Bio-interfacial engineering of electrophoretic coacervate microcapsules for cytocompatible on-skin displays

Electrophoretic ink (e-ink) holds significant potential for energy-efficient displays and sustainable alternatives to print media. However, their use in wearables is hindered by hazardous materials which are cytotoxic and sensitizing to human tissue. While commercial e-inks encapsulate these fluids in rigid, synthetic microplastics, such as melamine-formaldehyde, they lack the mechanical conformability and environmental safety required for epidermal contact. We thus present a novel approach for producing e-ink microcapsules (EIMCs) for integration into biocompatible and free-form digital devices. Spherical, core-shell EIMCs were fabricated via complex coacervation (pH 4.1), using user-friendly biopolymers (gum acacia and gelatin). A core of white electrophoretic TiO2 particles (diameter approx. 30 nm) was marked with a darker violet dye, while being homodispersed in hexylsalicylate-tetrachloroethylene (HS-PCE), as the liquid medium. Characterization of the physico-chemical and performance properties of EIMCs showed remarkable nominal compression stress at rupture (3.9 ± 0.9 MPa). When subjected to a DC voltage of 20 V, the microcapsules created a writable display, potentially enabling users to write and rewrite. The response time was approximately 0.5 seconds (2 Hz refresh rate), comparable to commercial products. In vitro cytotoxicity and CD54 expression (h-CLAT) assays demonstrated that the microcapsule shell successfully mitigated solvent-induced immunotoxicity, maintaining high cellular viability (approx. 70%) and minimal sensitization (approx. 1.7%).