Fig. 2: Fluid-based assembly and performance characterization.

a Schematics showing the fluid-based assembly of linear magnets in a microfluidic system. The heated disperse phase and continuous phase (sodium alginate, SA) are simultaneously pumped into the microfluidic device. Subsequently, the disperse phase breaks into a sequence of uniformly sized droplets. Being exposed to a CaCl₂ solution at room temperature, the biphasic liquid solidifies owing to the chemical crosslinking of alginate hydrogel and fluid-to-solid PCM phase transition. b The production rate of the linear magnet in a linear relationship to the total flow rate. The inset shows a photograph of a roll of linear magnets. Scale bar: 5 mm. The error bar indicates the standard deviations (SDs) of three different measurements. c SEM images of the dehydrated linear magnet with uniformly sized micro cells. The inset shows the magnetization-reorientable micro cells and hydrogel structure (dehydrated). Scale bars: 400 μm, 100 μm (inset). d The optical microscopy images showing the in situ programming of the micro cells. The magnetization direction changes from vertical to horizontal after laser heating. The mass fraction of NdFeB particles in the cell is diluted tenfold for clear visualization of the particle rotation. Scale bar: 200 μm. e Thermal response of a single micro cell at different laser power. Blue and pink shaded regions, respectively, denote off and on states of the NIR laser. The heated cell is cooled naturally at room temperature. f Infrared thermal image showing the temperature distribution of a linear magnet heated by a 0.35 W cm⁻² laser. The corresponding temperature profile is extracted along the lengthwise centerline. The laser is positioned at x = 0 mm. Scale bar: 1 mm. g Spatial coordinates of a bending linear magnet under stepwise increasing magnetic flux densities, ranging from 0 to 30 mT. Gravitational acceleration is along with the positive y-axis. h The deflection of linear magnets of different length-diameter ratios (L/d) upon magnetic actuation. The inset shows the image of a bending linear magnet. The error bar indicates the SDs of three different measurements. Scale bar: 4 mm. Source data are provided as a Source Data file.