# Mechanistic Investigation of Microdroplet Formation in High-Viscosity Shear-Thinning Hydrogel Bioinks

## TL;DR

This paper introduces a new method for printing high-viscosity hydrogel bioinks with precise control, enabling better biofabrication without compromising the material's properties.

## Contribution

A transient shear-enabled jetting method is introduced to print high-viscosity shear-thinning hydrogel bioinks with controlled droplet formation.

## Key findings

- High-frequency electromagnetic microvalve enables localized shear to lower viscosity for stable droplet ejection.
- Coupled modeling framework links actuation parameters to droplet volume and stability.
- Experiments with sodium alginate show tunable droplet size and stable generation under optimized conditions.

## Abstract

High-resolution biofabrication requires precise microscale deposition, yet drop-on-demand (DOD) inkjet bioprinting is constrained by a narrow printable viscosity window. Many biocompatible hydrogel precursors display high zero-shear viscosity and strong shear-thinning, so stable droplet ejection typically requires dilution or reformulation that can compromise the biochemical microenvironment. We present a transient shear-enabled jetting method that exploits intrinsic shear-thinning by using a high-frequency electromagnetic microvalve to deliver short, high-pressure pulses. The resulting localized shear dynamically lowers apparent viscosity in the nozzle region and promotes controlled nucleation, ligament formation, necking, and pinch-off. A coupled, rheology-informed modeling framework (axisymmetric transient CFD, valve dynamics, and electromagnetic FEM) links actuation parameters to droplet volume and stability and guides hardware optimization. Experiments with 2.5% (w/v) sodium alginate validate stable droplet generation and tunable droplet size via stroke length and driving conditions. These results define a practical process window for high-resolution droplet printing of high-viscosity shear-thinning hydrogel inks.