Biomimicry and Materials in Medicine

LARISA-EMILIA CHERAN, ALIN CHERAN AND MICHAEL THOMPSON

1.1 Biomimicry

Past generations have attempted to tame, harness and conquer nature, and bring her to her knees. Such efforts have transformed over the years into a wiser, enthusiastic pursuit to unravel her secrets and copy the "engineering" afforded by nature. Implicit in this is the recognition that literally billions of years of evolution have resulted in the refining of biological structures at the macro- and nano-scales.

A living cell that grows and multiplies looks much like a robot that has learned to build itself. We are still not sure exactly how this machine was first assembled and programmed. In comparison, strident announcements with respect to scientists "creating life" in the lab are only lame self-promoting marketing strategies. The reality is that there is a failure to acknowledge the fact that the inception of such efforts were initiated with existing living cells before any major manipulations were made inside the cells. There is no clear definition of life; we even struggle to ascertain the difference between a living cell and a dead one. All the chemicals and physical properties are essentially identical, with the most remarkable feature being the inefiable force that controls those 100 000 chemical reactions in each second, in each living cell. For that matter, we are required to recognize that we do not even have a real definition of reality, only hypotheses which contradict the Copenhagen interpretation because of the need of an observer, explicitly the multiverse theory, Bohm's implicate order, Everett's parallel universes, and orchestrated quantum coherence. Troublesome as it is, we do not have a definition of time either, since the vibrations of the cesium atoms and quartz crystals only measure something that does not exist when we dream, for example; something that expands exasperatedly when we are bored and contracts when we are in the flow.

Leaving such big questions under the rug of modern science, where they belong, we turn to the somewhat more uncomplicated quest of mimicking nature by creating synthetic materials capable of both detecting biochemical and biophysical changes at the molecular level that reflect sensory mechanisms. Additionally, synthetic materials can replace natural biological components for medical corrective procedures and/or healing. Such activities are often characterized as being incorporated in the relatively new field of "biomimicry", a word coined by Schmitt some years ago.

1.2 Advanced Synthetic Materials

New materials which have features that can be modified under specific conditions are pushing forward the frontiers of scientific and technological capabilities. Synthetic materials designed to change their size or shape when exposed to heat, change from a liquid to a solid in magnetic fields, or dramatically change their volume, viscosity, conductivity, work function, etc., will allow new and interesting applications to emerge, not only for sensing and detection purposes, but also for everyday use. Smart materials with piezoelectric, magneto-rheostatic, electro-rheostatic, and memory hysteresis are already used in cars, coffee pots, glasses, or in space missions. Relevant to this chapter, intelligent biomaterials that respond to biological signals show great promise in regenerative medicine, diagnostics, and drug delivery.

Common examples include piezoelectric materials which produce a charge under mechanical stress and, conversely, contact or expand when a voltage is applied. In shape-memory alloys and polymers, large deformations can be induced and recovered by temperature changes or stress changes. This pseudoelasticity is the result of martensitic phase changes. In magnetostrictive materials, changes in shape can be instigated by magnetic fields; on the other hand, these materials also exhibit changes in magnetization under mechanical stress. Magnetic-shape memory alloys change their shape when under a magnetic field, and pH-sensitive polymers change in volume when exposed to alterations in the pH of the solution in which they are immersed. Halochromic materials change their color as a result of change in acidity; they are very suitable detection materials for applications such as corrosion detection. Chromogenic materials change color as a response to electrical, thermal, or optical changes. When an electric voltage is applied, electrochromic materials change their color or opacity, as in liquid crystal displays. Thermochromic materials change in color depending on temperature changes and photochromic materials respond to light changes (used in light-sensitive sunglasses which darken when exposed to bright sunlight). Photomechanical materials change shape when exposed to light. Dielectric elastomers produce large strains under external electrical fields. Magnetocaloric materials change reversibly in temperature under a changing magnetic field. Thermoelectric materials convert temperature differences into electricity. Such active materials are ideal for sensor devices and for such applications as resorbable bioceramics, adaptive bioglasses, biomimetic polymers and gels, active nanoparticles, smart textiles, and active optical fibers. Manipulation of material properties at the atomic and molecular scale is leading to self-assembling materials, nanolithography, DNA-based technologies (such as DNA computing), nano- and micro-engineered devices for diagnostics, pharmaceuticals, therapies, drug delivery systems, biocompatible implants and prostheses, and bio-functional systems.

We now turn to the employment of new materials in a plethora of applications in biology and medicine.

1.2.1 Nitinol and Memory Metals

A shape memory transformation was first observed in 1932 in an alloy of gold and cadmium, and then later in brass in 1938. The shape memory effect was seen in the gold–cadmium alloy in 1951, but this was of little use. Some 10 years later, in 1962, an equiatomic alloy of titanium and nickel was found to exhibit a significant shape memory effect and Nitinol (so named because it is made from nickel and titanium and its properties were discovered at the Naval Ordinance Laboratories) has become the most common shape memory alloy (SMA). Other SMAs include those based on copper (in particular...