Microcarriers in the engineering of cartilage and bone Jos Malda 1 and Carmelita G. Frondoza 2,3 1 Tissue Repair and Regeneration Program, Institute of Health and Biomedical Innovation, Queensland University of Technology, GPO Box 2434, Brisbane, QLD 4001, Australia 2 Department of Orthopaedic Surgery, Johns Hopkins University, Good Samaritan Hospital, 5601 Loch Raven Rd, Baltimore, MD 21239, USA 3 Research and Development, Nutramax Laboratories Inc, 2208 Lakeside Boulevard, Edgewood, MD 21040, USA A major problem in tissue engineering is the availability of a sufficient number of cells with the appropriate phenotype for delivery to damaged or diseased cartilage and bone; the challenge is to amplify cell numbers and maintain the appropriate phenotype for tissue repair and restoration of function. The microcarrier bioreactor culture system offers an attractive method for cell amplification and enhancement of phenotype expression. Besides serving as substrates for the propagation of anchorage-dependent cells, microcar- riers can also be used to deliver the expanded undifferentiated or differentiated cells to the site of the defect. The present article provides an overview of the microcarrier culture system, its utility as an in vitro research tool and its potential applications in tissue engineering, particularly in the repair of cartilage and bone. Introduction A wide range of tissue engineering approaches using cells from the recipient are currently being investigated for the repair of musculoskeletal tissues; nevertheless, major challenges still need to be addressed to make these approaches a clinical success [1]. To obtain sufficient numbers of cells of the desired phenotype for delivery to the damaged or degenerated site, tissue engineering approaches typically include cell expansion. In micro- carrier culture, anchorage-dependent cells grow on small suspended microspheres in a spinner or bioreactor vessel. Undifferentiated or differentiated cells propagated on the surface of microcarriers can then be retrieved in large numbers for subsequent implantation or analysis. An important advantage of this technology is that cell-seeded microcarriers can be delivered directly to the site that needs repair – aggregates can be injected or administered arthroscopically, thus eliminating the need for re-seeding the retrieved cells into a scaffold delivery system. Microcarriers Microcarriers offer the advantage of providing a large surface area for monolayer cell growth during propagation in a homogenous suspension culture system: one gram of microcarriers easily provides a surface area comparable with fifteen 75 cm 2 culture flasks. This culture system is, thus, space saving and cost effective with respect to the use of culture media and expensive additives such as growth factors and serum. The microcarrier culture technique also facilitates more efficient gas–liquid oxygen transfer and the maintenance of the crucial physical, biological and chemical milieu. Moreover, the system can be adjusted and monitored for the desired pH, pO 2 , levels of shear, agitation and nutrient components; conse- quently, more stringent regulation of the distinct pro- cesses of cellular proliferation and differentiation can be achieved. Periodic analysis of a small sample of cells can be performed with minimal disruption and without sacrificing the bulk of the cell-seeded microcarriers. Microcarrier culture was introduced by van Wezel in 1967 [2] to mass produce viral vaccines and biological cell products using mammalian cells. Since then, a wide range of commercially available microcarriers has been success- fully used for the production of a variety of biological products at the analytical and industrial scales [3]. This culture system is considered a crucial technology in the defense against future pandemics such as influenza [4]. The use of microcarriers in tissue engineering has only been explored to a limited extent, and Table 1 lists commercially available microcarriers that can potentially be used in tissue engineering. Several factors are crucial for the successful application of microcarriers in tissue engineering. The ease with which cells attach to the microcarrier surface depends on the chemical compo- sition, surface topography, degree of porosity and charge density. In addition, the number of cells that attach on the surface depends on the carrier diameter (typically in the range of 100–400 mm). The size distribution should be as narrow as possible to provide a homogeneous culture, and the specific density of the microcarrier beads should be slightly higher than that of the culture medium (typically between 1.02 and 1.10) to enable them to be maintained in suspension by gentle agitation. The chemical composition of the microcarriers determines the ease of retrieving viable cells. Enzymes, such as trypsin and collagenase, have been used successfully, but the efficiency of the recovery of viable cells varies according to chemical composition and degree of porosity. Separation of the Corresponding author: Malda, J. (jos@malda.nl). Available online 4 May 2006 Review TRENDS in Biotechnology Vol.24 No.7 July 2006 www.sciencedirect.com 0167-7799/$ - see front matter Crown Copyright Q 2006 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.tibtech.2006.04.009