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The term smart polymer embraces a wide range of different compounds that can change their color, transparency or shape in response to their environment; they are smart because of their ability to respond in a very dramatic way when small environmental changes are detected.
It only takes a small variation in temperature, humidity, pH, or light, for example, to induce a large change in smart polymer properties, and this non-linear response makes them unique. Smart polymers tend to have an all-or-nothing response, and are completely predictable, with the change being uniform throughout the material. They might alter their conformation, adhesiveness or water retention properties, and can return to their initial state when the stimulus ends.
These biocompatible, strong, resilient, and flexible polymers are classified according to their physical features, or the stimuli they respond to. If grouped by their physical shape, they may be classified as free linear chain solutions, reversible gels covalently cross-linked and polymer chain grafted on a surface.
When classified by stimuli they respond to, they may fall into one of three groups:
- Physical – temperature, ultrasounds, light, or mechanical strength.
- Chemical – pH and ionic strength.
- Biological – enzymes and biomolecules.
Polymers sensitive to pH accept or release protons in response to pH changed and contain acidic or basic groups. Thermo-responsive polymers are sensitive to temperature and change their microstructural features in response.
Light is considered a clean stimulus that allows remote control of the polymers without physical contact or mechanical apparatus. It enables scientists to change the geometry and dipole moment of the phot-switching molecule causing macroscopic variations in the structure. This can affect the final properties, such as wettability, permeability, and color.
Scientists have developed synthetic smart polymers with specific characteristics after studying how natural polymers behave in biological systems. These new polymeric materials are chemically formulated to sense a particular change in the environment in biological systems and adjust expectedly. They are becoming gradually more widespread as we discover the chemistry and triggers which alter the polymer structure and find new ways to take advantage of and control of them.
Smart polymers, also known as stimuli-responsive polymers or intelligent materials, appear in highly specialized items, and everyday things. Their uses include sensors and actuators like artificial muscles; production of hydrogels; biodegradable packaging; and in biomedical engineering.
Drug Delivery Systems
Smart polymers are of great interest for drug delivery systems where they can be used to control the release of drugs until they have reached their desired destination; not only are they inexpensive, they are easy to control.
Traditionally drugs have been encapsulated using lactic acid polymers, but more recently lattice-like matrices have been employed to hold the drug of interest by integrating or trapping them between polymer strands. This prevents the degradation of the drug delivery system as it journeys through the highly acidic stomach environment, while also avoiding any adverse effects in healthy bone and tissue.
A chemical or physiological structure-altering reaction controls the release of the drug, often a hydrolysis reaction, which cleaves the bonds and releases the drug as the matrix deteriorates into biodegradable components.
Protein Purification
Smart polymers may be used in protein purification where they undergo a rapid and reversible change in response to a change in medium properties. Such smart polymers are found in the conjugated systems used in physical and affinity separation and immunoassays. Microstructure alterations in the polymer structure are shown as precipitation formation.
They work when the protein to be separation forms a bioconjugate – a stable covalent link between the biomolecule and another molecule – with a polymer and precipitated with the polymer when the environment is changed. The precipitate can be removed, and the protein can be removed from the polymer and return to its original state.
Self-Adaptive Wound Dressings
A super-absorbent synthetic smart polymer immobilized in a 3D fiber matrix with hydration functionality – a hydrogel embedded in the core of the material – serves as a wound dressing that supports optimal healing.
Its mode of action relies on the ability of the polymer to detect and adapt to varying humidity and fluid content in all areas of the wound and to automatically and reversibly switch between absorption and hydration. This action ensures a synchronized response of the dressing to changes in and around the wound to achieve the best healing environment at all times.
This details just a few uses, there are many more potential applications, including tissue engineering, cell culture, gene carriers, in textile engineering and oil recovery. The more scientists learn about how smart polymers react, the more likely we are to see them becoming commonplace.
Sources and Further Reading
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