Twisted and coiled bamboo artificial muscles for moisture responsive torsional and tensile actuation

Structure and mechanical properties for bamboo yarns. (a) SEM images with low (left) and high (right) magnifications showing the cross-sectional monofilaments and porous structure in the micron- and nanometer scale. (b) SEM images of the pristine monofilament (upper), twisted yarn (middle), and two-plied bamboo yarn (lower). (c) Tensile stress–strain curves of bamboo yarns with different twist densities. The elongation rate was 25%⋅ min⁻¹. (d) Elongation at break and fracture strength as a function of twist density for non-plied bamboo yarns. (e) Mesopores of silk, bamboo, and cotton yarns, with surface area calculated by the BET method and pore volume calculated by the adsorption curve of N₂ gas. (f) Kinetics of water absorption and desorption of the bamboo yarn.

Fabrication and actuation performance of self-balanced two-ply torsional bamboo muscles. (a) Schematic illustrations of the fabrication of two-ply torsional bamboo muscles. (b) The torsional stroke as a function of time during the first 5 cycles. (c) The torsional stroke and rotation speed as a function of inserted twist. (d) The effect of actuation stress on the torsional actuation performance. For panels (b) and (c), the actuation stress was 2.01 MPa, and for panels (b) and (d), the twist density was 800 turns⋅m⁻¹.

Fabrication and actuation performances of mandrel coiled bamboo muscles for tensile actuation. (a) Schematic illustrations for the preparation of the tensile muscles. (b) The tensile stroke as a function of the number of actuation cycles for ZZ- and ZS-type muscles. (c) The tensile stroke for muscles with different twist densities and actuated at various humidities. (d) Work capacity as a function of applied stress for muscles with varied spring index. (e) Tensile strain normalized by loaded length for ZZ-type muscles. If not otherwise specified, the twist density was 900 turns⋅m⁻¹, the spring index was 4.3, the relative humidity was 90%, and the applied stress was 0.33 kPa for panels (b)–(e).

Tensile actuation normalized by the non-loaded bamboo muscle length. (a) Tensile strains for muscles with different C that actuated in ambient humidity from 50%–90%. (b) Tensile strains as a function of coil pitch. (c) Cyclability of the tensile stroke for ZS- and ZZ-type muscles. (d) The non-loaded length normalized tensile strain as a function of applied stress for ZZ muscles with varied spring index. If not otherwise specified, the twist density was 900 turns⋅m⁻¹, the spring index was 4.3, the relative humidity was 70%, and the applied stress was 0.33 kPa for panels (a)–(d).

The mechanism for shape fixing and actuation of mandrel coiled bamboo muscle. (a) X-ray diffraction curves for twisted muscles without treatments, followed by immersion in water, after shape fixing by drying out, and after multiple cycles of actuation. (b) The peak fitting profile for pristine bamboo fibers. (c) Crystallinity and the ratio of cellulose II to cellulose I for bamboo fibers with various treatment conditions. (d) Schematic illustration for the proposed mechanism of shape fixing and actuation for mandrel-coiled bamboo fibers.

Demonstration of the ZZ mandrel coiled bamboo fibers for intelligent smart skirts under moisture activation, with photos (a) before actuation, (b) during actuation, and (c) after actuation.