Professor James Economy's Group
Due to its high impact resistant, low wear rate, low friction, biocompatible in large pieces and sterilizable, ultrahigh molecular weight polyethylene (UHMWPE) has been successfully used as joint replace materials, such as socket - acetabular cup (see Figure 2). Today nearly 250,000 total hip replacements are done in the U.S. yearly. Approximately 10% of total hip replacements will fail in the first ten years, and another 30 to 40% over the next ten years. Gross wear can cause loosening directly, but wear particles can cause inflammation and bone erosion and thus indirectly cause loosening. It was not until the early 1990s that the production of UHMWPE debris at the articulating surface of joint replacements was widely recognized to play a central role in initiating osteolysis. Since that time, orthopedic research efforts have focused increasingly on improving UHMWPE, with the goals of reducing wear and, by implication, improving implant survival, especially for young active patients. Currently, improving UHMWPE is focusing on forming cross-linking structure by radiation. To provide a more stable structure and lower wear loss surface, UHMWPE and ATSP (aromatic thermosetting polyester ) blends were developed in our group. In the primary Dynamometer wear test, the blend system with small amount compatibilizer showed lower wear rate compared to UHMWPE. Further wear test is underway to confirm this result followed by the cell-culture experiment to identify its biocompatibility.
Figure. UHMWPE acetabular cup.
Aromatic Thermosetting Polyester (ATSP) / Polytetrafluoroethylene (PTFE) Composites
Polytetrafluoroethylene, PTFE (trade name Teflon®), has superb lubricity and has been widely used in tribological applications. Its mechanical properties, however, are low compared to other thermoplastic polymers, among which the wear rate and defomability are especially poor. PTFE is intractable, it can not be melt processed. It must be either sintered or extruded in a paste-like suspension under very high temperature (~400°C) and pressures (free sintering above 3000psi).
A novel type of PTFE composites have been developed, taking advantage of the unique characteristics of the aromatic thermosetting polyester, including solid state bonding and high temperature properties. The solid state bonding capability due to Interchain Transesterification Reactions (ITR), allows free blending and sintering with PTFE at the temperatures and times which PTFE is typically processed.
To enhance mixing and improve the interface adhesion, fine powders of ATSP were prepared by solution polymerization of trimesic acid, hydroquinone diacetate, isophthalic acid and acetoxybenzoic acid. It was found thermosetting polyester could indeed stiffen and strenghthen pure PTFE, besides, ATSP powders ( 20µm in diameter) improved the flow property of PFPE, hence, a broad processing window of 100°C was found , and ATSP/PFPE, ATSP/PFPE/Zonyl fluoropolymer composites were fabricated with excellent mechanical properties.