Biodegradable polymer scaffolds for cartilage tissue engineering

Cartilage tissue engineering using human auricular chondrocytes embedded in different hydrogel materials

To seek a suitable scaffold for cartilage tissue engineering, we compared various hydrogel materials originating from animals, plants, or synthetic peptides. Human auricular chondrocytes were embedded in atelopeptide collagen, alginate, or PuraMatrix, all of which are or will soon be clinically available. The chondrocytes in the atelopeptide collagen proliferated well, while the others showed no proliferation. A high-cell density culture within each hydrogel enhanced the expression of collagen type II mRNA, when compared with that without hydrogel. By stimulation with insulin and BMP-2, collagen type II and glycosaminoglycan were significantly accumulated within all hydrogels. Chondrocytes in the atelopeptide collagen showed high expression of beta1 integrin, seemingly promoting cell-matrix signaling. The N-cadherin expression was inhibited in the alginate, implying that decrease in cell-to-cell contacts may maintain chondrocyte activity. The matrix synthesis in PuraMatrix was less than that in others, while its Young's modulus was the lowest, suggesting a weakness in gelling ability and storage of cells and matrices. Considering biological effects and clinical availability, atelopeptide collagen may be accessible for clinical use. However, because synthetic peptides can control the risk of disease transmission and immunoreactivities, some improvement in gelling ability would provide a more useful hydrogel for ideal cartilage regeneration.

Articular cartilage repair by means of biodegradable scaffolds

INTRODUCTION

Articular cartilage has a limited capacity for self-repair; untreated injuries of cartilage may lead to osteoarthritis. In severe cases the only choice a total joint replacement, may be inadequate in young patients. This problem demands new effective methods to reconstruct articular cartilage. The aim of this study was to evaluate the application of collagen matrix for the reconstruction of articular cartilage.

MATERIALS AND METHODS

A group of 28 rabbits had a defect penetrating into the subchondral constructed and either filled with collagen scaffold (group I) or remained empty (group II). The results were observed after 4 and 12 weeks. Macroscopic and microscopic evaluations were performed.

RESULTS

In the first group we observed the presence of hyalinelike cartilage resembling normal articular cartilage. In the second group fibrous tissue dominated. The surface of regenerated tissue was smooth, intact, and the defect completely filled with regenerated tissue, showing good structural integrity. In the second group, superficial irregularities, disorders of structural integrity, and necrotic features were noticed.

CONCLUSIONS

This study showed better results of articular cartilage reconstruction by means of a biodegradable scaffold.

Tendon tissue engineering and gene transfer: the future of surgical treatment

Technologic improvements in the field of tissue engineering are leading to new potential developments in the currently used approaches to treat tendon injuries including difficult clinical scenarios such as zone II flexor tendon injuries of the hand and the mutilated hand with extensive tendon defects. A combination of mesenchymal (adult stem) cells, growth factors, and bioresorbable polymers can provide a solution for the treatment of difficult tendon injuries. Extensive research is needed to show that the extracellular matrix produced in response to the cell/growth factor/polymer composites in vivo is effective and functional as a regenerate tissue. Further exciting advances are foreseen in cell-based genetic engineering with the transfer of DNA to the site of tendon lacerations. These treatment modalities require improved safety precautions to reduce the risks and enhance the benefits of gene therapy.

Vascularized Tissue-Engineered Ears

BACKGROUND

A paucity of appropriate regional and local matching tissue can compromise the reconstruction efforts in areas of the body that require specialized tissue. The current study uses techniques of vascular prefabrication, tissue culturing, and capsule formation to form a vascularized ear construct that is reliably transferable on its blood supply.

METHODS

Thirty male Wistar rats (250 to 350 g) were anaesthetized. An incision was made over the right lower abdominal wall. A pocket was formed by blunt dissection just below the panniculus carnosus. A separate incision was made over the right femoral vessels, which were then isolated and transected distally. The vessels were transposed in a subcutaneous plane to the abdominal wound. A silicone mold in the shape of an ear (2 x 1.5 cm) was placed over the transposed vessels in the abdominal wound pocket. The wounds were closed. Auricular cartilage was minced, washed, and cultured. After 14 days, the chondrocyte culturing was complete and a vascularized capsule based on the incorporated, transposed femoral vessels was formed. The abdominal incision was then reopened, an incision was made in the lateral capsule, and the cultured chondrocytes were introduced into the molded capsule. Study groups included capsules filled with chondrocytes only, chondrocytes and a fibrin glue carrier, and the fibrin glue only. The capsule was closed and the wounds sutured. The prefabricated, prelaminated construct was isolated on its vascular pedicle 14 days later and traversed microsurgically to the contralateral leg vessels. Histologic analysis was performed.

RESULTS

All 30 capsules were completely vascularized and could be reliably isolated and transferred microsurgically on the transposed femoral vessels. The pedicle, being incorporated directly into the capsule, provided the dominant blood supply to the construct. None of the capsules with the fibrin glue only retained any shape and all were devoid of cartilage. Similarly, there was no evidence of retained cartilage in the capsules filled with chondrocytes alone. All capsules with the chondrocytes and the fibrin carrier had mature shaped cartilage preserved. External molds were required to maintain the shape of the ear. Extrusion, although almost uniform in the group with external molds, did not interfere with the end construct shape or vascularity. When molds were used, four of six had excellent maintenances of shape and two of six had only minor superior pole deformation. All constructs were reliably transferred as free flaps.

CONCLUSIONS

The authors have shown that transposing a vascular pedicle to a subcutaneously placed silicone block will result in a vascular capsule that can be mobilized and transferred based solely on the pedicle. Although the capsule provides vascularity to the chondrocytes, the cultured cartilage will fill the shape of the silicone mold only if an appropriate carrier such as fibrin glue is used and an external mold is applied.

A “room-temperature” injection molding/particulate leaching approach for fabrication of biodegradable three-dimensional porous scaffolds

A "room-temperature" injection molding approach combined with particulate leaching (RTIM/PL) has been, for the first time, developed in this work to fabricate three-dimensional porous scaffolds composed of biodegradable polyesters for tissue engineering. In this approach, a "wet" composite of particulate/polymer/solvent was used in processing, and thus the injection was not performed at melting state. Appropriate viscosity and flowability were facilely obtained at a certain solvent content so that the composite was able to be injected into a mould under low pressure at room temperature, which was very beneficial for avoiding thermal degradation of polyesters. As a demonstration, tubular and ear-shaped porous scaffolds were fabricated from biodegradable poly(D,L-lactide-co-glycolide) (PLGA) by this technology. Porosities of the resulting scaffolds were as high as 94%. The pores were well interconnected. Besides the well-known characteristics of injection molding to be suitable for automatization of a fabrication process with high repeatability and precision, this RTIM/PL approach is much suitable for tailoring highly porous foams with its advantages flexible for shaping complicated scaffolds, free of thermal degradation and high-pressure machine, etc.

Electrospun nanofibres: Biomedical applications

Synthetic and semi-synthetic polymeric materials were originally developed for their durability and resistance to all forms of degradation, including biodegradation. Nanotechnology has the potential to revolutionize many sectors, including pharmaceuticals, information technology, medical devices, materials science, chemicals, and energy. Nanofibres provide a connection between the nanoscale world and the macroscale world, since their diameters are in the range of 1 to 100 nanometres and several metres in length. Therefore, the current emphasis of research is to exploit such properties and focus on determining appropriate conditions for electrospinning various polymers and biopolymers for eventual applications including: multifunctional membranes; biomedical structural elements (scaffolds used in tissue engineering, wound dressing, drug delivery, artificial organs, vascular grafts); protective shields in specialty fabrics; and filter media for submicron particles in the separation industry. This paper reviews the research on recent biomedical applications of electrospun nanofibres.

Current concepts in periodontal bioengineering

Repair of tooth supporting alveolar bone defects caused by periodontal and peri-implant tissue destruction is a major goal of reconstructive therapy. Oral and craniofacial tissue engineering has been achieved with limited success by the utilization of a variety of approaches such as cell-occlusive barrier membranes, bone substitutes and autogenous block grafting techniques. Signaling molecules such as growth factors have been used to restore lost tooth support because of damage by periodontal disease or trauma. This paper will review emerging periodontal therapies in the areas of materials science, growth factor biology and cell/gene therapy. Several different polymer delivery systems that aid in the targeting of proteins, genes and cells to periodontal and peri-implant defects will be highlighted. Results from preclinical and clinical trials will be reviewed using the topical application of bone morphogenetic proteins (BMP-2 and BMP-7) and platelet-derived growth factor-BB (PDGF) for periodontal and peri-implant regeneration. The paper concludes with recent research on the use of ex vivo and in vivo gene delivery strategies via gene therapy vectors encoding growth promoting and inhibiting molecules (PDGF, BMP, noggin and others) to regenerate periodontal structures including bone, periodontal ligament and cementum.

Tissue engineering-based cartilage repair with allogenous chondrocytes and gelatin-chondroitin-hyaluronan tri-copolymer scaffold: a porcine model assessed at 18, 24, and 36 weeks

We previously showed that cartilage tissue can be engineered in vitro with porcine chondrocytes and gelatin/chondoitin-6-sulfate/hyaluronan tri-copolymer which mimic natural cartilage matrix for use as a scaffold. In this animal study, 15 miniature pigs were used in a randomized control study to compare tissue engineering with allogenous chondrocytes, autogenous osteochondral (OC) transplantation, and spontaneous repair for OC articular defects. In another study, 6 pigs were used as external controls in which full thickness (FT) and OC defects were either allowed to heal spontaneously or were filled with scaffold alone. After exclusion of cases with infection and secondary arthritis, the best results were obtained with autogenous OC transplantation, except that integration into host cartilage was poor. The results for the tissue engineering-treated group were satisfactory, the repair tissue being hyaline cartilage and/or fibrocartilage. Spontaneous healing and filling with scaffold alone did not result in good repair. With OC defects, the subchondral bone plate was not restored by cartilage tissue engineering. These results show that tri-copolymer can be used in in vivo cartilage tissue engineering for the treatment of FT articular defects.

Controlling alginate gel degradation utilizing partial oxidation and bimodal molecular weight distribution

Degradability is often a critical property of materials utilized in tissue engineering. Although alginate, a naturally derived polysaccharide, is an attractive material due to its biocompatibility and ability to form hydrogels, its slow and uncontrollable degradation can be an undesirable feature. In this study, we characterized gels formed using a combination of partial oxidation of polymer chains and a bimodal molecular weight distribution of polymer. Specifically, alginates were partially oxidized to a theoretical extent of 1% with sodium periodate, which created acetal groups susceptible to hydrolysis. The ratio of low MW to high MW alginates used to form gels was also varied, while maintaining the gel forming ability of the polymer. The rate of degradation was found to be controlled by both the oxidation and the ratio of high to low MW alginates, as monitored by the reduction of mechanical properties and corresponding number of crosslinks, dry weight loss, and molecular weight decrease. It was subsequently examined whether these modifications would lead to reduced biocompatibility by culturing C2C12 myoblast on these gels. Myoblasts adhered, proliferated, and differentiated on the modified gels at a comparable rate as those cultured on the unmodified gels. Altogether, this data indicates these hydrogels exhibit tunable degradation rates and provide a powerful material system for tissue engineering.

Evaluation of vacuum and dynamic cell seeding of polyglycolic acid and chitosan scaffolds for cartilage engineering

OBJECTIVES

To compare combined vacuum and rotation with the spinner flask technique for seeding chondrocytes on chitosan versus polyglycolic acid matrices.

SAMPLE POPULATION

Porcine chondrocytes.

PROCEDURE

A suspension containing 5 X 10(6) chondrocytes/scaffold was used to evaluate 2 seeding techniques, including a spinner flask and a custom-designed vacuum chamber used for 2 hours prior to transfer to a bioreactor. For each seeding technique, prewetted scaffolds were composed of polyglycolic acid (PGA) mesh or macroporous chitosan sponge. Constructs were collected at 48 hours for DNA quantification, measurement of water and gycosaminoglycan (GAG) content, and scanning electron microscopy.

RESULTS

Yield of both seeding techniques was similar for each type of scaffold. Percentage of cells contained in the center of PGA constructs was increased with seeding in the bioreactor (43% of total cell number), compared with the spinner flask (18%). The DNA content and cell number per construct were 10 times greater for PGA constructs, compared with chitosan constructs. Chitosan scaffolds seeded in the bioreactor yielded a significantly higher GAG:DNA ratio than did PGA scaffolds. Whereas chondrones formed on chitosan scaffolds, cell distribution was more uniform on PGA scaffolds.

CONCLUSIONS AND CLINICAL RELEVANCE

The vacuum-bioreactor technique allowed seeded chondrocytes to attach to PGA scaffolds within 48 hours and improved uniformity of cell distribution, compared with the spinner technique. Although formation of extracellular matrix may be stimulated by seeding chitosan scaffolds in the bioreactor, further evaluations of the seeding technique and characteristics of chitosan scaffolds are warranted.

Animal models for cartilage reconstruction

Animal models are widely used to develop and evaluate tissue-engineering techniques for the reconstruction of damaged human articular cartilage. For the purpose of this review, these model systems will include in vitro culture of animal cells and explants, heterotopic models of chondrogenesis, and articular cartilage defect models. The objectives for these preclinical studies are to engineer articular cartilage for the functional restoration of a joint surface that appears anatomically, histologically, biologically, biochemically, and mechanically to resemble the original joint surface. While no animal model permits direct application to humans, each is capable of yielding principles on which decisions can be made that might eventually translate into a human application. Clearly, the use of animal models has and will continue to play a significant role in the advancement of this field. Each animal model has specific advantages and disadvantages. The key issue in the selection of an appropriate animal model is to match the model to the question being investigated and the hypothesis to be tested. The purpose of this review is to discuss issues regarding animal model selection, the benefits and limitations of these model systems, scaffold selection with emphasis on polymers, and evaluation of the tissue-engineered articular cartilage.

Design of porous scaffolds for cartilage tissue engineering using a three-dimensional fiber-deposition technique

In this study, we present and characterize a fiber deposition technique for producing three-dimensional poly(ethylene glycol)-terephthalate-poly(butylene terephthalate) (PEGT/PBT) block co-polymer scaffolds with a 100% interconnecting pore network for engineering of articular cartilage. The technique allowed us to "design-in" desired scaffold characteristics layer by layer by accurately controlling the deposition of molten co-polymer fibers from a pressure-driven syringe onto a computer controlled x-y-z table. By varying PEGT/PBT composition, porosity and pore geometry, 3D-deposited scaffolds were produced with a range of mechanical properties. The equilibrium modulus and dynamic stiffness ranged between 0.05-2.5 and 0.16-4.33 MPa, respectively, and were similar to native articular cartilage explants (0.27 and 4.10 MPa, respectively). 3D-deposited scaffolds seeded with bovine articular chondrocytes supported a homogeneous cell distribution and subsequent cartilage-like tissue formation following in vitro culture as well as subcutaneous implantation in nude mice. This was demonstrated by the presence of articular cartilage extra cellular matrix constituents (glycosaminoglycan and type II collagen) throughout the interconnected pore volume. Similar results were achieved with respect to the attachment of expanded human articular chondrocytes, resulting in a homogeneous distribution of viable cells after 5 days dynamic seeding. The processing methods and model scaffolds developed in this study provide a useful method to further investigate the effects of scaffold composition and pore architecture on articular cartilage tissue formation.

Expansion of bovine chondrocytes on microcarriers enhances redifferentiation

Functional cartilage implants for orthopedic surgery or in vitro tissue evaluation can be created from expanded chondrocytes and biodegradable scaffolds. Expansion of chondrocytes in two-dimensional culture systems results in their dedifferentiation. The hallmark of this process is the switch of collagen synthesis from type II to type I. The aim of this study was to evaluate the postexpansion chondrogenic potential of microcarrier-expanded bovine articular chondrocytes in pellet cultures. A selection of microcarriers was screened for initial attachment of chondrocytes. On the basis of those results and additional selection criteria related to clinical application, Cytodex-1 microcarriers were selected for further investigation. Comparable doubling times were obtained in T-flask and microcarrier cultures. During propagation on Cytodex-1 microcarriers, cells acquired a spherical-like morphology and the presence of collagen type II was detected. Both observations are indicative of a differentiated chondrocyte. Pellet cultures of microcarrier-expanded cells showed cartilage-like morphology and staining for proteoglycans and collagen type II after 14 days. In contrast, pellets of T-flask-expanded cells had a fibrous appearance and showed abundant staining only for collagen type I. Therefore, culture of chondrocytes on microcarriers may offer useful and cost-effective cell expansion opportunities in the field of cartilage tissue engineering.

Preincubation of tissue engineered constructs enhances donor cell retention

Cartilage tissue engineering has been the focus of considerable research. However, the fate of transplanted donor cells rarely is explored directly. In the current study, the effect of preincubating perichondrial cells into a polylactic acid scaffold before implantation into an osteochondral defect was studied. The extracellular matrix produced during preincubation was characterized; the viability of the donor cells was assessed; and the retention of the donor cells in the repair tissue was determined using a gene marker on the Y chromosome, the gender-determining region Y gene. During in vitro incubation, the cells produced an extracellular matrix consisting of glycosaminoglycans, and Types I and II collagen, and the cell viability remained great. In vivo, preincubated constructs had significantly greater retention of donor cells in the host repair tissue in the short term when compared with nonincubated controls. This study shows the value of preincubating engineered constructs before implantation, and additionally validates the gender-determining region Y gene as an effective tool for assessing the fate of donor cells in cartilage tissue engineering.

Thermoreversible hydrogel scaffolds for articular cartilage engineering

Articular cartilage has limited potential for repair. Current clinical treatments for articular cartilage damage often result in fibrocartilage and are associated with joint pain and stiffness. To address these concerns, researchers have turned to the engineering of cartilage grafts. Tissue engineering, an emerging field for the functional restoration of articular cartilage and other tissues, is based on the utilization of morphogens, scaffolds, and responding progenitor/stem cells. Because articular cartilage is a water-laden tissue and contains within its matrix hydrophilic proteoglycans, an engineered cartilage graft may be based on synthetic hydrogels to mimic these properties. To this end, we have developed a polymer system based on the hydrophilic copolymer poly(propylene fumarate-co-ethylene glycol) [P(PF-co-EG)]. Solutions of this polymer are liquid below 25 degrees C and gel above 35 degrees C, allowing an aqueous solution containing cells at room temperature to form a hydrogel with encapsulated cells at physiological body temperature. The objective of this work was to determine the effects of the hydrogel components on the phenotype of encapsulated chondrocytes. Bovine articular chondrocytes were used as an experimental model. Results demonstrated that the components required for hydrogel fabrication did not significantly reduce the proteoglycan synthesis of chondrocytes, a phenotypic marker of chondrocyte function. In addition, chondrocyte viability, proteoglycan synthesis, and type II collagen synthesis within P(PF-co-EG) hydrogels were investigated. The addition of bone morphogenetic protein-7 increased chondrocyte proliferation with the P(PF-co-EG) hydrogels, but did not increase proteoglycan synthesis by the chondrocytes. These results indicate that the temperature-responsive P(PF-co-EG) hydrogels are suitable for chondrocyte delivery for articular cartilage repair.

The use of a novel PLGA fiber/collagen composite web as a scaffold for engineering of articular cartilage tissue with adjustable thickness

It has been a great challenge to make the thickness of engineered cartilage adjustable to cover the range of both partial-thickness and full-thickness articular cartilage defects. We developed a novel kind of composite web scaffold that could be used for tissue enginnering of articular cartilage with the thickness adjustable between 200 microm and 8 mm. The composite web showed a unique structure having web-like collagen microsponges formed in the openings of a mechanically strong knitted mesh of poly(lactic-co-glycolic acid). The knitted mesh served as a skeleton reinforcing the composite web, while the web-like collagen microsponges facilitated cell seeding, cell distribution, and tissue formation. Bovine chondrocytes cultured in the composite web showed a spatially even distribution, maintained their natural morphology, and produced cartilaginous extracellular matrices such as type II collagen and aggrecan. The thickness of the implant can be simply adjusted by laminating or rolling the web sheets. Not only did the histological structure of the engineered cartilage patches match the bovine native articular cartilage, but also their dynamic complex modulus, structural stiffness, and phase lag reached 37.8, 57.0, and 86.3% of those of native bovine articular cartilage, respectively. The composite web could be an important scaffold for tissue engineering.

Tissue engineering and cell therapy of cartilage and bone

Trauma and disease of bones and joints, frequently involving structural damage to both the articular cartilage surface and the subchondral bone, result in severe pain and disability for millions of people worldwide and represent major challenges for the orthopedic surgeons. Therapeutic repair of skeletal tissues by tissue engineering has raised the interest of the scientific community, providing very promising results in preclinical animal models and clinical pilot studies. In this review, we discuss this approach. The choice of a proper cell type is addressed. The use of terminally differentiated cells, as in the case of autologous chondrocyte implantation, is compared with the advantages/disadvantages of using more undifferentiated cell types, such as stem cells or early mesenchymal progenitors that retain multi-lineage and self-renewal potentials. The need for proper scaffold matrices is also examined, and we provide a brief overview of their fundamental properties. A description of the natural and biosynthetic materials currently used for reconstruction purposes, either of cartilage or bone, is given. Finally, we highlight the positive aspects and the remaining problems that will drive future research in articular cartilage and bone repair.

Articular cartilage bioreactors and bioprocesses

This review summarizes the major approaches for developing articular cartilage, using bioreactors and mechanical stimuli. Cartilage cells live in an environment heavily influenced by mechanical forces. The development of cartilaginous tissue is dependent on the environment that surrounds it, both in vivo and in vitro. Chondrocytes must be cultured in a way that gives them the proper concentration of nutrients and oxygen while removing wastes. A mechanical force must also be applied during the culturing process to produce a phenotypically correct tissue. Four main types of forces are currently used in cartilage-culturing processes: hydrostatic pressure, direct compression, "high"-shear fluid environments, and "low"-shear fluid environments. All these forces have been integrated into culturing devices that serve as bioreactors for articular cartilage. The strengths and weaknesses of each device and stimulus are explored, as is the future of cartilage bioreactors.