We demonstrate a new method for joining thermoplastic surfaces to produce microfluidic devices. The method takes advantage of the sharply defined permeation boundary of case-II diffusion to generate dimensionally controlled, activated bonding layers at the surfaces being joined. The technique is capable of producing bonds that exhibit cohesive failure, while preserving the fidelity of fine features in the bonding interface. This approach is uniquely suited to production of layered microfluidic structures, as it allows the bond-forming interface between plastic parts to be precisely manipulated at micrometre length scales. Distortions in microfluidic device channels are limited to the size scale of the permeant-swollen layer; 6 microm deep channels are routinely produced with no detectable cross-sectional distortions. Conventional thermal diffusion bonding of identical parts yields less strongly bonded microfluidic structures with increasingly severe dimensional compressions as bonding temperatures approach the thermoplastic glass-transition temperature: a preliminary rheological analysis is consistent with the observed compressions. The bond-enhancing procedure is easily integrated in standard process flows, uses inexpensive reagents, and requires no specialized equipment.

The strain-rate sensitivity exponent m and activation volume υ∗ are often used to characterize the strain-rate sensitivity of strength behavior in metals and alloys. Complications can arise when the m and υ∗ values become indeterminate, due to factors such as an inherent scatter in the mechanical property data. The study of commercial Ti-alloy wires is considered wherein to overcome this limitation, the formulation of the Kocks–Mecking (K–M) model is modified to provide a parameter cb that characterizes the microstructural scale responsible for the observed plasticity and work hardening behavior. The softening factor cb is found to be independent of strain-rate for the Ti-alloy wires of this study. It is proposed that cb !can offer a versatile and complementary computation to the activation volume υ∗ since its formulation includes the yield and ultimate strength values along with the plastic strain. For the tensile testing of Ti-alloy wires, a low cb-value of 14 is calculated for Ti-6Al-4V that is consistent with >10 % plasticity during work hardening whereas a high cb-value of 135 for Ti-6Al-7Nb corresponds with <4 % plasticity.