Biomaterials
Polymer Synthesis, Materials Chemistry, and Biomedicine
The use of fundamental chemistry to advance the fields of polymers, materials, and biomedicine is a major emphasis in modern research. Professor Harry Allcock and his students are exploring novel approaches to these subjects by the synthesis and study of new classes of high polymers and advanced materials using the techniques of organic, organometallic, and inorganic chemistry.
High polymers are long chain macromolecules that are the constituents of many useful materials. Depending on their molecular structures, different polymers can have properties, such as liquid crystallinity, high strength or elasticity, catalytic activity, unusual optical or electrical properties, or special biomedical qualities.
Most conventional polymers are derived entirely from petroleum. They are inexpensive, but they have a relatively restricted range of properties. For example, in general, they lack the thermal stability of ceramics, the long-term electrical behavior of silicon or metals, the ³optical switching² behavior of inorganic solids, or the biocompatibility of living tissues and ceramic materials.
The research in the Allcock group involves the design and synthesis of new polymers that contain organic components, together with heteroelements such as phosphorus, silicon, boron, or transition metals. The aim is to combine the most advantageous properties found in organic polymers with the special properties imparted by the heteroelements. For example, our research team has developed synthesis routes to a broad range of new polymers that have backbones of the types shown in 13, and with organic or organometallic side units attached to these backbones. By varying the side group structure, it is possible to bias the properties toward those of elastomers or structural materials, liquid crystalline materials, semiconductors, optical ³smart materials,² ceramics, inert biomedical materials, or biologically active polymers.There are three general aspects to nearly all the research topics in this program:
(1) The development of new synthesis methodology starting at the level of small molecules and progressing to macromolecules. Much of this work involves the development of organic substitution methods or organometallic reaction chemistry, and contains a high component of molecular design based on ongoing structure-property studies.
(2) Characterization and molecular structure determination of the new compounds by techniques such as NMR, IR, gel permeation chromatography, X-ray diffraction, molecular mechanicsmolecular graphics, etc. The aim of this aspect is to relate the unique properties found for the new polymers to their molecular structures.
(3) Examination of the materials¹ properties (i.e., solid state properties) of the new polymers, again with a view to developing structure-property relationships that will aid future research. Techniques such as thermal analysis, electrical and optical behavior, scanning electron microscopy, X-ray photoelectron spectroscopy, and biocompatibility studies are examples of the approaches used. This phase of each project often involves collaborations with other research groups that have specialized experience in materials¹ or medical-oriented techniques. For instance, our work on nonlinear optical materials, composite materials, ceramics, semiconductors, membranes, bioerodible polymers, bioactive surfaces, and solid polymeric battery electrolytes is conducted through collaborations with groups at other universities and in industrial laboratories. A recent example is the joint development with the group of Professor Robert Langer at MIT of a new polymer for the microencapsulation of mammalian cells as a starting point for the assembly of artificial liver or pancreas devices or for the biotechnology-type production of proteins from hybridoma cells.
Overall, the research in this program provides training in the ways that fundamental synthetic, mechanistic, and structural chemistry can be utilized in polymer chemistry and materials science. It also offers opportunities for an understanding of long-range practical topics that the student will almost certainly encounter in a professional scientific career.
One of the more promising applications for polyphosphazenes is their use in biocompatible materials. Degradation of the phosphorus-nitrogen backbone yields the biologically compatible hydrolysis products phosphate and ammonia. Even though most polyphosphazenes are stable to moisture, considerable effort has been focused on the synthesis of polymers which erode at biological pH to innocuous products.
Bioerodible polyphosphazenes
These polymers erode at different rates, and, in addition to phosphate and ammonia, yield biologically compatible hydrolysis products such as glycine, lactic acid, glycolic acid, glycerol or glucose. These polymers are excellent candidates for uses in medical applications such as devices for the erosion induced release of drug molecules, as sutures and as frameworks for tissue regrowth.
Other polyphosphazenes have been invesigated for other biomedically important purposes. Pictured here, on the cover of the Penn State Materials Research Institute directory, is a hydroxyapatite-polyphosphazene composite which has been invesitgated as a possible artificial bone replacement material.
Also recently investigated is the use of a calcium-crosslinked polyphosphazene used to microencapsulate mammalian liver cells. This would have application in the development of synthetic organs through tissue engineering.