Synthesis of Hydrogel Microparticles from Ultra‐High‐Viscosity Pre‐Gel Solution by Combining Centrifugal Force and Ultrasonic Vibration

Viscosity is a key physical parameter influencing droplet generation. Herein, the synthesis of hydrogel microparticles from an ultra-high-viscosity pre-gel (UHV-HMPs) solution using a microparticle synthesis device combining centrifugal force with ultrasonic vibration (MSDCU) is reported. The advantage of MSDCU is that it can synthesize closely spherical microparticles from ultra-high-viscosity pre-gel solutions with viscosities exceeding 10⁴ mPa·s in the air/water method. The ultrasonic droplet-shooting module embedded in the MSDCU ejects an ultra-high-viscosity pre-gel solution of sodium alginate through a capillary under ultrasonic vibration to overcome the viscosity limitations of using only centrifugal force. Based on an evaluation of the vibration modes of a multilayer transducer with a capillary at various resonance frequencies, a vibration mode with longitudinal and lateral vibrations of 100 kHz is selected. The combination of ultrasonic vibration at 100 kHz and a centrifugal force of 230g results in the ejection of pre-gel solution with a maximum viscosity of 1.5 × 10⁵ mPa·s. At a viscosity of 1.5 × 10⁴ mPa·s, control of the distance between the capillary and crosslinking solution improves particle circularity, enabling the formation of spherical UHV-HMPs with a diameter of ≈140 µm. With a higher viscosity solution (3.0 × 10⁴ mPa·s), smaller homogeneous satellite particles with a diameter of ≈31 µm are synthesized after filtering.