Artificial Muscles from Fishing Line and Sewing Thread

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Science  21 Feb 2014:
Vol. 343, Issue 6173, pp. 868-872
DOI: 10.1126/science.1246906

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  1. Fig. 1 Muscle and precursor structures using nylon 6,6 monofilament sewing thread.

    Optical images of (A) a nontwisted 300-μm-diameter fiber; (B) the fiber of (A) after coiling by twist insertion; (C) a two-ply muscle formed from the coil in (B); (D) a braid formed from 32 two-ply, coiled, 102-μm-diameter fibers produced as in (C); (E) a 1.55-mm-diameter coil formed by inserting twist in the fiber of (A), coiling it around a mandrel, and then thermally annealing the structure; and (F) helically wrapping the fiber of (A) with a forest-drawn CNT sheet and scanning electron microscope images of a CNT-wrapped, 76-μm-diameter nylon 6,6 monofilament (G) before and (H) after coiling by twist insertion.

  2. Fig. 2 Thermomechanical analysis of various muscles.

    (A) Comparison of the negative thermal expansion of braided polyethylene, nylon 6 monofilament, nylon 6,6 monofilament, and silver-coated nylon 6,6 multifilament fibers before twisting (inset) and after coiling by twist insertion. (B) Tensile stroke and nominal modulus versus temperature for a coiled, 300-μm-diameter nylon 6,6 monofilament muscle under 7.5 MPa static and 0.5 MPa dynamic load. During contraction, neighboring coils come into complete contact at ~130°C, which dramatically increases nominal elastic modulus and causes the thermal expansion coefficient to become positive. Optical micrographs (top) are shown of the coils before and after contact. (C) Tensile stroke versus load, as a percent of the loaded muscle length, for a 127-μm-diameter nylon 6,6 monofilament fiber that was coiled by twist insertion under loads of 10, 16, and 35 MPa, which resulted in spring indices of 1.7, 1.4, and 1.1, respectively. Optical images of the coils are inset. (D) The results of (C) when normalized to the initial nonloaded muscle length, indicating that the absolute displacement during actuation remains nearly constant at loads above those at which coils contact.

  3. Fig. 3 Performance of torsional and tensile artificial muscles.

    (A) Comparison of hysteresis for a 152-μm-diameter NiTi wire muscle and a coiled, 127-μm-diameter nylon 6,6 monofilament muscle, measured using a 2°C/min scan rate. Less than 1.2°C of hysteresis is observed for the nylon muscle versus the 27°C hysteresis for the NiTi muscle. (B) Tensile actuation versus cycle for a coiled, 76-μm-diameter, nylon 6,6 monofilament wrapped in a CNT sheet and driven electrothermally at 1 Hz under a 22-MPa load (each point averages 1000 cycles). The inset provides creep as a function of cycle. (C) The optically measured fiber bias angle induced by an applied torque and the torsional stroke and work during thermal actuation (between 20° and 160°C) as a function of this applied torque for a noncoiled torsional muscle made from 860-μm-diameter nylon 6 fishing line (1). The inset photograph was used to optically determine the fiber bias angle by measuring the displacement of a black line from its initial orientation parallel to the fiber axis.

  4. Fig. 4 Mechanism and applications for coiled polymer muscles.

    Hydrothermal actuation of a coiled 860-μm-diameter nylon 6 fishing line lifting a 500-g load by 12% when switched at 0.2 Hz between (A) ~25°C water (dyed blue) and (B) 95°C water (dyed red). (C) Calculated temperature dependence of tensile actuation (dashed lines) compared to experimental results (using an applied stress of 2.2 MPa, solid symbols, and 3.1 MPa, open symbols, respectively) for twisted 450-μm-diameter, nylon 6 monofilament fibers that are mandrel-wrapped to the indicated initial coil bias angles (1). Contracting and expanding coils were homochiral and heterochiral, respectively. From Eq. 3, fiber untwist during heating was calculated for the coiled fiber with a 17° bias angle to provide the data in the inset, which was then used to predict tensile actuation for the other coiled fibers. (D and E) Schematic illustration of the mechanism by which torsional fiber actuation drives large-stroke tensile actuation for (D) heterochiral and (E) homochiral coiled fibers. (F) Measured tensile actuation versus fiber bias angle for coiled, 860-μm-diameter nylon 6 muscles actuated between 25° and 95°C. These results show, for highly twisted fibers, that the homochiral muscles thermally contract when coils are noncontacting, and the heterochiral muscles expand. (G) An actuating textile woven from conventional polyester, cotton, and silver-plated nylon (to drive electrothermal actuation) yarn in the weft direction and coiled nylon monofilament muscle fibers in the warp direction.

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