Interactions between colloids, including macromolecules, are fundamental to biomaterials, and boundary conditions can have an important influence. Self-assembly into ordered phases is well known, for example, to couple to boundaries, such as the gratings used in the engineering of photonic crystals. Cells are also colloidal in some key respects, and recent studies have shown that the elasticity of a cell’s environment can influence aspects of cell morphology, assembly, and differentiation. Such biological experiments generally employ cross-linked polymer gels, but soft substrates might themselves induce interactions between inanimate colloids. The nature of such substrate-induced forces has just begun to be measured, and initial results show that colloidal interactions and assembly are indeed modulated by the softness of a substrate: at multiple length scales, kBT and entropy seem key. Whether soft boundaries can be understood sufficiently to exploit with cells, viruses, or biomolecules is unexplored and seems likely to require deeper insight and better material control at the nanoscale.
Many examples of natural self-assembling biomaterials illustrate the evolution of remarkable physical properties, with one well-known example being spider silk. Among the many known species of spiders, tens of thousands use a web of silk fibers to catch prey, and the fibers forming the frame of the web have a toughness and ductility exceeding that of many man-made materials. Spider silk has mechanical properties and biocompatibility (without inflammation) that have long been known and exploited by man for hunting, fishing and wound healing. Spiders are cannibals and cannot be farmed like silkworms, but spider silk proteins can be recombinantly expressed in insect cell lines or bacteria. The assembly behavior of these proteins is being studied by many groups with an aim to unravel the natural assembly mechanisms that generate a tough material out of a protein solution. Wet-spinning, electrospinning and some additional biomimetic processes are pursued, with subsequent analysis of properties permitting process optimization for various applications. Non-fiber morphologies also become possible and include films, gels, capsules, and nanofibers. With the correct molecular design and processing, films of spider silk might be useful as surface coatings or as thin and tough membranes. Spider silk that is engineered with additional functional sequences might be used to make microcapsules for use in colloidal recognition and medical application. Spider silk proteins could well be an important biomaterial in the near future due to their many potential applications, but basic questions in self-assembly as well as biology and biochemistry remain to be addressed in a materials science and engineering context. Of course, other self-assembling proteins might be studied and developed for similar as well as unforeseen applications.