Processing and quantitative analysis of biodegradable polymers (PLLA and PCL) thermal bonding

A quantitative analysis of the bond strength and microstructure integrity achieved when bonding the biodegradable polymers poly(L-lactide) (PLLA) and poly(epsilon-caprolactone) (PCL) has been performed using the response surface methodology. The respective influence of the bonding parameters (temperature, pressure, duration) on the bond strength and microchannel integrity was investigated. PLLA and PCL were identified as suitable candidates for packaging materials for bioelectronic circuits of conductive biodegradable polymers. For a future packaging application, the bonding parameters were adapted to optimize the bond strength; the estimated values for the bond strength and channel integrity that were predicted by the surface plots were 2.32 +/- 0.26 MPa and 33.7 +/- 12.9% for PLLA, and 0.81 +/- 0.11 MPa and 50.9 +/- 5.7% for PCL. These values were in good agreement with the experimentally determined bond strength of 2.00 +/- 1.10 MPa (PLLA) and 0.67 +/- 0.22 MPa (PCL) and deformation of 31.4 +/- 7.0% (PLLA) and 52.9 +/- 4.1% (PCL). Microchannels with an aspect ratio of 1:12.5 were successfully fabricated. The impact of the fabrication process on the PLLA and PCL chemical properties was also investigated through differential scanning calorimetry and gel permeation chromatography measurements. It was observed that the weight average molecular weight Mw decreases after each fabrication step, as much as 68% for PLLA and 59% for PCL. The strongest reduction was observed after the compression molding (above the melting temperature) which should be kept as short as possible. An annealing step allowed increasing the crystallinity and improved the overall polymer stiffness.