Читать книгу Materials for Biomedical Engineering - Mohamed N. Rahaman - Страница 40

The last 30–40 years have seen a rapid advance in the development and application of biomaterials. Prior to this period, biomaterials were selected from mechanically durable and chemically stable materials that were available off the shelf. These biomaterials were designed to serve a mainly mechanical (or physical) function. The last few decades have seen a significant shift in emphasis in the design of biomaterials (Section 1.3). Whereas materials science played a dominant role in biomaterials design and selection previously, this contemporary period is marked by biological sciences playing a role of significance comparable to that of materials science. Advances in biological sciences are utilized in designing biomaterials that serve not just a mechanical function but also a biological function.

Contemporary biomaterials are degradable or bioactive. Instead of remaining in their original chemically inert form in the body, implants are now being designed to degrade at a controllable rate, be replaced by newly forming tissue, and eventually disappear completely. Additionally, degradable implants can deliver ions and biomolecules to target specific cells and tissues in vivo, to control the behavior of cells, or to stimulate cells to create new tissues and organs in the body. In this way, these biomaterials can stimulate the body to heal itself.

Examples of contemporary biomaterials include composites composed of polylactic acid and β‐tricalcium phosphate for fracture fixation (Figure 1.1j) and calcium phosphate cements that set in situ for healing bone defects. The incorporation of a drug‐eluting function in a stent is now becoming common to reduce the tendency for endothelial proliferation and the adherence and clumping of blood cells, thereby improving the long‐term efficacy of the stent in treating vascular disease (Section 1.3.1).

Biomaterials are an important component in some tissue engineering (or regenerative medicine) approaches to create functional tissues and organs. At the same time, advances in science and technology are being used to design these biomaterials with greater complexity in structural and chemical characteristics to better mimic the structure of biological tissues. In addition to degradable or bioactive solids, water‐filled polymers known as hydrogels, synthetic, or natural, find considerable use in tissue engineering approaches, particularly for creating soft tissues. The stiffness of hydrogels and their structural characteristics can be modified to better mimic those of soft tissues. Functional skin substitutes for healing burns are already available commercially, while there are considerable research and development efforts to create bone, cartilage, liver, and pancreas for implantation in the body.

In addition to medical applications, contemporary biomaterials are playing a significant role in the pharmaceutical industry. In the form of 3D objects or particles of size ~1 μm to a few hundred microns (referred to as microparticles), degradable biomaterials have been used for several decades as devices to deliver drugs in a controlled manner to a particular site in the body. Hydrogels that respond to environmental conditions such as pH or temperature are another type of biomaterials that find use as carriers in drug delivery. Emphasis has been shifting in the last couple of decades toward more precise drug targeting in which the drug molecules are targeted specifically to the cells or tissues where they will exert their desired effect. This is achieved by combining the drug molecules with a suitable biomaterial to which is added a cell‐specific antibody or other targeting molecule that has an affinity for the targeted cells or tissues. Increasingly, the biomaterial in these applications is in the form of fine particles, of size smaller than 50–100 nm, referred to as nanoparticles. The combination of chemotherapy drugs and nanoparticles is under development as an alternative technique to radiotherapy or chemotherapy to treat cancer cells. Techniques that rely on the magnetic or optical properties of specially designed nanoparticles are also under development for treating tumors.

Contemporary biomaterials are now being designed and developed for use in diagnostic imaging of cells and tissues. One area of particular interest is the detection and real‐time imaging of cancer cells. Here again, biomaterials in the form of nanoparticles, particularly nanoparticles that become highly fluorescent when irradiated with light, are playing a significant role. Nanoparticles that simultaneously combine a drug delivery function and a diagnostic imaging function are under development to provide a technique for real‐time diagnosis and treatment of cancer.