Johnna S. Temenoff

Review: tissue engineering for regeneration of articular cartilage.

Joint pain due to cartilage degeneration is a serious problem, affecting people of all ages. Although many techniques, often surgical, are currently employed to treat this affliction, none have had complete success. Recent advances in biology and materials science have pushed tissue engineering to the forefront of new cartilage repair techniques. This review seeks to condense information for the biomaterialist interested in developing materials for this application. Articular cartilage anatomy, types of injury, and current repair methods are explained. The need for biomaterials, current commonly used materials for tissue-engineered cartilage, and considerations in scale-up of cell-biomaterial constructs are summarized.

Injectable biodegradable materials for orthopedic tissue engineering

The large number of orthopedic procedures performed each year, including many performed arthroscopically, have led to great interest in injectable biodegradable materials for regeneration of bone and cartilage. A variety of materials have been developed for these applications, including ceramics, naturally derived substances and synthetic polymers. These materials demonstrate overall biocompatibility and appropriate mechanical properties, as well as promote tissue formation, thus providing an important step towards minimally invasive orthopedic procedures. This review provides a comparison of these materials based on mechanical properties, biocompatibility and regeneration efficacy. Advantages and disadvantages of each material are explained and design criteria for injectable biodegradable systems are provided.

Biomaterials and bone mechanotransduction

Bone is an extremely complex tissue that provides many essential functions in the body. Bone tissue engineering holds great promise in providing strategies that will result in complete regeneration of bone and restoration of its function. Currently, such strategies include the transplantation of highly porous scaffolds seeded with cells. Prior to transplantation the seeded cells are cultured in vitro in order for the cells to proliferate, differentiate and generate extracellular matrix. Factors that can affect cellular function include the cell-biomaterial interaction, as well as the biochemical and the mechanical environment. To optimize culture conditions, good understanding of these parameters is necessary. The new developments in bone biology, bone cell mechanotransduction, and cell-surface interactions are reviewed here to demonstrate that bone mechanotransduction is strongly influenced by the biomaterial properties.

Formation of highly porous biodegradable scaffolds for tissue engineering

In recent years, lack of donor organs has caused many to consider tissue engineering methods as means to replace diseased or damaged organs. This newly-emerging field uses tissue-specific cells in a three-dimensional organization, provided by a scaffolding material, to return functionality of the organ. For these applications, the choice of scaffolding material is crucial to the success of the technique. In addition to the chemical properties of the material, physical properties such as surface area for cell attachment are essential. Various methods of creating pores in these materials to increase surface area are reviewed here. Scaffolds formed using the different techniques, which include fiber bonding, solvent casting/particulate leaching, gas foaming and phase separation, are compared on the basis of porosity, pore size, and promotion of tissue growth.

Effect of swelling ratio of injectable hydrogel composites on chondrogenic differentiation of encapsulated rabbit marrow mesenchymal stem cells in vitro.

An injectable, biodegradable hydrogel composite of oligo(poly(ethylene glycol) fumarate) (OPF) and gelatin microparticles (MPs) has been investigated as a cell and growth factor carrier for cartilage tissue engineering applications. In this study, hydrogel composites with different swelling ratios were prepared by cross-linking OPF macromers with poly(ethylene glycol) (PEG) repeating units of varying molecular weights from 1000 approximately 35000. Rabbit marrow mesenchymal stem cells (MSCs) and MPs loaded with transforming growth factor-beta1 (TGF-beta1) were encapsulated in the hydrogel composites to examine the effect of the swelling ratio of the hydrogel composites on the chondrogenic differentiation of encapsulated rabbit marrow MSCs both in the presence and in the absence of TGF-beta1. The swelling ratio of the hydrogel composites increased as the PEG molecular weight in the OPF macromers increased. Chondrocyte-specific genes were expressed at higher levels in groups containing TGF-beta1-loaded MPs and varied with the swelling ratio of the hydrogel composites. OPF hydrogel composites with PEG repeating units of molecular weight 35000 and 10000 with TGF-beta1-loaded MPs exhibited a 159 +/- 95- and a 89 +/- 31-fold increase in type II collagen gene expression at day 28, respectively, while OPF hydrogel composites with PEG repeating units of molecular weight 3000 and 1000 with TGF-beta1-loaded MPs showed a 27 +/- 10- and a 17 +/- 7-fold increase in type II collagen gene expression, respectively, as compared to the composites with blank MPs at day 0. The results indicate that chondrogenic differentiation of encapsulated rabbit marrow MSCs within OPF hydrogel composites could be affected by their swelling ratio, thus suggesting the potential of OPF composite hydrogels as part of a novel strategy for controlling the differentiation of stem cells.

Expansion of human nasal chondrocytes on macroporous microcarriers enhances redifferentiation

Articular cartilage has a limited capacity for self-repair. To overcome this problem, it is expected that functional cartilage replacements can be created from expanded chondrocytes seeded in biodegradable scaffolds. Expansion of chondrocytes in two-dimensional culture systems often results in dedifferentiation. This investigation focuses on the post-expansion phenotype of human nasal chondrocytes expanded on macroporous gelatin CultiSpher G microcarriers. Redifferentiation was evaluated in vitro via pellet cultures in three different culture media. Furthermore, the chondrogenic potential of expanded cells seeded in polyethylene glycol terephthalate/ polybuthylene terephthalate (PEGT/PBT) scaffolds, cultured for 14 days in vitro, and subsequently implanted subcutaneously in nude mice, was assessed. Chondrocytes remained viable during microcarrier culture and yielded doubling times (1.07+/-0.14 days) comparable to T-flask expansion (1.20+/-0.36 days). Safranin-O staining from pellet culture in different media demonstrated that production of GAG per cell was enhanced by microcarrier expansion. Chondrocyte-polymer constructs with cells expanded on microcarriers contained significantly more proteoglycans after subcutaneous implantation (288.5+/-29.2 microg) than those with T-flask-expanded cells (164.0+/-28.7 microg). Total collagen content was similar between the two groups. This study suggests that macroporous gelatin microcarriers are effective matrices for nasal chondrocyte expansion, while maintaining the ability of chondrocyte differentiation. Although the exact mechanism by which chondrocyte redifferentiation is induced through microcarrier expansion has not yet been elucidated, this technique shows promise for cartilage tissue engineering approaches.

Biodegradable Orthopedic Implants

Over the past 30 years, there have been significant advances in the development of biodegradable materials [79]. In particular, these materials have received attention for use as implants to aid regeneration of orthopedic defects [49, 91]. Every year more than 3.1 million orthopedic surgeries are performed in the United States alone [1]. However, although current treatments using nondegradable fixation materials have proven efficacious, tissue-engineering approaches with biodegradable implants are being considered as promising future alternatives [8, 49]. One possible advantage of these systems is that biodegradable implants can be engineered to provide temporary support for bone fractures, and because they can degrade at a rate matching new tissue formation, their use can eliminate the need for a second surgery [49].

Cytotoxicity of redox radical initiators for encapsulation of mesenchymal stem cells

We have developed a novel in situ crosslinkable hydrogel system based on oligo(poly(ethylene glycol) fumarate) that is being investigated as an injectable carrier for mesenchymal stem cells (MSCs) for orthopaedic tissue engineering applications. This hydrogel is crosslinked using the redox radical initiators ammonium persulfate and ascorbic acid. However, it is believed that, when oxidized, ascorbic acid produces a radical anion that decreases the pH of the solution during crosslinking. Therefore, in this study, combinations of two different persulfate oxidizing agents with three reducing agents derived from ascorbic acid were examined to determine the relationship between pH and cytotoxicity for rat MSCs. pH was recorded over two hours for solutions of the various reagents in cell culture media. After two hours of exposure, viability of MSCs was determined on a plate reader using the LIVE/DEAD fluorescent assay. pH and cell viability data for samples from 1-1000 mM showed that there was a smaller change in pH and a corresponding higher viability at lower concentrations, regardless of the reagent used. This indicates that low pH, even without the generation of radicals, has a negative impact on cell viability. Further results from combinations of oxidizing and reducing agents support the hypothesis that radical anion formation contributes to the pH drop in these solutions.

Controlled release of a tissue inducing peptide from oligo(poly(ethylene glycol) fumarate) hydrogels for orthopedic tissue engineering

The objective of this research is to develop injectable, in situ polymerizable polymer scaffolds for the controlled release of inductive factors for bone and cartilage tissue engineering. To that end, the novel polymer oligo(poly(ethylene glycol) fumarate) (OPF) was synthesized with varying poly(ethylene glycol) (PEG) chain lengths to create crosslinked networks of varying mesh size. A 23 amino acid peptide, TP508, was incorporated into OPF networks either directly or embedded in poly(DL-lactic-co-glycolicacid) (PLGA) microparticle carriers and the release kinetics of the TP508 was examined. After 30 hours, hydrogels of PEG chains of molecular weight 10,000 (PF10K) had released greater percentages of the total TP508 (53/spl plusmn/3 wt% of directly loaded TP508) than those fabricated with PEG chain of molecular weight 1,000 (PF1K) (31/spl plusmn/7 wt%). This effect was also observed upon the inclusion of PLGA microparticle carriers. For hydrogels of either PEG chain length, release of TP508 was greatly reduced with the use of microparticle carriers (6/spl plusmn/1 and 2/spl plusmn/1 wt% for PF10K and PF1K, respectively). Our results demonstrate that TP508 can be incorporated into OPF hydrogels and that the release kinetics of the peptide can be modulated through alterations in the scaffold mesh size and the use of a microparticle carrier.