Researchers engineered polymorphous sperm-like magnetic microswimmers that could potentially revolutionize targeted drug delivery. A new report described a novel fabrication method using vortex turbulence-assisted microfluidics to create these biocompatible microrobots in a single step.
The team developed polymorphous sperm-like magnetic microswimmers (PSMs) featuring a core-shell head and flexible tail inspired by spermatozoa morphology and function. These microswimmers demonstrated effective motion, drug loading capacity, and controlled release capabilities, according to a newly published paper in Nature Communications.
Key findings included:
- One-step fabrication process using vortex turbulence-assisted microfluidics (VTAM)
- Biodegradable composition with magnetic properties
- Controllable morphology based on vortex flow rotation speed and calcium chloride concentration
- Effective propulsion under remote magnetic actuation
- Sustained drug release capability after alginate-chitosan-alginate (ACA) layer coating.
Fabrication Process
The VTAM platform consisted of a cross-shaped microfluidic chip connected to a vortex container by a glass capillary. The process involved:
- Monodispersed magnetic alginate/oil droplet production
- Slender tail extraction
- Sperm-like morphology solidification.
Researchers optimized channel dimensions (maximum width: 50 μm, narrow junction: 25 μm) and flow rates (disperse:continuous = 0.5:1.5 μL/min) to produce droplets with diameters of 35 to 40 μm.
Morphology Control and Dimensional Characteristics
PSM morphology was classified into three types: helix, regular, and irregular.
Morphology was controlled by adjusting calcium chloride concentration (1-2 wt%) and magnetic stirrer rotation speed (600-1400 rpm). Under optimal conditions (1000 rpm, 1.0-1.5% CaCl2), the production rate of irregular head microswimmers reached 75%. Regular head microswimmers achieved a 50% production rate at 1,000 rpm and 2% CaCl2 concentration.
Irregular head was defined as a cross-sectional diameter of 5 to 8 μm and a length of 80 to110 μm; regular head was defined as a cross-sectional diameter of 10 to 15 μm and a length of 90 to 120 μm; and helix head was defined as a cross-sectional diameter of 2 to 4 μm and a length of 100 to 120 μm.
Motion Control
PSMs were actuated using a rotating magnetic field (strength: 10 mT, tilt angle: 30°, frequency: 0.5-3.0 Hz).
Helix-head PSMs demonstrated superior locomotion efficiency (14.4 μm/cycle) compared to regular (8.2 μm/cycle) and irregular (10.7 μm/cycle) heads. The maximum velocities were ~22 μm/s for helix head, ~18 μm/s for regular head, and ~20 μm/s for irregular head at 1.5 Hz. Average motion straightness was 90% for helix head, 40% for regular head, and 30% for irregular head.
Propulsive Force
Under a magnetic field of 10 mT, 2 Hz frequency, and 30° tilt angle, the propulsive force was:
- Regular head: 46.1 × 10^-4 μN total force (9.3% from head)
- Irregular head: 68.6 × 10^-4 μN total force (13.8% from head)
- Helix head: 97.3 × 10^-4 μN total force (34.4% from head).
Drug Release Capabilities
Researchers coated PSMs with an ACA membrane to enable sustained release. ACA membrane thickness was controlled by reaction time (linear relationship). Optimal thickness for sustained release was ~6 μm. In simulated physiological conditions, an acid environment (pH = 1.2) resulted in a <12.6% release over 8 hours, while an alkaline environment with a pH of 6.8 resulted in a 45% release, and an alkaline environment with a pH of 7.4 resulted in a 60% release.
ACA-coated PSMs had a sustained release between 30 minutes and 4 hours, while uncoated PSMs had an 88% release within 30 minutes and a full release within 1 hour.
In extreme alkaline environments (pH = 10-12), ACA-coated PSMs maintained effective release for nearly 10 minutes, while uncoated PSMs lasted only 8 seconds.
Notably, the microswimmers maintained a relatively stable form in physiological saline solution for 45 minutes, suggesting potential durability in human body–like conditions.
The study presented a novel approach to fabricating sperm-like microswimmers with potential applications in targeted drug delivery. The one-step VTAM process offered a method for creating sophisticated polymorphous structures. The researchers noted that further optimization and in vivo testing would be necessary before clinical application.
The authors declared no competing interests.
