In order to test the biodegradability of the PLA/silk composites the samples were immersed into phosphate buffered saline solution and kept in an incubator at 37 °C that was shaking at 100 rpm. PLA has a slow degradation rate and no changes in mass were observed for the pure PLA scaffolds during a 4 month period in the PBS, however, the composite materials exhibited a slight increase of ~2.3%. This was due to the silk fiber hydrophilicity, which attracted water molecules that could be stored between the gaps at the silk-PLA interface, and therefore higher moisture absorption was present for these samples, which was reflected in a weight increase. Furthermore the interface between the fibers and PLA, which contained voids and flaws could enhance capillary transports, and also the fabrication process could result in cracks within the PLA allowing for better diffusion of the water molecules to the voids.
The mechanical properties of the samples were tested after 1, 2, 3, and 4 months after immersion. The tensile strength was reduced to ~55 MPa and ~38 MPa for PLA and PLA/silk materials after 2 months and the elastic modulus was reduced to ~3 GPa for both samples after 2 months. Measuring these properties throughout the 4 month period did not show any further decrease in their value. The flexural modulus, however, continuously decreased throughout 4 months, dropping to ~3.2 and ~2.7 GPa, for both samples after 2 and 4 months, respectively. Hence, although the PLA/silk samples had a higher water uptake, only their tensile strength decreased significantly faster than that of pure PLA. It should be noted that the 5% silk fiber content of these materials is rather low and results in a localized strain to occur at the matrix regions surrounding the fiber. Hence the interface can easily fracture and affect the overall strength of the composite. It would therefore be important to examine the mechanical properties of higher content composites, as it is anticipated they would be able to maintain their strength for a longer period of time. However, care must be made in selecting the optimum fiber content, as increasing above certain levels can result in composites that are unsuitable for bioapplications, due to high rigidity.