geometry. In the present study, we investigated how the specific surface area of electrospun scaffolds affected cell attachment and spreading. Number of cells attached to the scaffold was measured by the relative intensity of a metabolic dye (MTS) and cell spreading was analysed for individual cells by measuring the area of projected F-actin cytoskeleton. We varied the fibre diameter to obtain a specific surface area distribution in the range 2.24-18.79 microm(-1). In addition, we had one case where the scaffolds had beads in them and therefore had non-uniform fibres. For each of these different geometries, we varied the cell-seeding density (0.5-1 x 10(5)) and the serum concentration (0-12%) over the first 8 h in an electrospun polycaprolactone NIH 3T3 fibroblast system. Cells on beaded scaffolds showed the lowest attachment and almost no F-actin spreading in all experiments indicating uniform fibre diameter is essential for electrospun scaffolds. For the uniform fibre scaffolds, cell attachment was a function of scaffold specific surface area (SSA) (18.79-2.24 microm(-1)) and followed two distinct trends: when scaffold SSA was < 7.13 microm(-1), cell adhesion rate remained largely unchanged; however, for SSA > 7.13 microm(-1) there was a significant increase in cellular attachment rate with increasing SSA. This indicated that nanofibrous scaffolds increased cellular adhesion compared to microfibrous scaffolds. This phenomenon is true for serum concentrations of 7.5% and higher. For 5% and lower serum concentration, cell attachment is low and higher SSA fails to make a significant improvement in cell attachment. When cell attachment was investigated at a single-cell level by measuring the projected actin area, a similar trend was noted where the effect of higher SSA led to higher projected area for cells at 8 h. These results indicate that uniform electrospun scaffolds with SSA provide a faster cell attachment compared to lower SSA and beaded scaffolds. These results indicate that continuous electrospun nanofibrous scaffolds may be a good substrate for rapid tissue regeneration.

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.

Beyond molecular recognition: using a repulsive field to tune interfacial valency and binding specificity between adhesive surfaces

Surface-bound biomolecular fragments enable "smart" materials to recognize cells and other particles in applications ranging from tissue engineering and medical diagnostics to colloidal and nanoparticle assembly. Such smart surfaces are, however, limited in their design to biomolecular selectivity. This feature article demonstrates, using a completely nonbiological model system, how specificity can be achieved for particle (and cell) binding, employing surface designs where immobilized nanoscale adhesion elements are entirely nonselective. Fundamental principles are illustrated by a model experimental system where 11 nm cationic nanoparticles on a planar negative silica surface interact with flowing negative silica microspheres having 1.0 and 0.5 microm diameters. In these systems, the interfacial valency, defined as the number of cross-bonds needed to capture flowing particles, is tunable through ionic strength, which alters the range of the background repulsion and therefore the effective binding strength of the adhesive elements themselves. At high ionic strengths where long-range electrostatic repulsions are screened, single surface-bound nanoparticles capture microspheres, defining the univalent regime. At low ionic strengths, competing repulsions weaken the effective nanoparticle adhesion so that multiple nanoparticles are needed for microparticle capture. This article discusses important features of the univalent regime and then illustrates how multivalency produces interfacial-scale selectivity. The arguments are then generalized, providing a possible explanation for highly specific cell binding in nature, despite the degeneracy of adhesion molecules and cell types. The mechanism for the valency-related selectivity is further developed in the context of selective flocculation in the colloidal literature. Finally, results for multivalent binding are contrasted with the current thinking for interfacial design and the presentation of adhesion moieties on engineered surfaces.

Biomimetic hemocompatible coatings through immobilization of hyaluronan derivatives on metal surfaces

Biomimetic coatings offer exciting options to modulate the biocompatibility of biomaterials. The challenge is to create surfaces that undergo specific interactions with cells without promoting nonspecific fouling. This work reports an innovative approach toward biomimetic surfaces based on the covalent immobilization of a carboxylate terminated PEGylated hyaluronan (HA-PEG) onto plasma functionalized NiTi alloy surfaces. The metal substrates were aminated via two different plasma functionalization processes. Hyaluronan, a natural glycosaminoglycan and the major constituent of the extracellular matrix, was grafted to the substrates by reaction of the surface amines with the carboxylic acid terminated PEG spacer using carbodiimide chemistry. The surface modification was monitored at each step by X-ray photoelectron spectroscopy (XPS). HA-immobilized surfaces displayed increased hydrophilicity and reduced fouling, compared to bare surfaces, when exposed to human platelets (PLT) in an in vitro assay with radiolabeled platelets (204.1 +/- 123.8 x 10 (3) PLT/cm (2) vs 538.5 +/- 100.5 x 10 (3) PLT/cm (2) for bare metal, p < 0.05). Using a robust plasma patterning technique, microstructured hyaluronan surfaces were successfully created as demonstrated by XPS chemical imaging. The bioactive surfaces described present unique features, which result from the synergy between the intrinsic biological properties of hyaluronan and the chemical composition and morphology of the polymer layer immobilized on a metal surface.

Photoencapsulation of bone morphogenetic protein-2 and periosteal progenitor cells improve tendon graft healing in a bone tunnel

BACKGROUND

Tissue-engineered solutions for promoting the tendon graft incorporation within the bone tunnel appear to be promising.

HYPOTHESIS

To determine the feasibility that conjugation of hyaluronic acid-tethered bone morphogenetic protein-2 can be used to stimulate periosteal progenitor cells direct fibrocartilagenous attachment and new bone formation in an extra-articular tendon-bone healing model.

STUDY DESIGN

Controlled laboratory study.

METHODS

A total of 42 mature New Zealand White rabbits were used. The long digitorum extensor tendon was transplanted into a bone tunnel of the proximal tibia. The tendon was pulled through a drill hole in the proximal tibia and attached to the medial aspect of the tibia. Photopolymerizable hydrogel based on poly (ethylene glycol) diacrylate with hyaluronic acid-tethered bone morphogenetic protein-2 was injected and photogelated in a bone tunnel. Histological and biomechanical examination of the tendon-bone interface was evaluated at postoperative weeks 3 and 6.

RESULTS

Histological analysis showed an interface fibrocartilage and new bone formed by photoencapsulation of bone morphogenetic protein-2 and periosteal progenitor cells at 6 weeks. Biomechanical testing revealed higher maximum pullout strength and stiffness in experimental groups with a statistically significant difference at 3 and 6 weeks after tendon transplantation.

CONCLUSION

The healing tendon-bone interface undergoes a gradual remodeling process; it appears that photoencapsulation of bone morphogenetic protein-2 and periosteal progenitor cells possesses a powerful inductive ability between the tendon and the bone to incorporate the healing in a rabbit model.

CLINICAL RELEVANCE

Novel technologies, such as those described in this study, including photopolymerization and tissue engineering, may provide minimally invasive therapeutic procedures via arthroscopy to enhance biological healing after reconstruction of the anterior cruciate ligament.

Self-assembly and applications of biomimetic and bioactive peptide-amphiphiles

Peptide-amphiphiles are amphiphilic structures with a hydrophilic peptide headgroup that incorporates a bioactive sequence and has the potential to form distinct structures, and a hydrophobic tail that serves to align the headgroup, drive self-assembly, and induce secondary and tertiary conformations. In this paper we review the different self-assembled structures of peptide-amphiphiles that range from micelles and nanofibers, to patterned membranes. We also describe several examples where peptide-amphiphiles have found applications as soft bioactive materials for model studies of bioadhesion and characterization of different cellular phenomena, as well as scaffolds for tissue engineering, regenerative medicine, and targeted drug delivery.

Biomimicking Extracellular Matrix: Cell Adhesive RGD Peptide Modified Electrospun Poly(D,L-lactic-co-glycolic acid) Nanofiber Mesh

A cell adhesive peptide, Arg-Gly-Asp (RGD), was immobilized onto the surface of electrospun poly(D,L-lactic-co-glycolic acid) PLGA nanofiber mesh in an attempt to mimic an extracellular matrix structure. A blend mixture of PLGA and PLGA-b-PEG-NH(2) di-block copolymer dissolved in a 1:1 volume mixture of dimethylformamide and tetrahydrofuran was electrospun to produce a nanofiber mesh with functional primary amino groups on the surface. Various electrospinning parameters, such as polymer concentration and the blend ratio, were optimized to produce a nanofiber mesh with desirable morphology, surface characteristics, and fiber diameter. A cell adhesive peptide, GRGDY, was covalently grafted onto the aminated surface of the electrospun mesh under a hydrating condition. The amounts of surface primary amino groups and grafted RGD peptides were quantitatively determined. Cell attachment, spreading, and proliferation were greatly enhanced in the RGD modified electrospun PLGA nanofiber mesh compared with that of the unmodified one.

Evaluation of modifying collagen matrix with RGD peptide through periodate oxidation

The aim of the study is to evaluate the effect of modifying collagen matrices with Arg-Gly-Asp (RGD) peptide through periodate oxidation. The collagen matrices were modified with RGD peptide, by periodate activation. The modified collagen matrices and unmodified matrices were characterized by scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and electron spectroscopy for chemical analysis (ESCA). Mesenchymal stem cells (MSCs) were used to evaluate the cell compatibility of collagen matrices. In terms of cell growth, the MSCs attached much better on the modified matrix than on the unmodified one. But there was no significant difference between two groups regarding the MSC proliferation. Compared to the unmodified matrices, the mechanical strength of the modified matrix decreased sharply, and its 3D structure was destroyed. Introducing specific RGD receptor-mediated adhesion sites on matrices obviously enhanced the MSC adhesion on collagen matrices, but the coupled method of periodate oxidation would likely result in the declination of the mechanical strength of the matrix, as well as the destruction of the matrix structure. This would affect the cell growth on the matrix, and decrease the histocompatibility of the matrices.

Crosslinked hyaluronic acid hydrogels: a strategy to functionalize and pattern

The physiological activity of hyaluronic acid (HA) polymers and oligomers makes it a promising material for a variety of applications. The development of HA-hydrogel scaffolds with improved mechanical stability against degradation and biochemical functionality may enhance their application to tissue engineering. In this report, a crosslinking strategy targeting the alcohol groups via a poly(ethylene glycol) diepoxide crosslinker was investigated for the generation of degradable HA hydrogels. To provide support for cell adhesion in vitro, collagen was incorporated into the HA solution prior to the crosslinking process. The hydrogels have a continuous exterior and a porous interior, with pore diameters ranging from 6 to 9 microm. HA and HA-collagen hydrogels degrade in the presence of hyaluronidase and collagenase enzymes, indicating that the chemical modification does not prevent biodegradation. Complete degradation of the hydrogels occurred within 14 days in hyaluronidase (100 U/ml) and 3 days in collagenase (66 U/ml). Pattern transfer was employed to introduce a surface topography onto the hydrogel, which was able to orient cell growth. Furthermore, the hydrogels could be functionalized with the biomolecule neutravidin by incorporation of biotin along the HA backbone. This biotinylation approach may allow attachment of bioactive molecules that are conjugated to avidin.

Evaluation of Chitosan-alginate-hyaluronate Complexes Modified by an RGD-containing Protein as Tissue-engineering Scaffolds for Cartilage Regeneration

In this study, a series of natural biodegradable materials in the form of chitosan (C)-alginate (A)-hyaluronate (H) complexes are evaluated as tissue-engineering scaffolds. The weight ratio of C/A is 1 : 1 or 1 : 2. Sodium hyaluronate is mixed in 2%. The complexes can be cast into films or fabricated as scaffolds. Their surface can be further modified by an Arg-Gly-Asp (RGD)-containing protein, a cellulose-binding domain-RGD (R). Cytocompatibility tests of the films are conducted using immortalized rat chondrocyte (IRC) as well as primary articular chondrocytes harvested from rabbits. The neocartilage formation in cell-seeded scaffolds is examined in vitro as well as in rabbits, where the scaffolds are implanted into the defect-containing joints. The results from cytocompatibility tests demonstrate that R enhances cell attachment and proliferation on C-A and C-A-H complex films. Complex C1A1HR (C : A = 1 : 1 with H and R) has better performance than the other formulation. Cells retain their spherical morphology on all C-A and C-A-H complexes. The in vitro evaluation of the seeded scaffolds indicates that the C1A1HR complex is the most appropriate for 3-D culture, manifested by the better cell growth as well as higher glycosaminoglycan and collagen contents. When the chondrocyte scaffolds are implanted into rabbit knee cartilage defects, partial repair is observed after 1 month in C1A1HR as well as in C1A1 (C : A = 1 : 1 without H and R) scaffolds. The defects are completely repaired in 6 months when C1A1HR constructs are implanted. It is concluded that C1A1HR is a potential tissue-engineering scaffold for cartilage regeneration.

Contact profilometry and correspondence analysis to correlate surface properties and cell adhesion in vitro of uncoated and coated Ti and Ti6Al4V disks

A fundamental goal in the field of implantology is the design of specific devices able to induce a controlled and rapid "osseointegration". This result has been achieved by means of surface modifications aimed at optimizing implant-to-bone contact; furthermore, bone cell adhesion on implant surface has been directly improved by the application of biomolecules that stimulate new tissue formation, thus controlling interactions between biological environment and implanted materials. Actually, methods for biochemical factor delivery at the interface between implant surface and biological tissues are under investigation; a reliable technique is represented by the inclusion of biologically active molecules into biocompatible and biodegradable materials used for coating implant surface. This paper focuses the application of three polymeric materials already acknowledged in the clinical practice, i.e. poly-L-lactic acid (PLLA), poly-DL-lactic acid (PDLA), and sodium alginate hydrogel. They have been used to coat Ti (Ti2) and Ti6Al4V (Ti5) disks; their characteristics have been determined and their performances compared, with specific regard to the ability in allowing osteoblast adhesion in vitro. Moreover, profilometry data analysis permitted to identify a specific roughness parameter (peak density) which mainly controls the amount of osteoblast adhesion.

Biomimetic Peptide−Amphiphiles for Functional Biomaterials: The Role of GRGDSP and PHSRN

The study we present involves the use of a biomimetic system that allows us to study specific interactions in the alpha(5)beta(1) receptor-GRGDSP ligand system with an atomic force microscope (AFM). Bioartificial membranes that mimic the adhesion domain of the extracellular matrix protein fibronectin are constructed from peptide-amphiphiles. A novel peptide-amphiphile is designed that contains both GRGDSP (Gly-Arg-Gly-Asp-Ser-Pro, the primary recognition site for alpha(5)beta(1)) and PHSRN (Pro-His-Ser-Arg-Asn, the synergy binding site for alpha(5)beta(1)) sequences in a single peptide formulation, separated by a spacer. Two different antibodies are used to immobilize and activate isolated alpha(5)beta(1) integrins on the AFM tip. The interaction measured between immobilized alpha(5)beta(1) integrins and peptide-amphiphiles is specific for integrin-peptide binding and is affected by divalent cations in a way that accurately mimics the adhesion function of the alpha(5)beta(1) receptor. The strength of the PHSRN synergistic effect depends on the accessibility of this sequence to alpha(5)beta(1) integrins. An increase in adhesion is observed compared to surfaces displaying only GRGDSP peptides when the new biomimetic peptide-amphiphiles are diluted with lipidated poly(ethylene glycol), which provides more space for the peptide headgroups to bend and expose more of the PHSRN at the interface.

Disulfide-crosslinked hyaluronan-gelatin sponge: growth of fibrous tissue in vivo

The modification of hyaluronan (HA) and gelatin using dithiobis(propanoic dihydrazide) (DTP) has provided two thiolated macromolecular components of the extracellular matrix (ECM), specifically HA-DTPH and gelatin-DTPH. Blends of these thiolated ECM components were crosslinked in air to form hydrogels that were interpenetrating disulfide-crosslinked networks. Lyophilization of the hydrogels afforded sponge-like macroporous scaffolds suitable for cell attachment and proliferation. Increasing percentages of gelatin-DTPH (0, 25, 50, and 75%) were blended with HA-DTPH, and the resulting sponges were evaluated in vitro and in vivo as scaffolds for tissue engineering by seeding with human tracheal scar (HTS) fibroblasts. While cells failed to attach and grow in HA-only sponges, the gelatin-modified HA sponges promoted cell adhesion, proliferation, and spreading in vitro. Optimal attachment and growth was observed with 50% gelatin-HA sponges. Cell attachment to the gelatin-HA sponge could be blocked by preincubation of cells with a soluble fibronectin peptide Gly-Arg-Gly-Asp (GRGD). Finally, HTS fibroblast-seeded gelatin-HA sponges were implanted into the flanks of nude mice and evaluated at 2 and 8 weeks postimplantation. The sponges were fully biocompatible and new fibrous tissue formed, gradually replacing the sponge-like scaffold. The gelatin-HA sponges act as synthetic, macroporous, covalent mimics of the ECM and constitute novel scaffolds for cell growth and tissue augmentation.

Development of photocrosslinkable hyaluronic acid-polyethylene glycol-peptide composite hydrogels for soft tissue engineering

Hyaluronic acid (HA; also called hyaluronan) is a naturally derived, nonimmunogenic, nonadhesive glycosaminoglycan that has important roles in several wound-healing processes. In previous work, we created photocrosslinkable glycidyl methacrylate-HA (GMHA) hydrogel biomaterials that were cytocompatible, biologically active, and had a decreased rate of hyaluronidase degradation compared with native HA. The goal of the studies presented herein was to explore peptide conjugation techniques to further adjust the material and biological properties of the GMHA hydrogels. We conjugated GMHA with acrylated forms of polyethylene glycol (PEG) and PEG-peptides to yield GMHA-PEG-peptide composite hydrogels. By varying the reactant concentrations, we created stable hydrogels with high peptide conjugation efficiencies (up to 80%), controllable peptide concentrations (in the range of 1-6 micromol peptide per milliliter of hydrogel), and defined physicochemical properties (e.g., swelling ratio, enzymatic degradation rate). These composite hydrogels may prove to be a promising scaffolding biomaterial for a variety of soft tissue engineering applications.

RGD modified polymers: biomaterials for stimulated cell adhesion and beyond

Since RGD peptides (R: arginine; G: glycine; D: aspartic acid) have been found to promote cell adhesion in 1984 (Cell attachment activity of fibronectin can be duplicated by small synthetic fragments of the molecule, Nature 309 (1984) 30), numerous materials have been RGD functionalized for academic studies or medical applications. This review gives an overview of RGD modified polymers, that have been used for cell adhesion, and provides information about technical aspects of RGD immobilization on polymers. The impacts of RGD peptide surface density, spatial arrangement as well as integrin affinity and selectivity on cell responses like adhesion and migration are discussed.

Molecular signaling in bioengineered tissue microenvironments

Biological tissues and organs consist of specialized living cells arrayed within a complex structural and functional framework known generally as the extracellular matrix (ECM). The great diversity observed in the morphology and composition of the ECM contributes enormously to the properties and function of each organ and tissue. For example, the ECM contributes to the rigidity and tensile strength of bone, the resilience of cartilage, the flexibility and hydrostatic strength of blood vessels, and the elasticity of skin. The ECM is also important during growth, development, and wound repair: its own dynamic composition acts as a reservoir for soluble signaling molecules and mediates signals from other sources to migrating, proliferating, and differentiating cells. Artificial three-dimensional substitutes for ECM, called tissue scaffolds, may consist of natural or synthetic polymers or a combination of both. Scaffolds have been used successfully alone and in combination with cells and soluble factors to induce tissue formation or promote tissue repair. Appropriate numbers of properly functioning living cells are central to many tissue-engineering strategies, and significant efforts have been made to identify and propagate pluripotent stem cells and lineage-restricted progenitor cells. The study of these and other living cells in artificial microenvironments, in turn, has led to the identification of signaling events important for their controlled proliferation, proper differentiation, and optimal function.

Adhesion of α5β1 receptors to biomimetic substrates constructed from peptide amphiphiles

Biomimetic membrane surfaces functionalized with fragments of the extracellular matrix protein, fibronectin, are constructed from mixtures of peptide and polyethylene glycol (PEG) amphiphiles. Peptides from the primary binding loop, GRGDSP, were used in conjunction with the synergy site peptide, PHSRN, in the III(9-10) sites of human fibronectin. These peptides were attached to dialkyl lipid tails to form peptide amphiphiles. PEG amphiphiles were mixed in the layer to minimize non-specific adhesion in the background. GRGDSP and PEG amphiphiles or GRGDSP, PHSRN, and PEG amphiphiles were mixed in various ratios and deposited on solid substrates from the air-water interface using Langmuir-Blodgett techniques. In this method, peptide composition, density, and presentation could be controlled accurately. The effectiveness of these substrates to mimic native fibronectin is evaluated by their ability to generate adhesive forces when they are in contact with purified activated alpha5beta1 integrin receptors that are immobilized on an opposing surface. Adhesion is measured using a contact mechanical approach (JKR experiment). The effects of membrane composition, density, temperature, and peptide conformation on adhesion to activated integrins in this simulated cell adhesion setup were determined. Addition of the synergy site, PHSRN, was found to increase adhesion of alpha5beta1, to biomimetic substrates markedly. Increased peptide mobility (due to increased experimental temperature) increased integrin adhesion markedly at low peptide concentrations. A balance between peptide density and steric accessibility of the receptor binding face to alpha5beta1 integrin was required for highest adhesion.

Peptoid-containing collagen mimetics with cell binding activity

Collagen mimetic peptides containing the peptoid residue Nleu (Goodman Bhumralkar, Jefferson, Kwak, Locardi. Biopolymers 1998;47:127-142) were tested for interactions with epithelial cells and fibroblasts. Molecules containing the sequence Gly-Pro-Nleu with a minimum of nine repeats showed cell binding activity. The activity of these molecules appeared to be conformationally sensitive, with the triple-helical form being preferred. When immobilized on a surface, the (Gly-Pro-Nleu)(10)-Gly-Pro-NH(2) sequence stimulated the attachment and growth of corneal epithelial cells and fibroblasts and the migration of epithelial tissue. The peptide sequence KDGEA inhibited cell attachment to the (Gly-Pro-Nleu)(10)-Gly-Pro-NH(2) sequence, suggesting that cell binding to this collagen mimetic involves the alpha2beta1 heterodimer integrin receptor. Interestingly, peptides containing the sequence (GlyNleu-Pro-)(10)-NH(2) did not have cell binding activity. The discovery that triple-helical peptides containing the Gly-Pro-Nleu sequences interact with cells opens up new opportunities in the design of collagen mimetic biomaterials.

Surface microarchitectural design in biomedical applications: in vivo analysis of tissue ingrowth in excimer laser-directed micropored scaffold for cardiovascular tissue engineering

A micropatterned microporous segmented polyurethane film (20 x 12 mm in size, 30 micrometer thick) with four regions was prepared by excimer laser microprocessing to provide an in vivo model of transmural tissue ingrowth in an open cell-structured scaffold specially designed for cardiovascular tissue engineering. Three microporous regions had the same circular micropores (30 micrometer diameter) but different pore density arrangements (percentage of total pore area against unit area was 0.3%, 1.1%, and 4.5%), and the other region remained nonporous. The covered stent, prepared by wrapping the regionally different density-microporous film on an expandable metallic stent (approximately 3.1 mm in diameter), was delivered to the luminal surface of canine common carotid arteries and placed after expansion of the stent to a diameter of approximately 8 mm using a balloon catheter. At 4 weeks of implantation, all the covered stents (n = 10) were patent. The luminal surfaces of the covered stents were almost confluently endothelialized both in nonporous and microporous regions. Histological examination showed that the neointimal wall was formed by tissue ingrowth from host through micropores (transmural) and anastomotic sites. Thrombus formation occurred frequently in the lowest density porous region and nonporous region. With an increase in pore density, the thickness of the neointimal wall decreased. This study demonstrated how the micropore density of implanted devices influences tissue ingrowth in arterial implantation.

Platelet derived growth factor releasing chitosan sponge for periodontal bone regeneration

With an aim of improving bone regeneration, chitosan sponge containing platelet-derived growth factor-BB (PDGF-BB) were developed. For fabrication of chitosan sponge, chitosan solution was freeze-dried, crosslinked and freeze-dried again. PDGF-BB was incorporated into the chitosan sponge by soaking chitosan sponge into the PDGF-BB solution. Release kinetics of PDGF-BB, cell attachment, proliferation capacity and bony regenerative potentials of PDGF-BB-loaded chitosan sponge were investigated. Prepared chitosan sponge retained porous structure with 100 microm pore diameter that was suitable for cellular migration and growth. Release rate of PDGF-BB could be controlled by varying initial loading content of PDGF-BB to obtain optimal therapeutic efficacy. PDGF-BB-loaded chitosan sponge induced significantly high cell attachment and proliferation level, which indicated good cellular adaptability. PDGF-BB-loaded chitosan sponge demonstrated marked increase in new bone formation and rapid calcification. Degradation of the chitosan sponge was proceeded at defect site and subsequently replaced with new bone. Histomorphometric analysis confirmed that PDGF-BB-loaded chitosan sponge significantly induced new bone formation. These results suggested that chitosan sponge and PDGF-BB-loaded chitosan sponge may be beneficial to enhance periodontal bone regeneration.

Enhancement of cell interactions with collagen/glycosaminoglycan matrices by RGD derivatization

The interaction of three cell types important to the wound repair process with collagen/glycosaminoglycan (GAG) dermal regeneration matrices covalently modified with an Arg-Gly-Asp (RGD)-containing peptide was characterized. Function-blocking monoclonal antibodies directed against various integrin subunits were used to demonstrate that human fibroblasts attached to the unmodified matrix through the integrin, alpha2beta1. Human endothelial cells and human keratinocytes, however, attached minimally to the unmodified matrix. After modification of the collagen/GAG matrix with RGD-containing peptide, endothelial cells and keratinocytes attached and spread well on the matrix. This attachment was RGD dependent as evidenced by essentially complete inhibition with competing soluble peptide. In terms of overall cell number, fibroblast cell attachment remained unchanged on the RGD peptide-modified matrix compared to the unmodified material. Antibody and peptide inhibition studies demonstrate, however, that attachment to the modified matrix was mediated by both alpha2beta1 and RGD-binding integrins. We have successfully introduced a specific RGD receptor-mediated attachment site on collagen/GAG dermal regeneration matrices, resulting in enhanced cell interaction of important wound healing cell types. This modification could have important implications for the performance of these matrices in promoting dermal regeneration.