ABSTRACT

Cells can be grown in culture to generate cell products, to produce functional new tissue, or to act as model systems for the characterization of cell function. To accomplish these goals, it is essential for the cells to survive and grow; additionally, it is often important for the cells to express a specific phenotype. Surfaces and interfaces are essential for cell survival since most normal cells derived from animal tissue are not viable in suspension and will require a surface for attachment. The mechanical properties of this surface can affect phenotypic expression of adherent cells and are therefore a significant factor to consider as part of the culturing conditions. The purpose of this chapter is to summarize the different

mechanical properties of substrates that have been used for cell culture and to describe how these surfaces can be prepared or modified. The importance of the substrate mechanical properties with regard to cell development will be illustrated with examples, and examples will be given for the manner in which the mechanical properties affect the cell phenotype. Animal cells were first cultivated in vitro more than 100 years ago, initially as fragments of tissue and shortly after as dispersed cells [1]. The significance of the mechanical properties of the matrix adjacent to the cells during cultivation has been known for nearly this length of time [1] but has gained more prominence in recent years [2]. The importance of the matrix mechanical properties for cell growth in vitro reflects the importance of mechanical stimulation in vivo. Animal cells do not grow in isolation; instead, they are typically in contact with neighboring cells or tissue and are stretched, compressed, or sheared during the normal course of cell growth. These external mechanical forces that act on cells are an important stimulus that affects the biological response. Differentiation of stem cells is affected by external forces and by the mechanical properties of the environment [3] and can affect the development of tissue such as muscle [4, 5]. The response of stem cells to the mechanical properties of tissue is a significant factor in disease conditions [6] and may control the biological response in which stem cells home in at the site of injuries [4, 7]. Differentiated cells such as neurons [8] and epithelial cells [2, 9] can also be affected by the substrate mechanical properties during in vitro cultivation. Cells in vivo will be associated with tissue having a range of stiffness values. Animal tissues such as brain can be soft, while tissues such as bone are much harder (Fig. 25.1). The mechanical properties of tissues are often described by a single elastic modulus E, known as Young´s modulus, which is a measure of the ratio of uniaxial stress to uniaxial strain (with stress being the applied force and strain being the relative deformation). The values of E range from 0.2-0.5 kPa for brain through 10-100 kPa for muscle to 950 kPa for articular cartilage [4, 10]. If artificial substrates are intended to emulate living tissue, it will therefore be desirable to produce substrates having an equal range of stiffness values (Fig. 25.1).