Electrically Induced Bursting of Aqueous Capsules Made from Biopolymers: ‘Switching On’ the Release of Payloads

Abstract

The use of electric fields to stimulate the delivery of drugs or other active ingredients is of great interest for wearable electronics and other applications. Most attempts at electrically induced delivery with soft materials in water have focused on electronically conducting polymers (e.g., polypyrroles) or conductive nanocomposites (e.g., polymers with carbon nanotubes). Here, electrical responses are induced even in structures made from nonconducting biopolymers that are widely available, biocompatible, and biodegradable. The materials studied here are spherical capsules created from the anionic polysaccharide alginate by cross-linking with cations like Ca2+ or Cu2+. When these capsules are placed in an aqueous solution and subjected to an electric field (direct current) of ≈8 V cm−1, they deform within a couple of minutes and then burst and disintegrate into pieces within ≈5 min. Capsules across a range of length scales (200 µm to 2 cm) respond in the above manner, and the electroresponse persists even if the capsules are embedded in a nonionic gel matrix. This electroresponse is due to electrophoretic migration of charged species (ions and/or polyelectrolyte chain-segments) within (or out of) the capsules. In an alginate capsule, the cations are induced to migrate away from the positive electrode, which creates a weakly cross-linked region of the capsule that swells appreciably. This anisotropic swelling continues until the capsule eventually bursts. Applications for electroresponsive capsules that highlight the spatial and temporal accuracy possible with an electrical stimulus are discussed. The bursting of capsules can be used to release solutes loaded inside these structures. Also, even the deformation of intact capsules can be used to create electrically actuatable valves, where a liquid flows out through the valve only when a capsule plug is dislodged.