Preparation of MR yarns
We selected LDPE as the flexible matrix because of its high filler capacity enabled by highly branched chains with low molecular packing46 and good flowability indicated by a broad molecular weight distribution (Supplementary Fig. 23) and significant shear thinning (Extended Data Fig. 1a). CIP was chosen over other soft magnetic materials for its high susceptibility and low remnant magnetization, availability in micro-spherical particles, cost-effectiveness and resistance to oxidation with SiO 2 coating (Supplementary Table 2). CIPs coated with SiO 2 (SQ, BASF SE; Supplementary Fig. 24) were dispersed within LDPE (1700 MN 18C, total energies SE; melt flow rate of 70 g/10 min at 190 °C/2.16 kg) by melt compounding by a twin-screw extruder (Thermo Fisher Hot Melt Extruder Pharma 11). The temperature profile, ranging from the hopper to the die, was set at 10, 80, 130, 150, 150, 150, 150, 150, and 150 °C, respectively, while the screw speed was kept at 50 rpm. Four LDPE/CIP composites were prepared, varying in CIP content at 30, 50, 70, and 80 wt%. Interfacial interactions between LDPE molecules and CIPs (Supplementary Note 8 and Supplementary Fig. 25) promote the wetting of polymer on CIP surfaces, ensuring the uniform CIP dispersion of composites with filler content up to 80 wt% (Extended Data Fig. 7). Among these, the composite containing 70 wt% CIPs was chosen for fibre spinning.
This selected composite was introduced into the barrel of a laboratory melt spinning machine (AT225, Anytester Hefei) to produce MR fibres. The three heating zones of the barrel were set at 140 °C, 150 °C and 160 °C, respectively, allowing the composites to melt for 10 min until reaching a stable temperature of 160 °C. Nitrogen was then filled into the barrel to provide a pressure of 1.2 MPa. The molten polymer strands were extruded through a spinneret and guided by a ceramic wheel onto a collection roller with a diameter of 8.2 cm. A cooling fan positioned between the spinneret and the guide wheel facilitated solidification of the molten composite fibres. The diameter of the resulting MR fibre could be controlled by adjusting the winding speed, and in this case, the extruded molten filaments with a diameter of 750 μm underwent rapid thinning over a short range of about 20 cm (Extended Data Fig. 3). MR fibres with a diameter of 57 μm were consistently produced at a winding speed of 130 rpm. Then, seven of these fully drawn fibres were twisted clockwise to form a yarn, which was subsequently heat-set in an oven at 60 °C for 1 h to alleviate residual stress and stabilize the twist configuration. The primary goal in optimizing the structure of MR yarn was to maximize A y /I y , which is inversely related to the diameter of the fibres and directly related to the helical angle of the yarn (Supplementary Note 1). Given that the fibre diameter directly correlates with its modulus, our evaluation focused on optimizing A y /EI y to refine the MR yarn design.
Fabrication of MR fabrics
Woven MR fabrics were fabricated by interlacing MR yarns as weft and sewing thread as warp at right angles, using a hand weaving machine. Float lengths of 1, 2 and 4 were specifically chosen to fabricate plain-, twill-, and satin-weave MR fabrics that exhibit packing densities of 65 yarns cm−1, 83 yarns cm−1, and 111 yarns cm−1, respectively (Extended Data Fig. 8). These selections enable the examination of the spatial freedom of interlacing MR yarns concerning their float length and packing density. The plain weave has the highest number of interlacing points, followed by the twill weave, and the satin weave.
Cut-pile MR fabrics were fabricated by inserting MR yarns into holes of a base plain-weave fabric using a punch needle kit. On pulling the needle out, a loop of MR yarn formed on the opposite side, its length controlled by the depth of needle insertion. This process repeated until the predefined area was uniformly filled at yarn densities of 250 yarns cm−2 or 500 yarns cm−2. Subsequently, a thin layer of silicone glue was applied to secure the inserted yarns. The loops were then cut at their centre points, followed by the application of silicone precursor at the cut tips. After curing overnight at room temperature, the silicone effectively prevented the free ends from untwisting.
Coefficient of friction
The as-spun filaments, each with a length of 3 cm, were arranged in parallel without any gaps to form a 1-cm wide region and adhered onto a square glass measuring 3 cm × 3 cm and weighing 1.8 g using a double-sided tape. Two identical samples were prepared for the measurement: one with the filaments facing upwards, and the glass side fixed onto a horizontal linear platform, and the other sample with the filaments facing downwards, aligned face-to-face and parallel to the filament region of the first sample. At the left edge of the top glass packed with filament tips, the centre point of the glass edge was horizontally connected to a force gauge using a nylon filament. A series of weights (1 g, 2 g, 5 g, 10 g, 20 g, 50 g and 100 g) were separately placed on top of the sample to provide normal force F nf = m tm g, where m tm is the total mass of the weight and top glass substrate, and g is the acceleration due to gravity. The linear platform moved away from the force gauge at a velocity of 5 µm s−1. The maximum reading of the force gauge was recorded as the maximum static friction F friction , and the static friction coefficient a was calculated using the formula: F friction = aF nf . The static coefficient of friction between MR fibres was measured in the same way, by replacing the filaments with fibres.
Differential scanning calorimetry
Differential scanning calorimetry experiments were conducted using a Mettler Toledo DSC3 in a nitrogen atmosphere. Each sample, weighing approximately 1.5 mg, was analysed within a temperature range of 55–135 °C at a heating rate of 10 °C min−1 to record the endothermic curves. The heat of fusion (ΔH f ) was determined by integrating the heat flow between 60 °C and 115 °C. This parameter was then used to calculate the crystallinity degree (χ) of both pure LDPE and the composite matrix. The crystallinity degree χ was defined as the ratio of ΔH f /(1 − x), where x represents the content of CIPs in the composite, to the heat of fusion (289.9 J g−1) of the purely crystalline form of polyethylene47.
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