Byung-Soo Kim á Carlos E. Baez á Anthony Atala Biomaterials for tissue engineering Abstract Biomaterials play a critical role in the engi- neering of new functional genitourinary tissues for the replacement of lost or malfunctioning tissues. They provide a temporary scaolding to guide new tissue growth and organization and may provide bioactive signals (e.g., cell-adhesion peptides and growth factors) required for the retention of tissue-speci®c gene expres- sion. A variety of biomaterials, which can be classi®ed into three types ± naturally derived materials (e.g., col- lagen and alginate), acellular tissue matrices (e.g., blad- der submucosa and small-intestinal submucosa), and synthetic polymers [e.g., polyglycolic acid, polylactic acid, and poly(lactic-co-glycolic acid)] ± have proved to be useful in the reconstruction of a number of genito- urinary tissues in animal models. Some of these mate- rials are currently being used clinically for genitourinary applications. Ultimately, the development or selection of appropriate biomaterials may allow the engineering of multiple types of functional genitourinary tissues. Lost or malfunctioning genitourinary tissues have tra- ditionally been reconstructed with native nonurologic tissues (e.g., gastrointestinal segments [6, 9], skin [30], peritoneum [40], fascia [67], omentum [34], and dura [44, 45]) or synthetic prostheses (silicone [19, 37, 88], poly- vinyl [56, 92], and Te¯on [20, 52, 70, 79, 80]). Although reconstructive therapies using these materials have saved and improved countless lives, they remain imperfect solutions. Reconstruction with nonurologic native tis- sues rarely replaces the entire function of the original tissue and bears the risk of complications, including metabolic abnormalities, infection, perforation, and malignancy [5, 47, 58, 62]. Furthermore, the limited amount of autologous donor tissue con®nes these types of reconstruction. The use of synthetic prostheses has usually failed due to the wide array of complications (e.g., device malfunction, infection, and stone forma- tion) associated with mechanical or biocompatibility problems [4]. Tissue engineering has emerged as a potential alter- native to the current therapies for genitourinary tissue reconstruction [50]. In this approach, new functional genitourinary tissues are reconstructed by transplanta- tion of cells using biocompatible biomaterials or by in- duction of tissue ingrowth from the surrounding tissue onto the biomaterials. Biomaterials play a central role in the engineering of functional genitourinary tissues. In this article we discuss the design criteria as well as the types of biomaterial that serve as scaolds for the en- gineering of genitourinary tissues and review the current concepts of genitourinary tissue engineering using vari- ous types of biomaterials. Importance of biomaterials In genitourinary-tissue engineering, biomaterials func- tion as an arti®cial extracellular matrix (ECM) and elicit biologic and mechanical functions of native ECM found in tissues in the body. Native ECM brings cells together into tissue, controls the tissue structure, and regulates the cell phenotype [1]. Biomaterials facilitate the local- ization and delivery of cells and/or bioactive factors (e.g., cell-adhesion peptides and growth factors) to desired sites in the body, de®ne a three-dimensional space for the formation of new tissues with appropriate structure, and guide the development of new tissues with appropriate function [48]. Direct injection of cell suspensions without biomaterial matrices has been used in some cases [21, 81], but it is dicult to control the World J Urol (2000) 18: 2±9 Ó Springer-Verlag 2000 B.-S. Kim á C. E. Baez á A. Atala (&) Laboratory for Tissue Engineering and Cellular Therapeutics, Department of Urology, Children's Hospital and Harvard Medical School, 300 Longwood Avenue, Boston, MA 02115, USA e-mail: atala@a1.tch.harvard.edu Tel: +1-617-3556169; Fax: +1-617-3556587