Mimicking the Stimulus-Responsiveness of Natural Polymers
Scientists studying the natural polymers found in living organisms (proteins, carbohydrates and nucleic acids) have learned how they behave in biological systems as they perform their structural and physiological roles. That information is being put to use to develop similar man-made polymeric substances with specific properties and the ability to respond to changes in their environment. These synthetic polymers are potentially very useful for a variety of applications including some related to biotechnology and biomedicine.
Smart polymers are becoming increasingly more prevalent as scientists learn about the chemistry and triggers that induce conformational changes in polymer structures and devise ways to take advantage of, and control them. New polymeric materials are being chemically formulated that sense specific environmental changes in biological systems, and adjust in a predictable manner making them useful tools for drug delivery or other metabolic control mechanisms.
The nonlinear response of smart polymers is what makes them so unique and effective. A significant change in structure and properties can be induced by a very small stimulus. Once that change occurs, there is no further change, meaning a predictable all-or-nothing response occurs, with complete uniformity throughout the polymer. Smart polymers may change conformation, adhesiveness or water retention properties, due to slight changes in pH, ionic strength, temperature or other triggers.
Another factor in the effectiveness of smart polymers lies in the inherent nature of polymers in general. The strength of each molecule’s response to changes in stimuli is the composite of changes of individual monomer units which, alone, would be weak. However, these weak responses, compounded hundreds or thousands of times, create a considerable force for driving biological processes.
Classification and Chemistry
Currently, the most prevalent use for smart polymers in biomedicine is for specifically targeted drug delivery. Since the advent of timed-release pharmaceuticals, scientists have been faced with the problem of finding ways to deliver drugs to a particular site in the body without having them first degrade in the highly acidic stomach environment. Prevention of adverse effects to healthy bone and tissue is also an important consideration. Researchers have devised ways to use smart polymers to control the release of drugs until the delivery system has reached the desired target. This release is controlled by either a chemical or physiological trigger.
Linear and matrix smart polymers exist with a variety of properties depending on reactive functional groups and side chains. These groups might be responsive to pH, temperature, ionic strength, electric or magnetic fields, and light. Some polymers are reversibly cross-linked by noncovalent bonds that can break and reform depending on external conditions. Nanotechnology has been fundamental in the development of certain nanoparticle polymers such as dendrimers and fullerenes, that have been applied for drug delivery. Traditional drug encapsulation has been done using lactic acid polymers. More recent developments have seen the formation of lattice-like matrices that hold the drug of interest integrated or entrapped between the polymer strands.
Smart polymer matrices release drugs by a chemical or physiological structure-altering reaction, often a hydrolysis reaction resulting in cleavage of bonds and release of drug as the matrix breaks down into biodegradable components. The use of natural polymers has given way to artificially synthesized polymers such as polyanhydrides, polyesters, polyacrylic acids, poly(methyl methacrylates), and polyurethanes. Hydrophilic, amorphous, low-molecular-weight polymers containing heteroatoms (i.e., atoms other than carbon) have been found to degrade fastest. Scientists control the rate of drug delivery by varying these properties thus adjusting the rate of degradation.