Photo-Cross-Linking of Type I Collagen Gels in the Presence of Smooth Muscle Cells: Mechanical Properties, Cell Viability, and Function William T. Brinkman, ² Karthik Nagapudi, ² Benjamin S. Thomas, ² and Elliot L. Chaikof* ,²,‡ Departments of Surgery and Biomedical Engineering, Emory University School of Medicine, Atlanta, Georgia 30322, and School of Chemical Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332 Received November 23, 2002; Revised Manuscript Received March 19, 2003 The effectiveness of photomediated cross-linking of type I collagen gels in the presence of rat aortic smooth muscle cells (RASMC) as a method to enhance gel mechanical properties while retaining native collagen triple helical structure and maintaining high cell viability was investigated. Collagen was chemically modified to incorporate an acrylate moiety. Collagen methacrylamide was cast into gels in the presence of a photoinitiator along with RASMC. The gels were cross-linked using visible light irradiation. Neither acrylate modification nor the cross-linking reaction altered collagen triple helical content. The cross-linking reaction, however, moved the denaturation temperature beyond the physiologic range. A twelve-fold increase in shear modulus was observed after cross-linking. Cell viability in the range of 70% (n ) 4, p > 0.05) was observed in the photo-cross-linked gels. Moreover the cells were able to contract the cross-linked gel in a manner commensurate with that observed for natural type I collagen. Methacrylate-mediated photo-cross-linking is a facile route to improve mechanical properties of collagen gels in the presence of cells while maintaining high cell viability. This enhances the potential for type I collagen gels to be used as scaffolds for tissue engineering. Introduction The failure of the current generation of commercially available synthetic vascular prostheses is related to maladap- tive biological reactions at both the blood-material and tissue-material interfaces. In response to these problems, alternative strategies have endeavored to design an arterial prosthesis through the mimicry of some or all of the characteristics of the arterial wall. For example, several investigators have sought to develop a functional endothelial monolayer on the luminal surface of a synthetic vascular prosthesis by the transplantation of autologous or allogeneic endothelial cells onto the prosthetic surface prior to graft implantation. 1-4 Likewise, alternative approaches have fo- cused on the development of synthetic prostheses that contain angiogenic factors in order to regenerate an endothelial lining after graft implantation. Although promising results have been reported with both strategies, vascular grafts that are composed, either in whole or in part, from synthetic polymeric materials remain at risk for bacterial colonization and subsequent graft infection and, in addition, are capable of promoting a low-level, chronic inflammatory response that may contribute to the development of neointimal hyperpla- sia. 5 Moreover, a “compliance mismatch” that characteristi- cally exists between high modulus prosthetic grafts and the host artery may also lead to neointimal hyperplasia and late graft failure. 6,7 Thus, the inherent limitations of a biohybrid prosthesis that consists, in part, of nonbiological elements have motivated investigators to develop arterial constructs that are comprised exclusively of biological components. In 1986, Weinberg and Bell 8 generated an arterial construct consisting of a cell populated collagen gel. Although cell- mediated reorganization of the surrounding collagen matrix enhanced the mechanical integrity of the construct, this model did not display adequate mechanical properties necessary for in vivo applications. In response to this limitation, other investigators have sought to improve the mechanical char- acteristics of tissue engineered media/advenitial equivalents by the introduction of techniques that promote cell-assisted matrix protein assembly or endogenous collagen cross- linking. The latter approach has been pursued primarily by the supplementation of culture media with ascorbic acid or ribose. 9-13 Despite encouraging results, months of incubation are required before any of these constructs are suitable for implantation. To accelerate the time period for the maturation of a mechanically robust media equivalent, we postulated that the introduction of photochemically cross-linkable moieties into native collagen might facilitate rapid and controllable matrix protein cross-linking. We report, herein, visible light mediated photo-cross-linking of smooth muscle cell containing collagen matrixes as a mechanism to improve the mechanical properties of tissue engineered media equiva- lents. * To whom correspondence should be addressed. Elliot L. Chaikof, M. D., Ph.D. 1639 Pierce Drive 5105 WMB, Emory University, Atlanta, GA 30322. Phone: (404) 727-8413. Fax: (404) 727-3660. E-mail: echaiko@ emory.edu. ² Emory University School of Medicine. ‡ Georgia Institute of Technology. 890 Biomacromolecules 2003, 4, 890-895 10.1021/bm0257412 CCC: $25.00 © 2003 American Chemical Society Published on Web 05/07/2003