So Hyun Chung
Investigation of scaffold materials for cartilage tissue engineering
Cartilage is one of the expectative targets for tissue engineering due to its limitation in self-repair capacity. The lack of a sufficient supply of healthy chondrocytes to the defective sites or the low productivity of matrices in regenerated chondrocytes seem to cause the difficulty in the self-repair of cartilage. Cartilage tissue engineering could overcome such limitations by using ex vivo culture techniques and supportive artificial materials.
In cartilage tissue engineering, the chondrocytes isolated from their original tissues need to be conditioned in a 3D environment, mimicking the physiological situation with favorable scaffolds in order to reproduce their functions and enhance protein synthesis. The scaffold provides the seeded cells with the space for function and supports their activities. Because every scaffold possesses specific properties that fit some kinds of cells or mimic the interested tissues in mechanical strength, we should choose the optimal scaffold based on the characteristics of the target cells and tissues.
The scaffold can be a naturally occurring tissue, such as collagen or it may be a synthetic polymer. Naturally occurring substances have the advantages of known biocompatibility and fewer regulatory constraints, but autograft material leads to an additional surgical wound. Also, allograft tissue can transmit disease including AIDS and hepatitis. Synthetic polymers can be made in different shapes and sizes or as an injectable substance. It is possible to manipulate polymers to change their mechanical strength, modulus of elasticity, cell adhesiveness, porosity, degradation rate, and mode of degradation (surface or bulk eroding) to tailor their properties to the clinical application. The scaffolds can be synthesized with factors or drugs bonded directly to the polymer structure or encapsulated into microspheres that reside within pores in its structure.[A]
"A cartilage defect must be filled with some scaffold material to allow organized repair to occur. Additional bioactive factors could be added to the construct to enhance the process of regeneration. When the cartilage defect is a discrete, contained lesion, strategies involving an injectable polymer are desirable because they can be done arthroscopically or with other minimally invasive techniques. If a large portion of the joint surface is destroyed, however, a technique using a preformed, shape-specific polymer will most likely prove to be more successful."[A]
Here, I will compare different kinds of scaffold materials for tissue engineered cartilage.
Collagen type I gel, atelopeptides of collagen, fibrin glue, gelatin, agarose, or alginate,[7,8] are classified as hydrogel. Although the hydrogel lacks mechanical strength in itself, it has advantages that it can be mixed with cells and can surround the seeded cells in all directions. Many previous articles reported that those hydrogel materials were more effective in retaining chondrocyte functions or promoting matrix synthesis, when compared with monolayer culture. [1,3,6,9]
On this website, I will study three materials: atelopeptides of collagen, alginate and PuraMatrix¢â based on the paper of Yamaoka et al. Atelopeptides of collagen and alginate have been used as materials for medical devices. They have been used widely for the 3D culture studies of chondrocytes because the atelopeptide collagen spontaneously polymerizes into a stable gel at neutral pH and physiological temperature, and the alginate undergoes instant ionotrophic gelation in the presence of divalent cations. In addition to these two materials, a composition of systhetic peptides, PuraMatrix¢â is considered as a potential candidate for a clinically available hydrogel scaffold. In PuraMatrix¢â, the motif RAD was incorporated to mimic the known cell adhesion motif RGD that is found in many ECM proteins. It assembles to form an in vivo-like 3D extracellular matrix hydrogel around 37¡ÆC at pH 7, suggesting characteristics similar to the atelopeptide collagen. This has been used for research on hepatic regeneration or nerve regeneration and has been reported to promote neurite outgrowth and synaptic formation in neural cells, as well as functional differentiation in hepatocyte-like cells.[11,12] At present, clinical trials of the PuraMatrix¢â for use in the orthopedic field have been planned by the suppliers.
As an experimental design in the paper of Yamaoka et al., human auricular chondrocytes were embedded within each hydrogel material at the identical concentration of 0.5% by weight. They examined the proliferation ability of the chondrocytes at Passage 2, which should be more expanded when large sizes of engineered tissues would be made. The cell/hydrogel constructs at low cell density (2 x 10000 cells/mL) were incubated in a medium containing 10% fetal bovine serum (FBS). To evaluate matrix synthesis, they used the dedifferentiated chondrocytes of Passage 4, and made cell/hydrogel constructs at high cell density (10,000,000 cells/mL). The constructs were incubated with BMP(Bone morphogenetic protein)-2 and insulin, either of which was reported to induce the redifferentiation of chondrocytes or effectively enhance matrix synthesis.[14,15] The properties of hydrogel or cell/hydrogel constructs were examined cytologically, biochemically, histologically, and biomechanically. Moreover, to clarify the molecular effects of each hydrogel material, cell-matrix interactions or cell-to-cell contacts were examined through the gene expression pattern of beta-1 integrin or N-cadherin, both of which are major adhesion molecules in chondrocytes or their progenitors. [1,16,17]
Alginate ( www-biol.paisley.ac.uk/.../ Chapter1/alginate.htm )
PuraMatrix ( www.puramatrix.com/ technology/nanofibers.html )
At first, the effects of atelopeptide collagen, alginate, and PuraMatrix¢â on chondrocyte proliferation were examined. Human auricular chondrocytes of Passage 2 were encapsulated in the three kinds of hydrogel materials and cultured for 2 weeks.The chondrocytes in atelopeptide collagen proliferated at ~10-fold with 10% FBS, although no proliferation was seen without FBS. The cells rather decreased in number with or without FBS, in alginate or PuraMatrix¢â.
Gene expression of chondrocytes in each hydrogel
Next, they observed the effects of those hydrogel materials on the gene expression of chondrocytes when the dedifferentiated chondrocytes (Passage 4) were embedded in the hydrogel at a high cell density of 10,000,000 cells/mL. At 1 week after incubation, the expression of Col1A1 in all of hydrogel materials ranged between half and 2-fold of that in high density culture without any hydrogel (gel(-)), when they were incubated in medium without any factors (control). Because of the induction of redifferentiation by a medium containing insulin and BMP-2, the Col1A1expression in all hydrogels decreased, when compared with that in the control medium. The expression of Col2A1 was more than 100 times larger than that of gel(-), when the cells were embedded in every kind of hydrogel material, even in the control medium. With the redifferentiation medium, those in all hydrogel exhibited over a 105 fold increase, when compared with that of gel(-). Among the three kinds of materials, the expression of both Col1A1 and Col2A1 in PuraMatrix¢â tended to be high in the control medium, but response to the redifferentiation medium was rather low in Col2A1 expression.
To examine the molecular effects of the hydrogel on chondrocytes, they measured the gene expression of beta-1 integrin and N-cadherin, which play major roles in cell-matrix interactions and cell-to-cell contacts, respectively, in chondrocytes. At 1 week, beta-1 integrin was abundantly expressed in gel(-). Within all kinds of hydrogel materials, the beta-1 integrin expression was upregulated in the redifferentiation medium when compared with that in the control. beta-1 integrin of the atelopeptide collagen constructs showed the highest expression among three materials, not only in the redifferentiation media, but also in the control, suggesting abundant extracellular signaling through chondrocyte/collagen interaction. The enhancement of beta-1 integrin expression in both control and redifferentiation media seemed lower within PuraMatrix¢â than within atelopeptide collagen, although PuraMatrix ¢â abundantly contains the RAD motif, which is regarded as the analog of RGD, the major integrin ligand in collagen. Alginate did not enhance beta-1 in-tegrin expression, perhaps due to the lack of cell? matrix interactions. Although the expression of N-cadherin was enhanced in the gel(-), its expression decreased in all hydrogel materials. Particularly, the upregulation of N-cadherin expression was inhibited within alginate in both control and redifferentiation media, suggesting that alginate can maintain isolation of each chondrocyte. In the atelopeptide collagen, although both control and redifferentiation media promoted N-cadherin expression when compared with that in alginate, it was downregulated according to the induction of redifferentiation. Contrarily, the chondrocytes in PuraMatrix¢â rather increased the expression of N-cadherin, when the redifferentiation was induced.
Matrix synthesis of chondrocytes in each hydrogel
At 3 weeks after the incubation of chondrocytes at a high density (10,000,000/mL), gel(-) accumulated collagen type I, although it was diminished in encapsulation with each hydrogel. Moreover, the protein content of collagen type I was decreased in the atelopeptide collagen and in PuraMatrix¢â with the redifferentiation media, when compared with that in the control. The content of collagen type II or GAG in gel(-) and each hydrogel was hardly detectable in the control medium, but the redifferentiation medium significantly gained amounts of GAG and collagen type II, in all hydrogels. The accumulation of collagen type II was remarkable in the redifferentiation medium within the atelopeptide collagen hydrogel, while the GAG content was abundantly determined in alginate. Also in PuraMatrix¢â, the effects of the redifferentiation medium were noted on the synthesis of collagen type II and GAG, although the amount of both matrices was less than that in atelopeptide collagen or alginate.
The constructs of chondrocytes with encapsulation in atelopeptide collagen, alginate, and PuraMatrix¢â were examined histologically. The findings for PuraMatrix¢â showed a sparse and loose appearance in the middle area of the constructs, even in the redifferentiation medium, suggesting that the ability to support cells and newly synthesized matrices in PuraMatrix¢â was weak.
While the hydrogel of atelopeptide collagen or alginate was firm, the PuraMatrix¢â gel seemed rather soft when the hydrogel was probed by the tip of a microspatula. To examine mechanical properties of each hydrogel quantitatively, they measured Young¡¯s modulus by a Venustron tactile sensor. Each hydrogel reached maximal elasticity within 24 h of incubation in the culture media. Young¡¯s modulus of atelopeptide collagen was the highest (65.5 4.1 kPa), following those of alginate (36.3 +- 5.4 kPa) and PuraMatrix¢â (16.7 +- 1.0 kPa). 
The atelopeptides of type I collagen are a fiber protein made from a collagen, which is solubilized by protease. The atelopeptide collagen seems to possess the ligands for integrins, RGD. This cause more cell-matrix interaction which are crucial for cell survival and function. The expression of beta-1-integrin was enhanced in high cell density culture without any hydrogel, although it did not contain detectable amount of collagen type II and GAG. The reason why this method of culture did not accumulate cartilaginous matrices in spite of high beta-1-integrin expression may be that isolation of each chondrocytes was extremely diminished with cell-to-cell contacts increased, and that the 3D environment promoting chondrocyte activity could not be reproduced.
PuraMatrix¢â consists of repeated sequences of RAD. The motif RAD mimics the known cell adhesion motif RGD.Those abundant reactive motives may also transduce the extracellular signaling into chondrocytes and induce cell growth or the gene expression of a cartilage matrix marker, such as collagen type II, in PuraMatrix¢â. However, the proliferation of chondrocytes or the accumulation of cartilaginous matrices, collagen type II and GAG, was rather smaller in PuraMatrix¢â than in atelopeptide collagen or alginate. Histological findings also showed that the interior structure was sparse and loose in the cell/hydrogel construct using PuraMatrix¢â. One of the reasons can be the weakness of the gelling ability and preservability for chondrocytes and matrices produced by chondrocytes within the cell/hydrogel constructs. Indeed, the gel of PuraMatrix¢â showed the lowest Young¡¯s modulus among all hydrogel materials.
On the other hand, alginate forms a firm gel, however, it does not possess common adhesion molecules for mammalian cells. Because this kind of material hardly provides cell?matrix interactions for cells, extracellular signaling was reduced, resulting in the inhibition of chondrocyte proliferation. In contrast, a firm gel of alginate could preserve the isolation of each chondrocyte with cell-to-cell contacts decreased. Particularly, alginate helps the chondrocytes reduce cell-to-cell contacts and maintain the cell shape and function. These properties may enhance the expression and accumulation of cartilaginous matrices such as collagen type II and GAG, in the alginate constructs.
In spite of this property of alginate, the clinical use of alginate is not preferred yet because of uncertainty of immunoreactivity in long term usage. On the other hand, the atelopeptide collagen has been confirmed that it shows even lower immunogenicity than native collagen, because telopeptides determining the antigenicity are removed from the collagen in protease digestion. The atelopeptide collagen hydrogel showed some advantage over other materials for proliferation and matrix synthesis, especially collagen type II. Also in histology, the findings of cell/ hydrogel constructs using the atelopeptide collagen showed abundant matrices metachromatically stained with toluidine blue, embedding round-shaped and isolated chondrocytes, which resemble physiological cartilage tissues. Therefore, the atelopeptide collagen may be accessible for clinical use in cartilage tissue engineering from the standpoints of biological properties and clinical availability.
However, the atelopeptide collagen has been prepared from animals. Although it is quality-controlled as a medical device to prevent disease transmission or to maintain a aseptic state, the future risks for unknown disease transmission need to be figured out. On this point, synthetic materials will have merits, because they have big possibility of controlling contamination or immunoreactivity. Therefore, the expectation of synthetic peptides would increase as substitutes for materials originated from organism products. Increase in gelling ability could improve supportability of cells or matrices. Usage of transwell dishes may induce immediate neutralization of the PuraMatrix¢â and more rapid and firm gel formation. Some improvement of synthetic peptides would provide more useful hydrogel materials to create ideal regenerated cartilage. 
Thus, from now on, I will talk about synthetic polymer materials for scaffold.
References for Hydrogel part
 Yamaoka et al. CARTILAGE TISSUE ENGINEERING USING HUMAN AURICULAR CHONDROCYTES, Journal of Biomedical Materials Research Part A DOI 10.1002/jbm.a
 Chaipinyo K, Oakes BW, Van Damme MP. The use of debrided human articular cartilage for autologous chondrocyte implantation: Maintenance of chondrocyte differentiation and proliferation in type I collagen gels. J Orthop Res 2004;22:446-455.
 Uchio Y, Ochi M, Matsusaki M, Kurioka H, Katsube K. Human chondrocyte proliferation and matrix synthesis cultured in Atelocollagen gel. J Biomed Mater Res 2000;50:138 ?143.
 Ting V, Sims CD, Brecht LE, McCarthy JG, Kasabian AK, Connelly PR, Elisseeff J, Gittes GK, Longaker MT. In vitro prefabrication of human cartilage shapes using fibrin glue and human chondrocytes. Ann Plast Surg 1998;40:413- 420.
 Ibusuki S, Fujii Y, Iwamoto Y, Matsuda T. Tissue-engineered cartilage using an injectable and in situ gelable thermoresponsive gelatin: Fabrication and in vitro performance. Tissue Eng 2003;9:371-384.
 Benya PD, Shaffer JD. Dedifferentiated chondrocytes reexpress the differentiated collagen phenotype when cultured in agarose gels. Cell 1982;30:215-224.
 Stevens MM, Qanadilo HF, Langer R, Prasad Shastri V. A rapid-curing alginate gel system: Utility in periosteum-derived cartilage tissue engineering. Biomaterials 2004;25:887- 894.
 Gerard C, Catuogno C, Amargier-Huin C, Grossin L, Hubert P, Gillet P, Netter P, Dellacherie E, Payan E. The effect of alginate, hyaluronate and hyaluronate derivatives biomaterials on synthesis of non-articular chondrocyte extracellular matrix. J Mater Sci Mater Med 2005;16:541-551.
 Malemud CJ, Stevenson S, Mehraban F, Papay RS, Purchio AF, Goldberg VM. The proteoglycan synthesis repertoire of rabbit chondrocytes maintained in type II collagen gels. Osteoarthritis Cartilage 1994;2:29-41.
 Sakai D, Mochida J, Yamamoto Y, Nomura T, Okuma M, Nishimura K, Nakai T, Ando K, Hotta T. Transplantation of mesenchymal stem cells embedded in Atelocollagen gel to the intervertebral disc: A potential therapeutic model for disc degeneration. Biomaterials 2003;24:3531-3541.
 Holmes TC, de Lacalle S, Su X, Liu G, Rich A, Zhang S. Extensive neurite outgrowth and active synapse formation on self-assembling peptide scaffolds. Proc Natl Acad Sci USA 2000;97:6728-6733.
 Prieto AL, Edelman GM, Crossin KL. Multiple integrins mediate cell attachment to cytotactin/tenascin. Proc Natl Acad Sci USA 1993;90:10154 ?10158.
 Semino CE, Merok JR, Crane GG, Panagiotakos G, Zhang S. Functional differentiation of hepatocyte-like spheroid structures from putative liver progenitor cells in three-dimensional peptide scaffolds. Differentiation 2003;71:262?270.
 Grunder T, Gaissmaier C, Fritz J, Stoop R, Hortschansky P, Mollenhauer J, Aicher WK. Bone morphogenetic protein (BMP)-2 enhances the expression of type II collagen and aggrecan in chondrocytes embedded in alginate beads. Osteoarthritis Cartilage 2004;12:559 -567.
 Kato Y, Gospodarowicz D. Growth requirements of low-density rabbit costal chondrocyte cultures maintained in serumfree medium. J Cell Physiol 1984;120:354 ?363.
 Aszodi A, Hunziker EB, Brakebusch C, Fassler R. Beta1 integrins regulate chondrocyte rotation, G1 progression, and cytokinesis. Genes Dev 2003;17:2465-2479.
 Woodward WA, Tuan RS. N-Cadherin expression and signaling in limb mesenchymal chondrogenesis: Stimulation by polyl-lysine. Dev Genet 1999;24:178 ?187.
-Lu et al., Biodegradable Polymer Scaffolds for Cartilage Tissue Engineering, Clinical Orthopaedics and Related Research, No.391S, pp. S257-S261-
The most widely investigated synthetic biodegradable polymers for articular cartilage repair are poly(-hydroxy esters) such as PGA, PLA, and their copolymers PLGA.[A1] These materials have good biocompatibility and are approved by the Food and Drug Administration for certain human clinical uses such as bioresorbable surgical sutures, surgical screws, plates, and rods.[A2,A3,A4,A5] Random hydrolysis of the ester bonds in the polymer chain leads
to bulk degradation of the materials into lactic and glycolic acid, which are removed from the body by natural metabolic pathways.
The physical, mechanical, and degradative properties of the amorphous PLGA copolymers can be controlled by varying the copolymer ratio.[A1]
PLA - PGA bioresorbable polymer (www.cytoplast.com/ products.htm)
Cell and polymer composites based on chondrocytes and fibrous non-woven PGA scaffolds have been used extensively for articular cartilage tissue engineering.[A8-A26] In a short-term in vitro study, PGA was shown to be a suitable scaffold for articular cartilage regeneration, as seen by high rates of initial cell growth, maintenance of differentiated chondrocyte function, and secretion of an extracellular matrix similar to that of normal hyaline cartilage.[A27] The relatively fast degradation rate and high porosity are favorable for efficient cell to cell contact at high cell densities, which has been associated with high rates of extracellular matrix synthesis.[A15,A21] Neocartilage formed after in vitro culture of bovine chondrocytes on PGA scaffolds for as many as 12 weeks showed similar morphologic features and biomechanical properties as compared with native bovine cartilage.[A18] The low mechanical strength of these nonwoven fibrous discs, however, render them unsuitable for immediate implantation. Other sources of cells such as avian bone marrow stromal cells and mammalian bone marrow cells have been cultured on PGA scaffolds. The former resulted in the formation of constructs consisting of an organized extracellular matrix with high concentrations of GAG and collagen,[A20] whereas the latter constructs first contracted and then collapsed.[A29] Porous sponges fabricated from PLGA and PEG were found to be a better scaffold for mammalian bone marrow stromal cells by maintaining the construct size and shape during culture.[A29] Poly(glycolic acid) scaffolds have been used in vivo as carriers of rabbit articular cartilage chondrocytes to repair full-thickness osteochondral defects in knees in adult rabbits.[A10] The repair tissue induced by implanted chondrocyte and PGA constructs was qualitatively better than that formed using PGA scaffolds alone, as indicated by columnar orientation of chondrocytes, surface smoothness, uniform GAG distribution, and bonding of the repair tissue to the underlying bone. Other studies have reported poor cartilage healing for defects filled with scaffolds without cells.[A30,A23]
Chemical formula [repeating unit] of PLA ( www.abdn.ac.uk/physics/ px4007/2004/material.hti )
Cartilage repair also has been studied using PLA scaffolds.[A31-A35,A12,A39,A38,A23,A59] Chondrocytes cultured on PLA scaffolds in vitro also proliferated and produced GAG, although the cell growth and matrix synthesis were lower than on PGA meshes.[A12] Similar to PGA, PLA scaffolds seeded with chondrocytes were successful in healing cartilage in vivo, as assessed by morphologic, histologic, and biochemical properties of the repair tissue. Constructs combining periosteum and PLA were used to repair full-thickness osteochondral defects in the rabbit knee, resulting in the formation of hyalinelike neocartilage with a high content of Type II collagen.[A59] However, the biomechanical testing showed noticeable differences in aggregate modulus when compared with normal cartilage. Allogenic perichondrial cells seeded on PLA were used for treating osteochondral defects in a rabbit model.[A31-33] A high percentage of smooth, firm repair tissue with Type II collagen was present after 1 year of implantation. However, the poor subchondral bone reconstitution and a low GAG content were indicative of structurally abnormal neocartilage. The results obtained using autologous perichondral cells also were suboptimal.[A34,A35]
Chemical structure of PLGA polymer (
www.shu.ac.uk/research/ meri/pcas/pics/plga.gif )
Because of their low mechanical strength, the use of PLGA scaffolds for articular cartilage regeneration is limited.[A29] Nonarticular chondrocytes, cultured on PLGA scaffolds and implanted subcutaneously in nude mice, resulted in the formation of neocartilage that retained the shape of the original template.[A36, A37]
In addition to PGA, PLA, and PLGA, two dimensional polymer films of PCL, PLA/PCL, PGTMC, and PDO have been studied as substrates for the attachment and proliferation of human articular chondrocytes.[A38] A polymer sponge based on PLA/PCL has been explored for cartilage repair.[A39] A short-term study showed that chondrocyte-seeded constructs resulted in neocartilage formation when implanted subcutaneously.[A39] Among the injectable polymers, PEO gels combined with chondrocytes induced neocartilage formation after 12 weeks of subcutaneous injection in nude mice.[A40] To obtain sufficient mechanical strength of the formed scaffolds, PEO and chondrocytes were injected subcutaneously in athymic mice and photopolymerized transdermally.[A41,A42] Formation of neocartilage was shown 7 weeks after implantation.
Additionally, polymers of sebacic acid alone, or copolymers of sebacic acid and 1,3-bis(p-carboxyphenoxy) propane, or 1,6-bis(p-carboxyphenoxy) hexane, form a group of photopolymerizable poly(anhydrides).[A43-A46] The mechanical properties and degradation time of these materials are dependent on the choice of monomer(s). Poly(anhydrides) degrade by surface erosion, thus resulting in a more controlled release of degradation products as compared with bulk degrading polyesters.[A1] In addition, the bulk mechanical properties can be maintained during the degradation process. However, the dependence on light for polymeriza-tion creates an extra step during surgery and may limit the use of these materials in deep defects. Moreover, these surface-eroding polymers may not facilitate cell adhesion and formation of the cell and polymer constructs.
Polymers that can be cured chemically, usually through cross-linking rather than polymerization, eliminate the need for light and are advantageous for use in minimally invasive surgery. Many studies in this area have focused on a new class of biomaterials based on PPF.[A47,A48] Poly(propylene fumarate) is an unsaturated linear polyester that can be modified easily or cross-linked through its fumarate double bonds. By combining PPF with a vinyl monomer (Nvinyl pyrrolidone) and a radical initiator (benzoyl peroxide), a new injectable, in situ polymerizable, and biodegradable orthopaedic material has been developed for filling skeletal defects.[A49] Biodegradable macromers such as PPF-DA52 and PEG-DA51 also can serve as cross-linking agents. Additionally, by altering the composite formulation, the handling characteristics of the injectable paste can be modulated, creating a mixture that hardens within 5 to 10 minutes and cures at body temperature.[A49] Poly(propylene fumarate) degrades by simple hydrolysis into propylene glycol, poly(acrylic acid-co-fumaric acid), and fumaric acid, a naturally occurring substance in the body.[A50] The degradation time and mechanical properties of PPF can vary greatly depending on the polymer structure, synthesis methods, type and density of cross-linking agent, and the presence of other components such as beta-TCP in the composite materials.[A51,A52] Despite the bulk degradation mechanism, the compressive strength of PPF scaffolds could increase in the short-term because of continued cross-linking.[A52] Poly(propylene fumarate) and beta-TCP composites also were shown to encourage bone growth in vitro.[A53,A54] When implanted subcutaneously in rats, PPF does not show a deleterious long-term inflammatory response.[A51]
To modulate cell attachment and migration additionally, PPF was modified with a GRGD peptide sequence using tethered PEG as spacer.[A55] Block copolymers of PPF and PEG [P(PF-co-EG)] hydrogels have been developed for cardiovascular applications.[A56-A58] The properties of P(PF-co-EG) and PPF crosslinked with PEG-DA could be engineered by varying the ratio of the PEG block in the copolymer, the molecular weight of PEG, or the cross-linking density, and could be functionalized with covalently bound peptide sequences.
These materials are biocompatible and hold promise as scaffolds for use in cartilage tissue engineering.
References for Biodegradable Synthetic Polymers
[A] Lu et al., Biodegradable Polymer Scaffolds for Cartilage Tissue Engineering, Clinical Orthopaedics and Related Research, No.391S, pp. S251-S270
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[A8] Cao YL, Rodriguez A, Vacanti M, et al: Comparative study of the use of poly(glycolic acid), calcium alginate and pluronics in the engineering of autologous porcine cartilage. J Biomater Sci Polym Ed 9:475-487, 1998.
[A9] Dunkelman NS, Zimber MP, LeBaron RG, et al: Cartilage production by rabbit articular chondrocytes on polyglycolic acid scaffolds in a closed bioreactor system. Biotechnol Bioeng 46:299-305, 1995.
[A10] Freed LE, Grande DA, Lingbin Z, et al: Joint resurfacing using allograft chondrocytes and synthetic biodegradable polymer scaffolds. J Biomed Mater Res 28:891-899, 1994.
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[A12] Freed LE, Marquis JC, Nohria A, et al: Neocartilage formation in vitro and in vivo using cells cultured on synthetic biodegradable polymers. J Biomed Mater Res 27:11-23, 1993.
[A13] Freed LE, Marquis JC, Vunjak-Novakovik G, et al: Composition of cartilage-cell polymer implants.
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[A14] Freed LE, Martin I, Vunjak-Novakovic G: Frontiers in tissue engineering: In vitro modulation of chondrogenesis. Clin Orthop 367(Suppl):S46-S58, 1999.
[A15] Freed LE, Vunjak-Novakovic G, Langer R: Cultivation of cell-polymer cartilage implants in bioreactors. J Cell Biochem 51:257-264, 1993.
[A16] Grande DA, Halberstadt C, Naughton G, et al: Evaluation of matrix scaffolds for tissue engineering of articular cartilage grafts. J Biomed Mater Res 34:211-220, 1997.
[A17] Kim WS, Vacanti JP, Cima L, et al: Cartilage engineered in predetermined shapes employing cell transplantation on synthetic biodegradable polymers. Plast Reconstr Surg 94:233-237, 1994.
[A18] Ma PX, Schloo B, Mooney D, et al: Development of biomechanical properties and morphogenesis of in vitro tissue engineered cartilage. J Biomed Mater Res 29:1587-1595, 1995.
[A19] Martin I, Obradovic B, Treppo S, et al: Modulation of the mechanical properties of tissue engineered cartilage. Biorheology 37:141?147, 2000.
[A20] Martin I, Padera RF, Vunjak-Novakovic G, et al: In vitro differentiation of chick embryo bone marrow stromal cells into cartilaginous and bone-like tissues. J Orthop Res 16:181?189, 1998.
[A21] Puelacher WC, Kim SW, Vacanti JP, et al: Tissue engineered growth of cartilage: The effect of varying the concentration of chondrocytes seeded onto synthetic polymer matrices. Int J Oral Maxillofac Surg 23:49-53, 1994.
[A22] Stading M, Langer R: Mechanical shear properties of cell-polymer cartilage constructs. Tissue Eng 5:241?250, 1999.
[A23] Vacanti CA, Kim W, Schloo B, et al: Joint resurfacing with cartilage grown in situ from cellpolymer structures. Am J Sports Med 22:485-488, 1994.
[A24] Vunjak-Novakovic-G, Freed LE, Biron R, et al: Effects of mixing on tissue engineered cartilage. AIChE J 42:850-860, 1996.
[A25] Vunjak-Novakovic G, Martin I, Obradovic B, et al: Bioreactor cultivation conditions modulate the composition and mechanical properties of tissueengineered cartilage. J Orthop Res 17:130-138, 1999.
[A26] Wu F, Dunkelman N, Peterson A, et al: Bioreactor development for tissue-engineered cartilage. Ann N Y Acad Sci 18:405-411, 1999.
[A27] Freed LE, Vunjak-Novakovic G: Tissue Engineering of Cartilage. In Bronzino JD (ed). Biomedical Engineering. Hartford, CRC Press 1788-1806, 1995.
[A28] Freed LE, Vunjak-Novakovic G, Langer R: Cultivation of cell-polymer cartilage implants in bioreactors. J Cell Biochem 51:257?264, 1993.
[A29] Martin I, Shastri V, Padera RF, et al: Bone marrow stromal cell differentiation on porous polymer scaffolds. Trans Orthop Res Soc 24:57, 1999.
[A30] Grande D, Pitman MI, Peterson L, et al: The repair of experimentally produced defects in rabbit articular cartilage by autologous chondrocyte transplantation. J Orthop Res 7:208-218, 1989.
[A31] Chu CR, Coutts RD, Yoshioka M, et al: Articular cartilage repair using allogeneic perichondrocyteseeded biodegradable porous polylactic acid (PLA):
A tissue-engineering study. J Biomed Mater Res 29:1147-1154, 1995.
[A32] Chu CR, Dounchis JS, Yoshioka M, et al: Osteochondral repair using perichondrial cells: A 1-year study in rabbits. Clin Orthop 340:220-229, 1997.
[A33] Chu CR, Monosov AZ, Amiel D: In situ assessment of cell viability within biodegradable polylactic acid polymer matrices. Biomaterials 16:1381?1384,
[A34] Dounchis JS, Bae WC, Chen AC, et al: Cartilage repair with autogenic perichondrium cell and polylactic acid grafts. Clin Orthop 377:248-264, 2000.
[A35] Dounchis JS, Harwood FL, Bae WC, et al: Repair of osteochondral defects with perichondrocytepolylactic acid scaffold grafts: Autogenic vs. Allogenic cells. Trans Orthop Res Soc 22:550, 1997.
[A36] Puelacher WC, Mooney D, Langer R, et al: Design of nasoseptal cartilage replacements synthesized from biodegradable polymers and chondrocytes. Biomaterials 15:774-778, 1994.
[A37] Vacanti CA, Langer R, Schloo B, et al: Synthetic polymers seeded with chondrocytes provide a template for new cartilage formation. Plast Reconstr
Surg 88:753-759, 1991.
[A38] Ishaug-Riley SL, Okun LE, Prado G, et al: Human articular chondrocyte adhesion and proliferation on synthetic biodegradable polymer films. Biomaterials 20:2245-2256, 1999.
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