Fig. 3. Picospring as a real-time indicator for piconewton-scale forces.
a, Schematic illustration of the microforcemeter application in measuring the propulsion forces of different types of microswimmer: tubular sperm–motor (biohybrid), actuated by the sperm flagella; microjet (chemical), actuated by the generated bubbles; microhelices (physical), actuated magnetically. All microswimmers are guided magnetically. The microforcemeter inherits the mechanical properties and fabrication accuracy of the cantilever picospring employed for the characterization purpose above, resulting in a high measurement consistency. The force-sensing range and limit are both tunable by the picospring configuration and the laser power of TPL. We use the photoresist without MNPs to fabricate the microforcemeter, to avoid the influence of the magnetic field on the measurement accuracy. b, Microforcemeter deformation under the propulsion force of: A, one-tube sperm–motor; B, one-tube sperm–motor at 25 °C; C, two-tube sperm–motor; D, six-tube sperm–motor. All microswimmers except B were measured at 37 °C. c, Peak propulsion forces and swimming velocities of different microswimmers (n =15 microswimmers for each group). Boxes plot minimum, first quartile, median, third quartile and maximum values. The sperm–motor trains, consisting of multiple tubes, show no significant difference from the one-tube sperm–motor in the propulsion force measured at a speed of 0 μm/s. This is consistent with the fact that the elastic force of the cantilever picospring accurately equals the propulsion force provided by the sperm when there is no fluid drag (otherwise the drag force is typically proportional to velocity with a coefficient of friction determined by geometry). Thus, in the static state, the influence of the volume and friction of the synthetic part of the sperm–motor can be neglected on the picospring-based measurement of the sperm’s propulsion force. d, Microforcemeter deformation under: E, chemical microjet; F, long microhelix; G, short microhelix. A longer microhelix has higher propulsion force and swimming velocity under the same rotating magnetic field. This is consistent with the increased frictional drag for the longer helix. Scale bar, 20 μm.