What are biomaterials?

Biomaterials are materials that can function as a whole or part of a device to treat, assist, repair, or replace any tissue, organ, or function of the body. They can be classified as polymers, metals, or ceramics.

Research Focus-Overview

Our research seeks to design, synthesize, and characterize novel polymeric biomaterials to improve their performance or function. Optimization of a specific behavior is controlled at the molecular-level by tailoring the structure of the polymer. Various synthetic strategies are used to prepare novel polymers with desired properties. Of particular interest are inorganic siloxane-based polymers because of their unique set of properties and bioinert character. We prepare polymeric biomaterials in many different forms: homo- and copolymers, polymeric micelles, coatings, colloidal nanoparticles, and hydrogels. Careful characterization of these systems allows determination of structure-property relationships and optimization of specific behavior.

Current Projects:

Silicones, particularly poly(dimethylsiloxane) (PDMS), have been utilized in many biomedical applications because of their thermal and oxidative stability, gas permeability, low modulus, flexibility, and good biocompatibility. Unfortunately, silicones generally exhibit poor resistance to blood proteins as a result of its extreme hydrophobicity An adsorbed blood protein layer can invoke subsequent platelet adhesion and activation of coagulation pathways leading to thrombosis thereby compromising device success. In order to improve the protein resistance of silicone surfaces, poly(ethylene oxide) (PEO, or poly(ethylene glycol) (PEG)) has been incorporated into silicone materials. The configurational mobility of PEO produces an entropic penalty of chain compression if protein adsorption were to occur. In our research, PEO chain mobility and hence protein resistance of silicone surfaces containing PEO is systematically altered by the incorporation of PEO via siloxane tethers. We have produced PDMS-block-PEO copolymers and introduce them into silicone coatings by bulk crosslinking or surface grafting methods.  Block copolymer structure is then correlated to surface properties and protein or platelet resistance.

Thermoresponsive polymers such as poly(N-isopropylacrylamide) (PNIPAAm) are of growing interest for controlled drug delivery, ophthalmic biomaterials, tissue engineering, and sensor membranes. Crosslinked PNIPAAm hydrogels display a reversible volume phase transition (~34 °C) from a water-swollen to a shrunken (deswollen) state by exuding water when heated above the volume phase transition temperature (VPTT). After shrinking, the surfaces of PNIPAAm become more hydrophobic. We have created “nanocomposite hydrogels” by the introduction of colloidal PDMS nanoparticles (200 nm diameter) into PNIPAAm hydrogels in order to alter their mechanical, thermal, and surface properties. Unlike when polymeric chains are introduced, the VPTT of the nanocomposite hydrogels is maintained near physiological temperature which is desirable for implantable medical devices. In addition, these nanocomposite hydrogels produce a more hydrophobic surface above the VPTT compared to pure PNIPAAm hydrogels. We are currently evaluating the ability of these nanocomposite hydrogels to control protein or cell adhesion onto the surfaces when modulated through their VPTT.

- PDMS-PEO Hybrid Hydrogel Scaffolds

Correlating scaffold material properties to the long-term mechanical properties of engineered tissues (e.g. blood vessels) would ultimately lead to more effective design of TE scaffolds. To do this, we are preparing a library of scaffolds whose material properties can be systematically altered over a broad range. Poly(ethylene oxide) (PEO) hydrogels have been widely studied as scaffolds (i.e. for tissue engineered vascular grafts (TEVGs)) due in part to their non-thrombogencity. However, the material properties of PEO hydrogels may be tuned over a relatively narrow range. Instead, we have prepared photocurable PDMS-PEO “hybrid” hydrogel scaffolds by incorporation of an inorganic macromer, methacrylated star polydimethylsiloxane (PDMS_star-MA), with PEO-diacrylate (PEO-DA). We are evaluating the impact compositional variables on the resulting hydrogel water content, morphology, and mechanical properties.

Conventional thermoplastic biodegradable polymers such as poly(lactic acid) (PLA), poly(glycolic acid) (PGA) and their copolymers are generally brittle and lack appreciable elasticity at physiological temperature. Thus, they fail to mimic the elastic nature of many soft tissues. Furthermore, semi-crystalline, thermoplastic polymers degrade in a non-homo­­geneous fashion such that mechanical properties are lost prior to significant loss of mass (i.e. non-linear loss in mechanical properties). Elastomeric biodegradable polymers are promising candidates to prepare TE scaffolds with elasticity which more closely parallels soft tissues. Furthermore, thermosetting biodegradable elastomers generally maintain their original dimension with a significant loss of mass and also exhibit a linear loss in mechanical properties during degradation. Thus, the objective of this research is to develop novel photo-crosslinked inorganic-organic elastomers as a new class of thermosetting elastomeric biodegradable TE scaffolds. These hybrid elastomers are designed to exhibit the elastic nature of soft tissues and demonstrate linear loss in mechanical properties during degradation. These elastomers will be formed by the photochemical crosslinking of a diacrylated triblock copolymer consisting of an inorganic block, poly(dimethylsiloxane) (PDMS), and terminal organic terminal blocks, poly(caprolactone) (PCL) or PCL-co-poly(glycolic acid) (PCL-co-PGA). The effect of copolymer molecular weight and PDMS:PCL:PGA ratio on the surface, thermal, and mechanical properties of the resulting elastomer are being studied.