Biodegradable, Selectively Permeable Membranes for Crosslinking of Polypeptides

Elastin-like polypeptides (ELPs) are bioinspired stimuli-responsive biopolymers; these polypeptides undergo a lower critical solution temperature phase transition in water. We aim to utilize this phase transition to create new particle structures at the microscale by crosslinking multiphase liquid states inside droplets. To do so, we have engineered microcapsules that contain ELPs surrounded by a nanoporous biodegradable polymeric membrane. The porous microcapsule acts as a semi-permeable membrane, maintaining the larger protein molecules (MW ≈ 30 kDa) inside the capsule while allowing smaller crosslinking molecules (MW ≈ 500 Da) to diffuse into the capsule through the membrane pores.

To fabricate the capsules, we use controlled biphasic flow within a capillary microfluidic device to form template liquid water-in-oil-in-water double emulsions; the inner drop consists of ELPs in an aqueous buffered solution, while the middle layer is a solution of polylactic acid, a biodegradable polymer, dissolved in toluene. The outer or continuous phase is a water solution containing a surfactant to promote capsule stability. Solvent evaporation is used to consolidate the ultra thin (500 nm – 2 µm) polymer solution middle layer to produce a biodegradable nanoporous microcapsule.

Following small molecule delivery, the dialysis membrane can be removed through an esterase enzymatic reaction that does not disrupt the inner ELP particles. ELP colloids can be further purified through multiple centrifugation and washing cycles. Liberated ELP microparticles can be used downstream in a variety of biomedical applications, from drug delivery to compartmentalized micro-reactions.