Materials for biological modulation, sensing, and imaging
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Introduction Biological cells are fundamental building blocks of tissues and organs of living organisms.1 Over their lifetimes, cells are responsible for the development, programmed remodeling, and regeneration of tissues and organs. In addition, cells play crucial physiological roles, such as transport of oxygen and carbon dioxide, signal transduction, muscle contraction/ relaxation, homeostasis, immune response, and metabolic activities. In addition, a number of chronic and malignant diseases result from a wide range of genetic abnormalities and the conversion of normal cells to cancer cells. Therefore, cells have been studied extensively as biomarkers for early detection, therapeutic targets, and even as sources for regenerative medicine that can take current levels of diagnosis and disease treatment to the next level.2–7 In these efforts, cells are typically isolated from tissues of interest and cultured in vitro. Alternatively, various pluripotent and multipotent stem cells are being studied to generate specific cell types for use both in understanding their diverse emergent behavior and for treating disease and tissue defects via their in vivo transplantation.8–10 Efforts to directly convert fibroblasts (main component of connective tissue) to neuronal cells (i.e., nerve cells) are also under way.11,12 Additionally, these cells are actively being used to assemble in vitro platforms, such as “body-on-a chip” that can screen newly designed drug molecules, including recombinant proteins and genes.13,14 From these studies, there
is a growing consensus that the successful use of cells in fundamental studies and translational clinical treatments greatly depends on the ability to retain cell viability and to regulate cellular activities in a controllable and predictable manner. It is well understood that the diverse activities of cells, including growth, gene expression, migration, differentiation, and even programmed death, are regulated by multifaceted extracellular microenvironmental factors and genetic influences (Figure 1). The characteristics of the microenvironment include various soluble biochemical factors, biochemical and biomechanical properties of the surrounding extracellular matrix, and intercellular adhesion.15–22 These factors also influence cellular binding and uptake of drug molecules and subsequent therapeutic activities.23,24 Major goals in the regenerative medicine field have been to recapitulate the extracellular microenvironment in vitro for specific diagnostic applications. In addition, a major goal for in vivo applications to regenerate damaged or diseased tissue has been to recapitulate specific tissue functions in vivo by first developing proper in vitro cell culture and subsequent in vivo transplantation tools and biomolecular carriers.21,25–27 Aligned with these efforts, various materials, either alone or integrated with other engineering devices such as microelectromechanical systems, are also being explored to analyze and control chemical, mechanical, and biological interactions inside and be
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