Attachment, proliferation, and migration of marrow stromal osteoblasts cultured on biomimetic hydrogels modified with an osteopontin-derived peptide

Sustained delivery of dibutyryl cyclic adenosine monophosphate to the transected spinal cord via oligo [(polyethylene glycol) fumarate] hydrogels

This study describes the use of oligo [(polyethylene glycol) fumarate] (OPF) hydrogel scaffolds as vehicles for sustained delivery of dibutyryl cyclic adenosine monophosphate (dbcAMP) to the transected spinal cord. dbcAMP was encapsulated in poly(lactic-co-glycolic acid) (PLGA) microspheres, which were embedded within the scaffolds architecture. Functionality of the released dbcAMP was assessed using neurite outgrowth assays in PC12 cells and by delivery to the transected spinal cord within OPF seven channel scaffolds, which had been loaded with Schwann cells or mesenchymal stem cells (MSCs). Our results showed that encapsulation of dbcAMP in microspheres lead to prolonged release and continued functionality in vitro. These microspheres were then successfully incorporated into OPF scaffolds and implanted in the transected thoracic spinal cord. Sustained delivery of dbcAMP inhibited axonal regeneration in the presence of Schwann cells but rescued MSC-induced inhibition of axonal regeneration. dbcAMP was also shown to reduce capillary formation in the presence of MSCs, which was coupled with significant functional improvements. Our findings demonstrate the feasibility of incorporating PLGA microsphere technology for spinal cord transection studies. It represents a novel sustained delivery mechanism within the transected spinal cord and provides a platform for potential delivery of other therapeutic agents.

Extracellular matrix-mimetic adhesive biomaterials for bone repair

Limited osseointegration of current orthopedic biomaterials contributes to the failure of implants such as arthroplasties, bone screws, and bone grafts, which present a large socioeconomic cost within the United States. These implant failures underscore the need for biomimetic approaches that modulate host cell-implant material responses to enhance implant osseointegration and bone formation. Bioinspired strategies have included functionalizing implants with extracellular matrix (ECM) proteins or ECM-derived peptides or protein fragments, which engage integrins and direct osteoblast adhesion and differentiation. This review discusses (1) bone ECM composition and key integrins implicated in osteogenic differentiation, (2) the use of implants functionalized with ECM-mimetic peptides/protein fragments, and (3) growth factor-derived peptides to promote the mechanical fixation of implants to bone and to enhance bone healing within large defects.

Biomimeticity in tissue engineering scaffolds through synthetic peptide modifications-altering chemistry for enhanced biological response

Biomimetic and bioactive biomaterials are desirable as tissue engineering scaffolds by virtue of their capability to mimic natural environments of the extracellular matrix. Biomimeticity has been achieved by the incorporation of synthetic short peptide sequences into suitable materials either by surface modification or by bulk incorporation. Research in this area has identified several novel synthetic peptide segments, some of them with cell-specific interactions, which may serve as potential candidates for use in explicit tissue applications. This review focuses on the developments and prospective directions of incorporating short synthetic peptide sequences onto scaffolds for tissue engineering, with emphasis on the chemistry of peptide immobilization and subsequent cell responses toward modified scaffolds. The article provides a decision-tree-type flow chart indicating the most probable cellular events on a given peptide-modified scaffold along with the consolidated list of synthetic peptide sequences, supports as well as cell types used in various tissue engineering studies, and aims to serve as a quick reference guide to peptide chemists and material scientists interested in the field.

Differentiation of stem cells: strategies for modifying surface biomaterials

Stem cells are a natural choice for cellular therapy because of their potential to differentiate into a variety of lineages, their capacity for self-renewal in the repair of damaged organs and tissues in vivo, and their ability to generate tissue constructs in vitro. Determining how to efficiently drive stem cell differentiation to a lineage of choice is critical for the success of cellular therapeutics. Many factors are involved in this process, the extracellular microenvironment playing a significant role in controlling cellular behavior. In recent years, researchers have focused on identifying a variety of biomaterials to provide a microenvironment that is conducive to stem cell growth and differentiation and that ultimately mimics the in vivo situation. Appropriate biomaterials support the cellular attachment, proliferation, and lineage-specific differentiation of stem cells. Tissue engineering approaches have been used to incorporate growth factors and morphogenetic factors-factors known to induce lineage commitment of stem cells-into cultures with scaffolding materials, including synthetic and naturally derived biomaterials. This review focuses on various strategies that have been used in stem cell expansion and examines modifications of natural and synthetic materials, as well as various culture conditions, for the maintenance and lineage-specific differentiation of embryonic and adult stem cells.

Mining the extracellular matrix for tissue engineering applications

Tissue engineering is a rapidly evolving interdisciplinary field that aims to regenerate new tissue to replace damaged tissues or organs. The extracellular matrix (ECM) of animal tissues is a complex mixture of macromolecules that play an essential instructional role in the development of tissues and organs. Therefore, tissue engineering approaches rely on the need to present the correct cues to cells, to guide them to maintain tissue-specific functions. Recent research efforts have allowed us to mine various sequences and motifs, which play key roles in these guidance functions, from the ECM. Small conserved peptide sequences mined from ECM molecules can mimic some of the biological functions of their large parent molecules. In addition, these peptide sequences can be linked to various biomaterial scaffolds that can provide the cells with mechanical support to ensure appropriate cell growth and aid the formation of the correct tissue structure. The tissue engineering field will continue to benefit from the advent of these mined ECM sequences which have two major advantages over recombinant ECM molecules: material consistency and scalability.

Phosphorylation-dependent mineral-type specificity for apatite-binding peptide sequences

Apatite-binding peptides discovered by phage display provide an alternative design method for creating functional biomaterials for bone and tooth tissue repair. A limitation of this approach is the absence of display peptide phosphorylation--a post-translational modification important to mineral-binding proteins. To refine the material specificity of a recently identified apatite-binding peptide, and to determine critical design parameters (net charge, charge distribution, amino acid sequence and composition) controlling peptide affinity for mineral, we investigated the effects of phosphorylation and sequence scrambling on peptide adsorption to four different apatites (bone-like mineral, and three types of apatite containing initially 0, 5.6 and 10.5% carbonate). Phosphorylation of the VTKHLNQISQSY peptide (VTK peptide) led to a 10-fold increase in peptide adsorption (compared to nonphosphorylated peptide) to bone-like mineral, and a 2-fold increase in adsorption to the carbonated apatite, but there was no effect of phosphorylation on peptide affinity to pure hydroxyapatite (without carbonate). Sequence scrambling of the nonphosphorylated VTK peptide enhanced its specificity for the bone-like mineral, but scrambled phosphorylated VTK peptide (pVTK) did not significantly alter mineral-binding suggesting that despite the importance of sequence order and/or charge distribution to mineral-binding, the enhanced binding after phosphorylation exceeds any further enhancement by altered sequence order. Osteoblast culture mineralization was dose-dependently inhibited by pVTK and to a significantly lesser extent by scrambled pVTK, while the nonphosphorylated and scrambled forms had no effect, indicating that inhibition of osteoblast mineralization is dependent on both peptide sequence and charge. Computational modeling of peptide-mineral interactions indicated a favorable change in binding energy upon phosphorylation that was unaffected by scrambling. In conclusion, phosphorylation of serine residues increases peptide specificity for bone-like mineral, whose adsorption is determined primarily by sequence composition and net charge as opposed to sequence order. However, sequence order in addition to net charge modulates the mineralization of osteoblast cultures. The ability of such peptides to inhibit mineralization has potential utility in the management of pathologic calcification.

Effect of functionalized polycaprolactone on the behaviour of murine preosteoblasts

The efficiency of biomaterials used in bone repair depends greatly on their ability to interact with bone cells. Hence, we have functionalized polycaprolactone (PCL) films by peptides derived from the bone sialoprotein containing RGD sequence (pRGD), to increase their ability to interact with murine MC3T3-E1 preosteoblasts, and favour cell response to recombinant human bone morphogenetic protein-2 (rhBMP-2). RGE peptides (pRGE) were used as negative controls. The PCL films were hydrolyzed with NaOH and then carboxylic acid groups were activated to allow chemisorption of the peptides. Alkaline treatment increased the hydrophilicity of PCL films without significantly change their roughness. Peptide immobilization on PCL was checked by X-ray photoelectron spectroscopy. Hydrolyzed PCL films (Hydro PCL), which adsorbed fibronectin and vitronectin from serum after 1 h incubation, prevented the spreading of MC3T3-E1 preosteoblasts, while films bearing pRGD or pRGE did not. In contrast, MC3T3-E1 preosteoblasts attached to pRGD and incubated for 1 h in serum-free medium spread better than cells on Hydro PCL or pRGE. Only cells on pRGD had organized cytoskeleton, phosphorylated focal adhesion kinase on Y(397) and responded to rhBMP-2 by activating Smad pathway. Thus, pRGD PCL may be used to favour bone cell cytoskeletal organization and response to rhBMP-2.

Enabling stem cell therapies through synthetic stem cell-niche engineering

Enabling stem cell-targeted therapies requires an understanding of how to create local microenvironments (niches) that stimulate endogenous stem cells or serve as a platform to receive and guide the integration of transplanted stem cells and their derivatives. In vivo, the stem cell niche is a complex and dynamic unit. Although components of the in vivo niche continue to be described for many stem cell systems, how these components interact to modulate stem cell fate is only beginning to be understood. Using the HSC niche as a model, we discuss here microscale engineering strategies capable of systematically examining and reconstructing individual niche components. Synthetic stem cell-niche engineering may form a new foundation for regenerative therapies.

Inspiration and application in the evolution of biomaterials

Biomaterials, traditionally defined as materials used in medical devices, have been used since antiquity, but recently their degree of sophistication has increased significantly. Biomaterials made today are routinely information rich and incorporate biologically active components derived from nature. In the future, biomaterials will assume an even greater role in medicine and will find use in a wide variety of non-medical applications through biologically inspired design and incorporation of dynamic behaviour.

Combined use of RGD-peptide modified PLGA and TGF-beta1 gene transfected MSCs to improve cell biobehaviors in vitro

In order to improve the surface properties of PLGA polymer for a better material/cell interface to modulate the cells behaviors, we prepared a novel three-block copolymer, PLGA-[ASP-PEG], and immobilized an RGD-containing peptide, Gly-Arg-Gly-Asp-Ser-Pro-Cys (GRGDSPC) on the surface of it. Transforming growth factor-beta1 (TGF-beta1) was transfected into bone marrow stromal cells (MSCs) employed as seeded cells. Cell adhesion, spreading, proliferation and differentiation on this material were investigated. The results showed that the cell adhesive ratio on RGD-modified materials was higher than on un-modified materials (P<0.05). The extent of cell spreading was also wider on RGD-modified materials than on un-modified materials. Cell proliferation indices of transfected MSCs were increased as compared with the un-transfected MSCs (P<0.05). The ALP activities in the MSCs cultured with RGD-modified materials were higher than on un-modified materials after 14 days (P<0.05), and those in transfected MSCs were higher than in un-transfected MSCs (P<0.05). It was suggested that the combined use of RGD-modification and TGF-beta gene transfection could improve the interaction of biomaterial and cells.

A 3D hybrid model for tissue growth: the interplay between cell population and mass transport dynamics

To provide theoretical guidance for the design and in vitro cultivation of bioartificial tissues, we have developed a multiscale computational model that can describe the complex interplay between cell population and mass transport dynamics that governs the growth of tissues in three-dimensional scaffolds. The model has three components: a transient partial differential equation for the simultaneous diffusion and consumption of a limiting nutrient; a cellular automaton describing cell migration, proliferation, and collision; and equations that quantify how the varying nutrient concentration modulates cell division and migration. The hybrid discrete-continuous model was parallelized and solved on a distributed-memory multicomputer to study how transport limitations affect tissue regeneration rates under conditions encountered in typical bioreactors. Simulation results show that the severity of transport limitations can be estimated by the magnitude of two dimensionless groups: the Thiele modulus and the Biot number. Key parameters including the initial seeding mode, cell migration speed, and the hydrodynamic conditions in the bioreactor are shown to affect not only the overall rate, but also the pattern of tissue growth. This study lays the groundwork for more comprehensive models that can handle mixed cell cultures, multiple nutrients and growth factors, and other cellular processes, such as cell death.

Denuded human amniotic membrane seeding bone marrow stromal cells as an effective composite matrix stimulates axonal outgrowth of rat neural cortical cells in vitro

BACKGROUND

Previous studies have shown that axonal outgrowth in the damaged central nervous system is closely related to the local microenvironment. Transplantation of bone marrow stromal cells (BMSC) or BMSC with some biomaterials has been used to treat various central nervous system diseases with some success. In the current study, we investigated if BMSC on denuded human amniotic membrane (DhAM) as a composite matrix could stimulate axonal outgrowth or not.

METHOD

After completely removing the cells on the amniotic membrane with a tryptic and mechanical approach, we seeded BMSC on it. The MTS was applied to test the cytotoxicity of DhAM compared with PLGA and PLL. The morphology of the BMSC was observed by light, electronic and laser confocal microscopy. We also used four kinds of substance (PLL, DhAM, BMSC + PLL, BMSC + DhAM) to coculturing with the cortical neurons. Finally, the lengths of axons in each group were studied using the positive axon-specific marker NF-H.

FINDINGS

The DhAM was devoid of cellular components and only its intact basement membrane was left. BMSC grew on the substrate and proliferated with a flat to fusiform morphology. In the MTS test, the results indicated that BMSC cultured in DhAM extract had a high survival rate (> 80%). Moreover, the cortical neural axons in the experimental group (BMSC + DhAM) were longer (287.37 +/- 12.72 microm) than in the other groups (P < 0.01).

CONCLUSIONS

This study demonstrates that the DhAM was a good carrier to support growth of BMSC and BMSC on DhAM was an effective composite matrix to support the outgrowth of the axons of rat cortical neurons in vitro. Future studies of the use of the composite matrix in disorders are planned.

Interaction of human mesenchymal stem cells with osteopontin coated hydroxyapatite surfaces

In vitro studies of the initial attachment, spreading and motility of human bone mesenchymal stem cells have been carried out on bovine osteopontin (OPN) coated hydroxyapatite (HA) and gold (Au) model surfaces. The adsorption of OPN extracted from bovine milk was monitored by the quartz crystal microbalance with dissipation (QCM-D) and the ellipsometry techniques, and the OPN coated surfaces were further investigated by antigen-antibody interaction. It is shown that the OPN surface mass density is significantly lower and that the number of antibodies binding to the resulting OPN layers is significantly higher on the HA as compared to the Au surfaces. The initial attachment, spreading and motility of human mesenchymal stem cells show a larger cell area, a faster arrangement of vinculin in the basal cell membrane and more motile cells on the OPN coated HA surfaces as compared to the OPN coated Au surfaces and to the uncoated Au and HA surfaces. These in vitro results indicate that there may be great potential for OPN coated biomaterials, for instance as functional protein coatings or drug delivery systems on orthopaedic implants or scaffolds for tissue-engineering.

Thermo-reversible protein fibrillar hydrogels as cell scaffolds

Hen egg white lysozyme has been exposed to various physical and chemical denaturing environments and the physical properties of the resulting gels have been examined and their potential for use as tissue engineering scaffolds has been explored. Transparent, self-supporting fibrillar hydrogels were obtained when lysozyme was heated at low pH, while opaque, particulate gels were obtained at high pH. No increase in viscosity was observed for lysozyme at pH 7 unless the native state was disrupted by reducing the disulfide bridges. This was achieved by adding 20 mM of the reductant dithiothreitol (DTT). Under these conditions the macroscopic critical gelation concentration, C(gel), was found to be approximately 3.0 mM and mechanical spectra obtained as a function of temperature revealed that the gelling and melting temperatures increased with increasing lysozyme concentration. The mechanical strength of the hydrogel measured as the plateau elastic modulus shows a scaling behavior of G(e) approximately c2.43 for concentrations > or = C(gel), which is in good agreement with the theoretical prediction for densely cross-linked semi-flexible networks. Infrared spectroscopy showed that an alpha-helix to beta-sheet molecular transition occurred during heating resulting in beta-sheet rich fibrils forming through the self-assembly of beta-sheet rich denaturated proteins. Cryo-transmission electron microscopy shows these fibres (6 nm in diameter) exist as single entities at low concentration, and at C(gel) associate to form the junctions of a well defined regular network. Our preliminary cell culture experiments show the gel matrix promotes cell spreading, attachment and proliferation; indicating our lysozyme hydrogels are cytocompatible and they provide a viable support for the cells.

The adsorption of preferential binding peptides to apatite-based materials

The objective of this work was to identify peptide sequences with high affinity to bone-like mineral (BLM) to provide alternative design methods for functional bone regeneration peptides. Adsorption of preferential binding peptide sequences on four apatite-based substrates [BLM and three sintered apatite disks pressed from powders containing 0% CO(3)(2-) (HA), 5.6% CO(3)(2-) (CA5), 10.5% CO(3)(2-) (CA10)] with varied compositions and morphologies was investigated. A combination of phage display, ELISA, and computational modeling was used to elucidate three 12-mer peptide sequences APWHLSSQYSRT (A), STLPIPHEFSRE (S), and VTKHLNQISQSY (V), from 243 candidates with preferential adsorption on BLM and HA. Overall, peptides S and V have a significantly higher adsorption to the apatite-based materials in comparison to peptide A (for S vs. A, BLM p=0.001, CA5 p<0.001, CA10 p<0.001, HA p=0.038; for V vs. A, BLM p=0.006, CA5 p=0.033, CA10 p=0.029). FT-IR analysis displayed carbonate levels in CA5 and CA10 dropped to approximately 1.1-2.2% after sintering, whereas SEM imaging displayed CA5 and CA10 possess distinct morphologies. Adsorption results normalized to surface area indicate that small changes in carbonate percentage at a similar morphological scale did not provide enough carbonate incorporation to show statistical differences in peptide adsorption. Because the identified peptides (S and V) have preferential binding to apatite, their use can now be investigated in bone and dentin tissue engineering, tendon and ligament repair, and enamel formation.

2007 AIChE Alpha Chi Sigma Award: From Material to Tissue: Biomaterial Development, Scaffold Fabrication, and Tissue Engineering

The need for techniques to facilitate the regeneration of failing or destroyed tissues remains great with the aging of the worldwide population and the continued incidence of trauma and diseases such as cancer. A 16-year history in biomaterial scaffold development and tissue engineering is examined, beginning with the synthesis of novel materials and fabrication of 3D porous scaffolds. Exploring cell-scaffold interactions and subsequently cellular delivery using biomaterial carriers, we have developed a variety of techniques for bone and cartilage engineering. In addition to delivering cells, we have utilized growth factors, DNA, and peptides to improve the in vitro and in vivo regeneration of tissues. This review covers important developments and discoveries within our laboratory, and the increasing breadth in the scope of our work within the expanding field of tissue engineering is presented.

Injectable hydrogels as unique biomedical materials

A concentrated fish soup could be gelled in the winter and re-solled upon heating. In contrast, some synthetic copolymers exhibit an inverse sol-gel transition with spontaneous physical gelation upon heating instead of cooling. If the transition in water takes place below the body temperature and the chemicals are biocompatible and biodegradable, such gelling behavior makes the associated physical gels injectable biomaterials with unique applications in drug delivery and tissue engineering etc. Various therapeutic agents or cells can be entrapped in situ and form a depot merely by a syringe injection of their aqueous solutions at target sites with minimal invasiveness and pain. This tutorial review summarizes and comments on this soft matter, especially thermogelling poly(ethylene glycol)-(biodegradable polyester) block copolymers. The main types of injectable hydrogels are also briefly introduced, including both physical gels and chemical gels.

Experimental characterization and computational modelling of two-dimensional cell spreading for skeletal regeneration

Limited cell ingrowth is a major problem for tissue engineering and the clinical application of porous biomaterials as bone substitutes. As a first step, migration and proliferation of an interacting cell population can be studied in two-dimensional culture. Mathematical modelling is essential to generalize the results of these experiments and to derive the intrinsic parameters that can be used for predictions. However, a more thorough evaluation of theoretical models is hampered by limited experimental observations. In this study, experiments and image analysis methods were developed to provide a detailed spatial and temporal picture of how cell distributions evolve. These methods were used to quantify the migration and proliferation of skeletal cell types including MG63 and human bone marrow stromal cells (HBMSCs). The high level of detail with which the cell distributions were mapped enabled a precise assessment of the correspondence between experimental results and theoretical model predictions. This analysis revealed that the standard Fisher equation is appropriate for describing the migration behaviour of the HBMSC population, while for the MG63 cells a sharp front model is more appropriate. In combination with experiments, this type of mathematical model will prove useful in predicting cell ingrowth and improving strategies and control of skeletal tissue regeneration.

Biomaterials for stem cell differentiation

The promise of cellular therapy lies in the repair of damaged organs and tissues in vivo as well as generating tissue constructs in vitro for subsequent transplantation. Unfortunately, the lack of available donor cell sources limits its ultimate clinical applicability. Stem cells are a natural choice for cell therapy due to their pluripotent nature and self-renewal capacity. Creating reserves of undifferentiated stem cells and subsequently driving their differentiation to a lineage of choice in an efficient and scalable manner is critical for the ultimate clinical success of cellular therapeutics. In recent years, a variety of biomaterials have been incorporated in stem cell cultures, primarily to provide a conducive microenvironment for their growth and differentiation and to ultimately mimic the stem cell niche. In this review, we examine applications of natural and synthetic materials, their modifications as well as various culture conditions for maintenance and lineage-specific differentiation of embryonic and adult stem cells.

Hydrogels for tissue engineering and delivery of tissue-inducing substances

This manuscript presents hydrogels (HGs) from a tissue engineering perspective being especially written for those who are approaching this field by offering a concise but inclusive review of hydrogel synthesis, properties, characterization methods, and applications.

Short-term effects of adhesion peptides on the responses of preosteoblasts to pBMP-9

Adhesion peptides are currently used to enhance the interactions of osteoblasts with biomaterials. However, little is known about the effects of adhesion peptides on cell responses to growth factors, especially the bone morphogenetic proteins (BMPs). We used adhesion peptides Ac-CGGNGERPRGDTYRAY-NH(2) (pRGD), derived from bone sialoprotein, and Ac-CGGDGEA-NH(2) (pDGEA), derived from collagen, which interact with alpha(v)beta(3) and alpha(2)beta(1) integrins, respectively. We analyzed the effects of pRGD- and pDGEA-coated polystyrene (PS) on the responses of murine MC3T3-E1 preosteoblasts to a peptide derived from human BMP-9 (pBMP-9) in serum-free medium. After 1h, pRGD favoured interactions with alpha(v) while pDGEA bound beta(1) integrin subunits. Adding pBMP-9 (400 ng/mL) increased the amount of alpha(v) integrin subunits in cell membranes on pRGD-coated PS, but had no effect on beta(1) integrin subunits. Only on this substratum, collagen type I mRNA was enhanced and the addition of pBMP-9 promoted the early cell differentiation, increasing their alkaline phosphatase (ALP) activity within 24 h. These cells also organized beta(1) integrin subunits at their focal adhesion points. Inhibiting alpha(2)beta(1) integrins by pDGEA pre-treatment decreased this ALP activity. It is therefore important to understand the impact of adhesion peptides on the early cell responses to growth factors in order to improve biomimetic materials.

Pre-culture period of mesenchymal stem cells in osteogenic media influences their in vivo bone forming potential

The objective of this study was to investigate if the in vitro pre-culture period in osteogenic media of rat mesenchymal stem cells (MSCs), influences their ability to regenerate bone when implanted in a critical size cranial defect. MSCs were harvested from the bone marrow of 6-8 weeks old male Fisher rats and expanded in vitro in osteogenic media for different time periods (4, 10, and 16 days) in tissue culture plates (TCP), seeded on sintered titanium fiber meshes without the extracellular matrix (ECM) generated in vitro, and implanted in the rat cranium after 12 h. Thirty two adult Fisher rats received the implants, divided in four groups. Three groups were implanted with cells cultured for 4, 10, or 16 days in osteogenic media and at that time their alkaline phosphatase activity and mineral deposition denoted that they were at different stages of their osteoblastic maturation (undifferentiated MSC, committed, and mature Osteoblasts, respectively). MSCs cultured without osteogenic media for 6 days were used as controls. The constructs were retrieved 4 weeks later and processed for histomorphometric analysis. Implants seeded with cells that have been cultured with osteogenic media for only 4 days revealed the highest bone formation. The lowest bone formation was obtained with the implants seeded with MSCs cultured for 16 days in the presence of osteogenic media. The results of this study suggested that the in vitro pre-culture period of MSCs is a critical factor for their ability to regenerate bone when implanted to an orthotopic site.

Matrices and scaffolds for delivery of bioactive molecules in bone and cartilage tissue engineering

Regeneration of bone and cartilage defects can be accelerated by localized delivery of appropriate growth factors incorporated within biodegradable carriers. The carrier essentially allows the impregnated growth factor to release at a desirable rate and concentration, and to linger at injury sites for a sufficient time to recruit progenitors and stimulate tissue healing processes. In addition, the carrier can be formulated to have particular structure to facilitate cellular infiltration and growth. In this review, we present a summary of growth factor delivery carrier systems for bone and cartilage tissue engineering. Firstly, we describe a list of growth factors implicated in repair and regeneration of bone and cartilage by addressing their biological effects at different stages of the healing process. General requirements for localized growth factor delivery carriers are then discussed. We also provide selective examples of material types (natural and synthetic polymers, inorganic materials, and their composites) and fabricated forms of the carrier (porous scaffolds, microparticles, and hydrogels), highlighting the dose-dependent efficacy, release kinetics, animal models, and restored tissue types. Extensive discussion on issues involving currently investigated carriers for bone and cartilage tissue engineering approaches may illustrate future paths toward the development of an ideal growth factor delivery system.

Biological designer self-assembling peptide nanofiber scaffolds significantly enhance osteoblast proliferation, differentiation and 3-D migration

A class of self-assembling peptide nanofiber scaffolds has been shown to be an excellent biological material for 3-dimension cell culture and stimulating cell migration into the scaffold, as well as for repairing tissue defects in animals. We report here the development of several peptide nanofiber scaffolds designed specifically for osteoblasts. We designed one of the pure self-assembling peptide scaffolds RADA16-I through direct coupling to short biologically active motifs. The motifs included osteogenic growth peptide ALK (ALKRQGRTLYGF) bone-cell secreted-signal peptide, osteopontin cell adhesion motif DGR (DGRGDSVAYG) and 2-unit RGD binding sequence PGR (PRGDSGYRGDS). We made the new peptide scaffolds by mixing the pure RAD16 and designer-peptide solutions, and we examined the molecular integration of the mixed nanofiber scaffolds using AFM. Compared to pure RAD16 scaffold, we found that these designer peptide scaffolds significantly promoted mouse pre-osteoblast MC3T3-E1 cell proliferation. Moreover, alkaline phosphatase (ALP) activity and osteocalcin secretion, which are early and late markers for osteoblastic differentiation, were also significantly increased. We demonstrated that the designer, self-assembling peptide scaffolds promoted the proliferation and osteogenic differentiation of MC3T3-E1. Under the identical culture medium condition, confocal images unequivocally demonstrated that the designer PRG peptide scaffold stimulated cell migration into the 3-D scaffold. Our results suggest that these designer peptide scaffolds may be very useful for promoting bone tissue regeneration.

In vitro assessment of motility and proliferation of human osteogenic cells on different isolated extracellular matrix components compared with enamel matrix derivative by continuous single-cell observation

OBJECTIVES

The composition of the extracellular matrix (ECM) plays a substantial role in bone remodelling, fracture healing and osseointegration of dental implants by regulating proliferation, migration and finally differentiation of osteogenic cell populations. Emdogain, a composition of an enamel matrix derivative (EMD), has been introduced as a potential candidate to promote tissue regeneration. We investigated whether EMD could serve as a potential promoter of cell proliferation and motility as a dynamic cell response and compared the results with the ubiquitous single ECM components type I collagen and laminin.

MATERIAL AND METHODS

In the investigation presented, we used a continuous observation method for the analysis of migratory and proliferative patterns of individual cells. We analyzed the response of four osteoblastic cell lines to specific extracellular ligands (type I collagen, laminin and EMD) over a period of 24 h compared with untreated glass surface and bovine serum albumin (BSA) as control groups.

RESULTS

Type I collagen and laminin promoted cell motility significantly compared with the control groups and, in part, compared with EMD as well. The analysis of all 451 investigated cells revealed the following mean values for cell motiliy: untreated glass (n=99): 5.46+/-2.74 microm/h, BSA (n=89): 6.35+/-2.43 microm/h, type I collagen (n=108): 8.77+/-3.42 microm/h, laminin (n=74): 9.89+/-5.10 microm/h and EMD (n=81): 7.92+/-3.35 microm/h. Proliferation rates on the different surfaces were heterogenous for all investigated cell lines and varied from 0% to 50% within 24 h without a correlation to cell motility.

CONCLUSION

In our study, EMD promotes cell motility better than the control groups. The two investigated single ECM components type I collagen and laminin promoted cell motility superior to EMD. This supports the hypothesis that EMD promotes a less mobile but more differentiated osteogenic phenotype.

Bone regeneration using hyaluronic acid-based hydrogel with bone morphogenic protein-2 and human mesenchymal stem cells

Acrylated hyaluronic acid (HA) was used as a scaffold for bone morphogenic protein-2 (BMP-2) and human mesenchymal stem cells (hMSCs) for rat calvarial defect regeneration. HA was acrylated by two-step reactions: (1) introduction of an amine group using adipic acid dihydrazide (ADH); (2) acrylation by N-acryloxysuccinimide. Tetrathiolated poly(ethylene) glycol (PEG-SH(4)) was used as a cross-linker by a Michael-type addition reaction and the hydrogel was formed within 10min under physiological conditions. This hydrogel is degraded completely by 100U/ml hyaluronidase in vitro. hMSCs and/or BMP-2 was added during gelation. Cellular viability in vitro was increased up to 55% in the hydrogels with BMP-2 compared with the control. For in vivo calvarial defect regeneration, five different samples (i.e., control, hydrogel, hydrogel with BMP-2, hydrogel with MSCs, and hydrogel with BMP-2 and MSCs) were implanted for 4 weeks. The histological results demonstrated that the hydrogels with BMP-2 and MSCs had the highest expression of osteocalcin and mature bone formation with vascular markers, such as CD31 and vascular endothelial growth factors, compared with the other samples. This study demonstrated that HA base hydrogel can be used for cell and growth factor carriers for tissue regeneration.

Application of a new bioresorbable film to guided bone regeneration in tibia defect model of the rabbits

Aim is to study the effect of calcium alginate film (CAF) on guided bone regeneration (GBR). Circular bone defects with 5 mm diameter were created in both tibias in 60 rabbits. The defects covered with CAF served as the experimental site, and with collagen membrane (CM) or with no membrane both served as the control site. Healing was analyzed by gross, X-ray, electromicroscopy, histology, immuno-histochemical studies, and an image pattern analysis system after 1, 2, 4, 6, and 8 weeks. The CM control sites showed more macrophages, and CM were absorbed more slowly while collecting fewer osteoinductive factors (p < 0.01) in the early weeks. CAF induced dense bone formation, whereas CM induced less new bone, and the blank control sites effected the worst. In conclusion, the effect of CAF group gave better results than blank control group and CM group on GBR in this animal model.

Biodegradable polymeric scaffolds. Improvements in bone tissue engineering through controlled drug delivery

Recent advances in biology, medicine, and engineering have led to the discovery of new therapeutic agents and novel materials for the repair of large bone defects caused by trauma, congenital defects, or bone tumors. These repair strategies often utilize degradable polymeric scaffolds for the controlled localized delivery of bioactive molecules to stimulate bone ingrowth as the scaffold degrades. Polymer composition, hydrophobicity, crystallinity, and degradability will affect the rate of drug release from these scaffolds, as well as the rate of tissue ingrowth. Accordingly, this chapter examines the wide range of synthetic degradable polymers utilized for osteogenic drug delivery. Additionally, the therapeutic proteins involved in bone formation and in the stimulation of osteoblasts, osteoclasts, and progenitor cells are reviewed to direct attention to the many critical issues influencing effective scaffold design for bone repair.

Matricellular proteins: Extracellular modulators of bone development, remodeling, and regeneration

Matricellular proteins are components of the extracellular matrix which are highly expressed in the developing and mature skeleton. Members of this protein class serve as biological mediators of cell function by interacting directly with cells or by modulating the activity of growth factors, proteases, and other extracellular matrix proteins. Although skeletons of matricellular protein-null mice are grossly normal, they each display unique deficiencies that are often magnified under pathological conditions. In addition, bone cells from wild-type and matricellular protein-null mice behave differently in various in vitro models of bone matrix synthesis and turnover. In this review, osteopontin, bone sialoprotein, tenascin C, SPARC, and thrombospondins 1 and 2 will each be discussed in the context of bone cell biology. Because the biological effects of matricellular proteins are largely context dependent, in vivo and in vitro results must be considered together in order to fully appreciate the specific contributions that matricellular proteins make to bone physiology and pathophysiology. In particular, it is clear that although matricellular proteins are not required for bone development and function, the proteins act to modulate post-natal bone structure in response to aging, ovariectomy, mechanical loading, and bone regeneration.

Characterization of DNA release from composites of oligo(poly(ethylene glycol) fumarate) and cationized gelatin microspheres in vitro

This research investigates the release of plasmid DNA from novel hydrogel composites of oligo(poly(ethylene glycol) fumarate) (OPF) and cationized gelatin microspheres (CGMS), as well as the swelling and degradation of these materials in vitro. The release of total DNA and of double-stranded DNA was measured fluorescently, and the swelling properties and polymer mass loss of the hydrogels were assessed. Further, the structural integrity of the released DNA was determined through electrophoresis. It was found that plasmid DNA can be released in a sustained fashion over the course of up to 49-140 days in vitro from hydrogels of OPF synthesized from poly(ethylene glycol) of nominal molecular weights of 10 kDa and 3 kDa, respectively, with the release kinetics depending upon the material composition and the method of DNA loading. Released DNA was predominately double-stranded DNA (dsDNA) in structure and of the open-circular conformation. The results suggest that DNA release from hydrogel composites of OPF and CGMS is dominated by the degradation of the OPF component of the gels. Electrophoresis results indicate that the released DNA retains suitable conformation for potential bioactivity over the course of at least 63 days of release. Thus, these studies demonstrate the potential of composites of OPF and CGMS in controlled gene delivery applications.

Evaluation of bone regeneration by DNA release from composites of oligo(poly(ethylene glycol) fumarate) and cationized gelatin microspheres in a critical-sized calvarial defect

This research examines the bone formation response to release of plasmid DNA encoding human Bone Morphogenetic Protein-2 from hydrogel composites consisting of cationized gelatin microspheres (CGMS) embedded within a crosslinked oligo(poly(ethylene glycol) fumarate) (OPF) hydrogel network in a critical-sized rat cranial defect model after 30 days. Four composite groups were investigated: (1) composites with 10 microg DNA loaded into the CGMS phase, (2) composites with 10 microg DNA loaded into the OPF phase, (3) composites with 100 microg DNA loaded into the OPF phase, and (4) composites without DNA (material control). Light microscopy revealed no enhancement in bone formation for groups releasing plasmid DNA, relative to the material control group. Limited formation of new bone was observed from the defect margins and within the defect for some samples. The hydrogels swelled appreciably and fragmentation of the implants was noted to varying degrees among samples within groups, with a presence of inflammatory cells related to the degree of fragmentation. The lack of enhancement in bone formation indicates that the release of plasmid DNA from the composites was not sufficient to elicit a bone regeneration response.

In vivo release of plasmid DNA from composites of oligo(poly(ethylene glycol)fumarate) and cationized gelatin microspheres

Composites of cationized gelatin microspheres (CGMS), crosslinked with either 3 mM or 6 mM glutaraldehyde solution, and a novel hydrogel material, oligo(poly(ethylene glycol)fumarate) (OPF) were fabricated and investigated toward prolonging the release of plasmid DNA in vivo relative to the constituent materials. The composites and constituent materials were investigated in a subcutaneous murine model to assess the release of 125I-labeled plasmid DNA and 125I-labeled cationized gelatin in vivo. The time profiles of the radioactivity remaining were employed to compare the profiles of DNA release and cationized gelatin degradation. Both composite formulations (incorporating either 3 mM or 6 mM CGMS) prolonged the bioavailability of plasmid DNA relative to both injected plasmid DNA solution and the respective non-embedded cationized gelatin microspheres. Injected plasmid DNA solution persisted in the subject for only 7-10 days, whereas the persistence of DNA from composites of OPF and either 3 mM or 6 mM CGMS extended to at least day 42. The 3 mM and 6 mM CGMS each increased the persistence of DNA slightly, relative to injection of DNA solution, to between 28 and 35 days. Interestingly, the release profile of plasmid DNA from composites was not significantly different from the release of DNA from OPF alone. The release of plasmid DNA from the composites was in accord with the degradation of the microspheres within the OPF. These results show that composites of OPF and cationized gelatin microspheres are able to prolong the availability of plasmid DNA in vivo relative to cationized gelatin microspheres alone and provide a promising candidate material for the sustained, controlled release of plasmid DNA.

The effect of irradiation modification and RGD sequence adsorption on the response of human osteoblasts to polycaprolactone

Using techniques of tissue engineering, synthetic substitutes can be applied for the repair and regeneration of damaged bone. It has been found that material surface properties are crucial for cell adhesion and spreading, i.e. cell activities that are related directly to the ability of osteoblasts to proliferate. This fact has promoted the strategy of creating an ECM-like layer onto materials, so as to influence the cell response. In this study human bone-derived osteoblasts have been used to test the effects of surface modification by low energy ion beams of a poly epsilon-caprolactone (PCL) substrate and subsequent RGD adsorption. Osteoblasts were seeded and grown onto untreated and irradiated poly epsilon-caprolactone films, with or without RGD-adsorption step, and viability, morphology, and spreading of the osteoblasts were studied at different time endpoints. Differences were observed in the organization of cytoskeleton within cells: stress fibers were more evident in irradiated samples vs. untreated and total cell adhesion was higher. Surface characterization by X-ray Photoelectron Spectroscopy, Atomic Force Microscopy, and surface free energy measurements showed that the polar character of PCL, i.e., the acid-base term, was increased following irradiation treatment. Moreover the irradiated PCL had a nano-sized topography, which also could improve osteoblasts adhesion. We found that the treatment of the surface with ion beam is per se improving osteoblasts adhesion and spreading onto PCL. Furthermore, also if a significant RGD adsorption was obtained for irradiated PCL surfaces, it was found that in the investigated conditions it seems to have only a minor effect on the cell response. This study suggests that new strategies involving irradiation-based treatments can be adopted to promote the initial steps of bone deposition onto synthetic surfaces, exploiting the surface-induced reorganization of the ECM matrix.

Bio-adhesive surfaces to promote osteoblast differentiation and bone formation

Binding of integrin adhesion receptors to extracellular matrix components, such as fibronectin and type I collagen, activates signaling pathways directing osteoblast survival, cell-cycle progression, gene expression, and matrix mineralization. Biomimetic strategies exploit these adhesive interactions to engineer bio-inspired surfaces that promote osteoblast adhesion and differentiation, bone formation, and osseointegration. These emerging initiatives focus on directing integrin binding through presentation of bio-adhesive motifs derived from extracellular matrices. These biomolecular approaches provide promising strategies for the development of biologically active implants and grafting substrates for enhanced bone repair.

Osteogenic differentiation of rat bone marrow stromal cells cultured on Arg–Gly–Asp modified hydrogels without dexamethasone and β-glycerol phosphate

In this study, we investigated the effect of signaling peptides incorporated into oligo(poly(ethylene glycol) fumarate) (OPF) hydrogels on in vitro differentiation and mineralization of marrow stromal cells (MSCs) cultured in media without soluble osteogenic supplements (dexamethasone and beta-glycerol phosphate). When MSCs were cultured for 16 days on OPF hydrogels modified with Arg-Gly-Asp (RGD) containing peptides, the normalized cell number was dependent on the peptide concentration between days 0 and 5 and reached comparable values at day 10 regardless of the concentration. The alkaline phosphatase (ALP) activity of MSCs on the peptide-modified OPF hydrogels was also concentration-dependent: ALP activity showed peaks on day 10 or day 13 on OPF hydrogels modified with 2.0 and 1.0 micromol peptide/g, which were significantly greater than those on the OPF hydrogels modified with 0.1 micromol peptides/g or no peptide. A characteristic marker of osteoblastic differentiation, osteopontin (OPN), was detected for all the test groups. However, OPN secretion between days 0 and 10 was significantly higher on the peptide modified hydrogels compared to that on tissue culture-treated polystyrene. Taken together, the results indicate that the presence of signaling peptide allows for a favorable microenvironment for MSCs to differentiate into osteoblasts and produce mineralized matrix, although the soluble factors may further enhance calcium deposition. These findings further support the usefulness of OPF hydrogels as scaffolds for guided bone regeneration, and represent an initial step in exploring the complex relationship between soluble and insoluble factors in osteogenic differentiation on biodegradable materials.

In vitro release of plasmid DNA from oligo(poly(ethylene glycol) fumarate) hydrogels

This research investigates the release of plasmid DNA in vitro from novel, injectable hydrogels based on the polymer oligo(poly(ethylene glycol) fumarate) (OPF). These biodegradable hydrogels can be crosslinked under physiological conditions to physically entrap plasmid DNA. The DNA release kinetics were characterized fluorescently with the PicoGreen and OliGreen Reagents as well as through the use of radiolabeled plasmid. Further, the ability of the released DNA to be expressed was assessed through bacterial transformations. It was found that plasmid DNA can be released in a sustained, linear fashion over the course of 45-62 days, with the release kinetics depending upon the molecular weight of the poly(ethylene glycol) from which the OPF was synthesized. Two formulations of OPF were synthesized from poly(ethylene glycol) of a nominal molecular weight of either 3.35K (termed OPF 3K) or 10K (termed OPF 10K). By the time the gels had completely degraded, 97.8+/-0.3% of the initially loaded DNA was recovered from OPF 3K hydrogels, with 80.8+/-1.9% of the initial DNA retaining its double-stranded form. Likewise, for OPF 10K gels, 92.1+/-4.3% of the initially loaded DNA was recovered upon complete degradation of the gels, with 81.6+/-3.8% of the initial DNA retaining double-stranded form. Experiments suggest that the release of plasmid DNA from OPF hydrogels is dominated by the degradation of the gels. Bacterial transformation results indicated that the DNA retained bioactivity over the course of 42 days of release. Thus, these studies demonstrate the potential of OPF hydrogels in controlled gene delivery applications.

Modulation of differentiation and mineralization of marrow stromal cells cultured on biomimetic hydrogels modified with Arg-Gly-Asp containing peptides

We synthesized biomimetic hydrogels modified with an osteopontin-derived peptide (ODP) and used them as a substrate for in vitro culture of marrow stromal cells (MSCs) to investigate the effect of the biomimetic surface on differentiation of MSCs into osteoblasts. Proliferation and biological assays for 16 days proved that MSCs became differentiated into osteoblasts secreting osteogenic phenotypic markers such as alkaline phosphatase (ALP), osteopontin, and mineralized calcium. In addition, there was an additive effect of the cell-binding peptide on differentiation and mineralization of MSCs cultured in the presence of soluble osteogenic supplements in cell culture media. For example, calcium content at day 16 on peptide-modified hydrogels was significantly higher than on tissue culture polystyrene. Two general trends were observed: (1) proliferation of MSCs decreased as the amount of differentiation markers increased, and (2) higher peptide concentrations accelerated the differentiation of MSCs. On the hydrogel modified with ODP, ALP activity exhibited a maximum value of 36.7 +/- 4.2 pmol/cell/h at day 10 for the concentration of 2 micromol/g while the culture time needed for maximum ALP activity occurred on day 13 for the lower concentrations. On the same hydrogel, the calcium content at day 10 was 21.4 +/- 2.3 ng/cell for the peptide concentration of 2 micromol/g and 1.0 +/- 0.3 ng/cell for 1.0 micromol/g. We used Gly-Arg-Gly-Asp-Ser (GRGDS) for modification of the hydrogel as a comparison to the results with ODP. However, osteoblast development was not significantly affected by the nature of the binding peptide sequences. These results suggest that MSC function can be modulated by variation of the peptide concentration in biomimetic hydrogels used for scaffold-based bone tissue engineering.

Thixotrophy property of hydrogels to evaluate the cell growing on the inside of the material bulk (Amber effect)

The realization of injectable hydrogels is one of the most challenging aims of biomedical research. Most injectable hydrogels are obtained by photopolymerization in situ. A hydrogel which is able to pass through a needle without losing its structure once crosslinked would be of great interest for several biomedical applications. In this work, hyaluronane and alginate-based 50% hydrogels were synthesized, their thixotrophic behavior was verified and their mechanical properties (G' and G'') were determined before and after the passage through the needle. A morphological analysis by scanning electron microscopy (SEM) was also performed in order to evaluate the effect of the passage of the hydrogel through the needle on the morphological structure of the material. The thixotrophic property of these hydrogels is used to realize a cell-containing material that supports cell proliferation and growth permitting in vivo engineering of new tissues. In fact, thixotrophic hydrogel can be sucked together with cell suspension and in this way cells remain entrapped inside the hydrogel structure (Amber effect). A comparison of the cell growth using this technique with the classical seeding technique (cell seeding onto the hydrogel) was performed. A different behavior was found between hyaluronane and alginate-based hydrogels.

Biomimetic polymers in pharmaceutical and biomedical sciences

This review describes recent developments in the emerging field of biomimetic polymeric biomaterials, which signal to cells via biologically active entities. The described biological effects are, in contrast to many other known interactions, receptor mediated and therefore very specific for certain cell types. As an introduction into this field, first some biological principles are illustrated such as cell attachment, cytokine signaling and endocytosis, which are some of the mechanisms used to control cells with biomimetic polymers. The next topics are then the basic design rules for the creation of biomimetic materials. Here, the major emphasis is on polymers that are assembled in separate building blocks, meaning that the biologically active entity is attached to the polymer in a separate chemical reaction. In that respect, first individual chemical standard reactions that may be used for this step are briefly reviewed. In the following chapter, the emphasis is on polymer types that have been used for the development of several biomimetic materials. There is, thereby, a delineation made between materials that are processed to devices exceeding cellular dimensions and materials predominantly used for the assembly of nanostructures. Finally, we give a few current examples for applications in which biomimetic polymers have been applied to achieve a better biomaterial performance.