Robust Sensing and Reversible Actuation Using Triblock Copolymer Stabilized Intradroplet Interfaces

Abstract

In this work, a recently developed method for forming copolymer-stabilized interfaces (CSI) between aqueous droplets is pursued to as a means to construct smart materials and systems. The ABA type copolymer employed consists of two hydrophilic (PEO) groups sandwiching a hydrophobic PDMS core. Aqueous droplets submerged in triblock copolymer (PEO-PDMS-PEO)-oil mixtures are rapidly coated in copolymer monolayers, however, unlike phospholipid-stabilized droplet interface bilayers (DIBs), electrical measurements reveal that there is no spontaneous formation of a “thinned” interface with droplet contact alone. The capacitance of the interface begins increasing significantly only upon application of sufficient voltage (>100mV), and capacitance then stabilizes within minutes. Further, the interfacial capacitance and area decreases when applied voltage is reduced back to 0mV, and droplets eventually return to their initial separated state. The fact that droplet adhesion and formation of the interface is voltage dependent and completely reversible clearly distinguishes a CSI from a DIB, and the novel polymer based interface is significantly more robust with average rupture potential of ≥ 800mV compared to 200–300mV with DIBs. Durable and stable CSIs could feasibly be used in applications ranging from sensing and energy harvesting to mechanical actuation. To demonstrate, this work introduces a new version of the DIB based hair cell sensor, now replacing lipids with block copolymers to provide greater durability, stability, and resistance to rupture when subjected to airflow. We calculate the current generated by the vibrating membranes in DIBs and CSIs to study the influence of surfactant selection on the hair cell durability and the related airflow operation range. We conclude that the hair cell constructed using triblock copolymer, as opposed to a DIB, withstands higher nominal airflow speeds (45m/s) and higher applied bias voltages (i.e. 0.1–1V) without rupturing. The ability to apply higher voltages provides a means of tuning the hair-cell sensitivity. Separately, the results of initial trials demonstrate the possibility for voltage-controlled shape change using networks of droplets and CSIs. The ability to apply large voltages and induce change in interfacial area leads to rearrangement of the droplet networks due to conservation of volume. Several embodiments of possible actuators based on this mechanism are discussed. In concert, the various aspects of this work highlight the potential use of CSIs in developing novel, reliable smart materials for sensing and actuation.