Design and evaluation of a paste applicator for laser tissue soldering

Abstract

Extrusion-based deposition of thermosensitive pastes is a key challenge in both bioprinting and emerging surgical applications. One such application is laser tissue soldering (LTS), a sutureless wound closure technique for robot-assisted minimally invasive surgery (RMIS) in which a solder paste is deposited along wound margins and thermally activated by laser illumination [1]. This work presents the design and evaluation of a thermally-controlled extrusion prototype [2]. The central challenge is the paste’s thermoreversible gelation: based on bovine serum albumin and gelatine with embedded nanoparticles [1], the paste forms a gel at room temperature but becomes extrudable only within a narrow, elevated temperature range.

Initial extrusion feasibility was validated using a CELLINK BIO X6 bioprinter, confirming that the paste can be reliably extruded through a nozzle. Based on these trials, a custom prototype was developed. A hexagonal aluminium block serves as a thermal mass to ensure homogeneous temperature distribution around the syringe barrel. Peltier elements enable bidirectional temperature control via PID regulation on an ESP32-C6 microcontroller, and a stepper motor coupled to a lead screw drives mechanical extrusion.

The prototype was characterised through two experimental test suites. An extrusion yield test quantified extruded paste mass at different temperatures, and a gravitational flow test evaluated paste stability on a 45° inclined surface.

An optimal operating window of 26–28 °C was identified. Within this range, paste extrusion was consistent (mean mass 0.147 g, coefficient of variation ? 4.4 %) and the paste remained stationary after deposition on a 45° inclined surface. Below 26 °C, extrusion became clumpy and unreliable, with frequent nozzle clogging. Above 28 °C, the paste transitioned to a free-flowing state: at 30 °C, it ran freely down the incline, rendering controlled deposition infeasible.

The prototype achieves stable temperature regulation (±0.03 °C steady-state accuracy, 55 s settling time), repeatable extrusion within the identified operating window, and homogeneous paste heating through its aluminium thermal mass, satisfying the core requirements for controlled paste deposition. A water-cooled variant is under development to enable miniaturisation towards an RMIS-compatible end-effector that integrates both paste deposition and laser activation.