Poly(lactic acid) (PLA) is used for medical devices such as sutures or orthopedic screws. A standard way to determine the loss of mechanical properties of a degradable polymer would be to soak the polymer in phosphate buffered saline (PBS) and test the desired property as a function of immersion time. This method is not sensitive enough to discern changes in mechanical properties through the cross-section of the polymer and neglects the degradation that is occurring at the molecular level. This article presents results of a nanoindentation study carried out with PLA. The modulus and hardness of PLA is characterized as a function of processing method, immersion time in PBS, and location of the indent. Measuring local properties with the nanoindenter allowed detection of differences in material properties as a function of all three of these variables. The mechanical properties on the edge were lower than the interior of the sample after in vitro degradation, and changes were seen earlier for nanoindentation than for traditional flexural or tensile tests. The nanoindenter is a valuable tool for quantifying changes in material properties and may have applicability for accelerated tests to screen biomaterials.

Poly(L-lactic acid) (PLLA) has been used for fracture fixation devices, but its use is limited because of its poor biocompatibility and mechanical properties. The effects of extrusion, incorporation of hydroxyapatite (HA) and self-reinforced composites (SRCs) on the resultant mechanical properties of PLLA were examined. Samples were conditioned for up to 52 weeks in PBS at 37 degrees C. Extrusion did not adversely affect the mechanical properties of PLLA. After in vitro conditioning, a slight but significant reduction in the strain to failure and modulus was seen. HA (10-40%) by weight was evenly distributed into PLLA using an intermeshing twin-screw extruder. As ceramic content increased, the initial modulus increased but flexural strength decreased. After immersion, the modulus of all HA-PLLA blends was lower than PLLA. HA particles did not form a strong bond with the PLLA, which promoted easier degradation of the HA-PLLA matrix. SRCs showed a higher modulus and strength when compared to all materials except the modulus of 30 and 40% HA-PLLA composites before immersion. Water preferentially attacked the matrix of the composite, leading to more fiber pullout, but the fiber orientation maintained the advantages in strength and modulus up to 24 weeks in vitro.

Understanding the wear characteristics of bone cement and its alternatives is critical to improving the quality and longevity of hip replacements. A novel composite material, self-reinforced composite poly(methyl methacrylate), has been previously developed for potential use as a pre-coat material for hip implants. The goal of this work was to examine the properties of self-reinforced composite poly(methyl methacrylate) as a function of processing temperature. Nanoindentation tests were performed to measure hardness and modulus of self-reinforced composite poly(methyl methacrylate) at the nanoscale. Nanoscratch tests were performed parallel, orthogonal, and longitudinal to composite fibers to measure residual scratch depths. Significant differences were found in the hardness, modulus, and residual scratch depth as a function of processing temperature when compared to poly(methyl methacrylate). As processing temperature is increased, hardness decreased and residual scratch depths increased. Data also showed that fiber orientation plays a critical role in scratch resistance. Scratching orthogonal to fiber orientation produced the least residual scratch depth ranging from 524 nm at 105 degrees C to 838 nm at 150 degrees C, compared to a residual scratch depth for poly(methyl methacrylate) of 842 nm.

In a composite material, the degree of adhesion between the fiber and the matrix plays an important role in the overall performance of the material. Because the load between the fiber and the matrix is realized throughout the interphase region material, a lot of effort has gone into characterizing the strength of the interphase. In this study, nanoscratch tests on the composite samples were used to provide a relative measure of adhesion in different composite materials. Carbon-filled nylon 6,6 and polycarbonate resins were evaluated with this method. The carbon fillers we used were polyacrylonitrile-based carbon fibers sized and surface-treated for the respective matrix and pitch-based carbon fibers without any sizing or surface treatment. Tensile and X-ray photoelectron spectroscopy data for the composites we considered are also presented to compare to the nanoscratch results. It is shown that nanoscratch testing on the composites, with the proposed data analysis, can be an effective tool for determining the relative degree of adhesion between different composites.

Uniaxial self-reinforced composite poly(methyl methacrylate) (SRC-PMMA) is being investigated as a pre-coat material for the femoral component of total hip replacements. Hot compaction of self-reinforced composites is largely an empirical process which varies the processing parameters of time, temperature and pressure until the desired properties are obtained. Previous work has shown that PMMA fibers have unique thermal relaxation properties dependent upon the retained molecular orientation in them. This work processed composites at times and temperatures that span the relaxation process for a single fiber. It was found that molecular orientation, as measured by birefringence, was lost in composites processed at times greater than relaxation times for single fibers. Flexural properties were also found to vary with processing conditions, with the highest values of 165 +/- 15 MPa and 168 +/- 3 MPa found at high and low processing times, respectively. These are significantly stronger than unreinforced PMMA which has a flexural strength of 127 +/- 14 MPa. It is hypothesized that diffusion between fibers occurs much more quickly than the loss of molecular orientation and it was seen that SRC-PMMA processing conditions can be predicted from the relaxation times and temperatures from single fibers.