Inorganic–organic hybrid scaffolds for osteochondral regeneration Dany J. Munoz-Pinto, 1 * Rebecca E. McMahon, 1 * Melissa A. Kanzelberger, 1 Andrea C. Jimenez-Vergara, 2 Melissa A. Grunlan, 3 Mariah S. Hahn 1,3 1 Department of Chemical Engineering, Texas A&M University, College Station, Texas 77843 2 Department of Materials Science and Engineering, Texas A&M University, College Station, Texas 77843 3 Department of Biomedical Engineering, Texas A&M University, College Station, Texas 77843 Received 11 May 2009; revised 11 August 2009; accepted 26 August 2009 Published online 2 February 2010 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jbm.a.32695 Abstract: Ligament graft failure frequently results from poor integration of the replacement tissue with associated bone. Thus, the ability to regenerate the bone-ligament osteochondral interface would be advantageous in liga- ment reconstruction. At the osteochondral interface, the tis- sue transitions from a bone-like matrix to fibrocartilage. Therefore, a scaffold which promotes a spatially regulated transition in cell behavior from osteoblast-like to chondro- cyte-like would be desirable. Previous research indicates that addition of inorganic components to organic scaffolds can enhance the deposition of bone-like matrix by associ- ated osteoblasts. We therefore reasoned that a gradient in the inorganic content of a hybrid inorganic–organic scaf- fold may induce an osteochondral-like transition in cell phenotype and matrix production. To test this hypothesis, hydrogels were prepared from poly(ethylene glycol) (PEG) and star poly(dimethylsiloxane) (PDMS star ). As anticipated, both the matrix deposition and phenotype of encapsulated osteoblasts varied with scaffold inorganic content, although the directionality of this modulation was contrary to expectation. Specifically, osteoblasts appeared to trans- differentiate into chondrocyte-like cells with increasing scaffold inorganic content, as indicated by increased chon- droitin sulfate and collagen type II production and by up- regulation of sox9, a transcription factor associated with chondrocytic differentiation. Furthermore, the deposition of bone-like matrix (collagen type I, calcium phosphate, and osteocalcin) decreased with increasing PDMS star con- tent. The resistance of the PDMS star -PEG scaffolds to pro- tein adsorption and/or the changes in gel modulus/mesh structure accompanying PDMS star incorporation may underlie the unexpected increase in chondrocytic pheno- type with increasing inorganic content. Combined, the present results indicate that PDMS star -PEG hybrid gels may prove promising for osteochondral regeneration. Ó 2010 Wiley Periodicals, Inc. J Biomed Mater Res 94A: 112–121, 2010 Key words: ligament tissue engineering; osteochondral interface; inorganic–organic scaffolds INTRODUCTION The cruciate ligaments of the human knee with- stand a variety of tensile and torsional forces during the course of normal daily routine and athletic activ- ity. Damage can result in pain, joint instability and dysfunction, and eventual degenerative joint disease. 1 As such, 150,000 surgical procedures are performed to treat injured anterior cruciate ligaments (ACLs) each year in the United States alone. 2 Currently, auto- logous tissue is the preferred graft material for ACL reconstruction. 3 However, the limited supply of auto- logous tissue suitable for grafting and the risk of do- nor site morbidity complicate the use of autologous grafts. 4 Tissue engineering is an alternative approach for ligament repair that may avoid many of limita- tions associated with autografts. Although rapid progress has been achieved in lig- ament tissue engineering over the past decade, engi- neered ligaments have, thus far, generally failed to achieve mechanical properties sufficiently similar to their native counterparts to serve as viable grafts. 5 In addition, these engineered tissues usually lack the osteochondral interface critical to the appropriate transfer of load between ligament and bone. 5 As graft failure frequently results from poor integration of the replacement tissue with associated bone, 6 the ability to regenerate the bone-ligament osteochondral interface would be advantageous in ligament recon- struction. *These authors contributed equally to this work. Correspondence to: M. S. Hahn; e-mail: mhahn@tamu. edu Contract grant sponsor: Texas Engineering Experimental Station Ó 2010 Wiley Periodicals, Inc.