The unit underlying the construction and functioning of muscle fibers is the sarcomere. Tension develops in fibers as thousands of sarcomeres arranged in series contract in unison. Shortening is due to the sliding of actin thin filaments along antiparallel arrays of myosin thick filaments. Remarkably, myosin catalytic heads situated across the center M-line of a sarcomere are separated by a distance that is a half integral of the 14.5 nm spacing between successive layers of myosin heads on the thick filaments. This results in the splitting of the 14.5 nm meridional reflection in X-ray diffraction patterns of muscle fibers. Following a quick drop in tension, changes in the relative intensities of the split meridional peaks provide a sensitive measure of myosin head movements. We use published data obtained with the x-ray interference method to validate a theory of muscle contraction in which cooperative structural transitions along force-generating actin filaments regulate the binding of myosin heads. The probability that an actin-bound myosin head will detach is represented here by a statistical function that yields a length-tension curve consistent with classical descriptions of the recovery of contracting muscle fibers subjected to millisecond drops in tension. ### Competing Interest Statement The authors have declared no competing interest.

Temperature is a global factor that affects the performance of all intracellular networks. Robustness against temperature variations is thus expected to be an essential network property, particularly in organisms without inherent temperature control. Here, we combine experimental analyses with computational modeling to investigate thermal robustness of signaling in chemotaxis of Escherichia coli, a relatively simple and well-established model for systems biology. We show that steady-state and kinetic pathway parameters that are essential for chemotactic performance are indeed temperature-compensated in the entire physiological range. Thermal robustness of steady-state pathway output is ensured at several levels by mutual compensation of temperature effects on activities of individual pathway components. Moreover, the effect of temperature on adaptation kinetics is counterbalanced by preprogrammed temperature dependence of enzyme synthesis and stability to achieve nearly optimal performance at the growth temperature. Similar compensatory mechanisms are expected to ensure thermal robustness in other systems.

Conducting polymer materials can be employed as actuation elements, length sensors, force sensors, energy storage devices, and electrical components. Combining the various functionalities of conducting polymers to create singlesubstrate, integrated systems remains a challenge, as chemical and electrical insulation barriers, adhesion techniques, and the possibility of scaling need to be taken into consideration. Here fabrication techniques for combining multiple conducting polymer components by means of electrically insulated, mechanical attachments are developed. Electrochemically synthesized polypyrrole substrates were coated with thin films of polystyrene, Parylene, and polyimide. The isotonic actuation performance of each coated film was evaluated in comparison to non-coated films, with an observed decrease in peak-to-peak maximum strain output near 95% (polystyrene and Parylene), 20% (vacuum, 0.8 Pa), 50% (curing at 110°C) and 20% (localized polyimide deposition). The chemical barrier properties of each manufacturing technique were evaluated by exposing the coated polypyrrole substrates to an oxidative chemical vapor deposition of Poly(3,4-ethylenedioxythiophene) (PEDOT). Vapor-deposited PEDOT made the insulation layers of polystyrene and Parylene conductive at thicknesses up to four microns. Spin-coated films of polyimide, greater than ten microns thick, maintained electrical insulation properties after PEDOT depositions. Conducting polymer film-to-film attachments using each manufacturing technique were attempted, with polyimide working successfully when employed under a specific deposition, drying, and curing protocol, as discussed. Three dimensional conducting polymer actuation systems composed of actuators, length sensors, and energy storage devices were constructed on flexible, single substrates. These results build a foundation upon which scalable, self-powered, polymer actuation systems can be developed.

Conducting polymer actuators such as polypyrrole can generate stresses over 10 times larger than skeletal muscle and have typical repeatable strains between 1% and 12%, making them potential candidates for lightweight, low-cost, robotic applications. Polypyrrole linear actuators under closed loop control have not been previously reported. Here we report the open and closed loop performance of polypyrrole linear contractile actuators evaluated at pre-loaded stresses of 1 MPa to 3 MPa. A standard PI control scheme driving a potentiostat was implemented in conjunction with positioning feedback from a DC/DC linear variable differential transformer (LVDT). A dynamic positioning range of 3400 is reported, with a positioning resolution of 125 nm (0.001% strain) and a maximum repeatable displacement of 427 microns (3.6% strain). The open loop frequency response of actuator strain shows characteristics of a first-order low pass filter with a log gain versus log frequency slope near -1 for frequencies tested between 0.05 Hz to 2 Hz. The closed loop frequency response of actuator strain when tracking a sinusoidal set-point signal of 0.5% strain shows characteristics of a first order system with one zero, with a corner frequency near 0.08 Hz and an operating bandwidth up to 1 Hz. Step responses at various controller output maximum voltages show a reduction in contractile response times by a factor of four, where higher voltages yield faster contractile responses.

Conducting polymers are employable as low-voltage actuators, sensors, energy storage and delivery components, structural elements, computational circuitry, memory, and electronic components, making them a versatile choice for creating integrated, multifunctional materials and devices. Here we show one such conducting polymer-based, multifunctional system, derived from the versatility of the conducting polymer polypyrrole. Three functions of polypyrrole (actuation, length sensation, and energy storage) have been individually evaluated and cooperatively combined in the synthesis of a multifunctional, polymeric system that actuates, senses strain deformation, and stores energy. The system operates whereby the strain of a polypyrrole actuator is measured by a polypyrrole length sensor, whilst being powered by an array of polypyrrole supercapacitors. Independently, polypyrrole actuators were evaluated at 250 discrete frequencies ranging from 0.01 to 10 Hz using fixed, ±1 V sinusoidal excitation. Polypyrrole length sensors were evaluated using a thin-film dynamic mechanical analyzer for the same range of frequencies with a 2% sinusoidal input strain. Polypyrrole supercapacitors were evaluated using cyclic voltammetry (−1.0 V to +1.0 V; 12.5 to 100 mV/sec) and galvanostatic charge-discharge cycling (0.5 to 2 mA/mg). As an actuator, polypyrrole samples showed measureable actuation strain between 0.001% and 1.6% for the frequency range tested, with amplitude versus frequency decay behavior similar to a first-order low-pass filter. As a length sensor, polypyrrole samples showed linearelastic behavior up to 3% strain and gauge factors near 4. As a symmetric supercapacitor, polypyrrole had capacitance values higher than 20 kF/kg, energy densities near 20 kJ/kg, and power densities near 2 kW/kg. The evaluation of each component, independently, justified creating a cooperative system composed of these three components operating simultaneously. Polypyrrole supercapacitors provided ample power to excite polypyrrole actuators. Polypyrrole length sensors attached in series to polypyrrole actuators were capable of measuring strain from coupled polypyrrole actuators. Performance metrics and future possibilities regarding conducting polymer-based multifunctional materials are discussed.

Swimming Escherichia coli responds to changes in temperature by modifying its motor behavior. Previous studies using populations of cells have shown that E. coli accumulate in spatial thermal gradients, but these experiments did not cleanly separate thermal responses from chemotactic responses. Here we have isolated the thermal response by studying the behavior of single, tethered cells. The motor output of cells grown at 33°C was measured at constant temperature, from 10° to 40°C, and in response to small, impulsive increases in temperature, from 23° to 43°C. The thermal impulse response at temperatures < 31°C is similar to the chemotactic impulse response: Both follow a similar time course, share the same directionality, and show biphasic characteristics. At temperatures > 31°C, some cells show an inverted response, switching from warm- to cold-seeking behavior. The fraction of inverted responses increases nonlinearly with temperature, switching steeply at the preferred temperature of 37°C.