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1. Tunable DNA Origami Motors Translocate Ballistically Over μm Distances at nm/s Speeds

   https://doi.org/10.1002/anie.201916281  

   Bazrafshan, Alisina; Meyer, Travis A.; Su, Hanquan; Brockman, Joshua M.; Blanchard, Aaron T.; Piranej, Selma; Duan, Yuxin; Ke, Yonggang; Salaita, Khalid (June 2020, Angewandte Chemie International Edition)

   Abstract Inspired by biological motor proteins, that efficiently convert chemical fuel to unidirectional motion, there has been considerable interest in developing synthetic analogues. Among the synthetic motors created thus far, DNA motors that undertake discrete steps on RNA tracks have shown the greatest promise. Nonetheless, DNA nanomotors lack intrinsic directionality, are low speed and take a limited number of steps prior to stalling or dissociation. Herein, we report the first example of a highly tunable DNA origami motor that moves linearly over micron distances at an average speed of 40 nm/min. Importantly, nanomotors move unidirectionally without intervention through an external force field or a patterned track. Because DNA origami enables precise testing of nanoscale structure‐function relationships, we were able to experimentally study the role of motor shape, chassis flexibility, leg distribution, and total number of legs in tuning performance. An anisotropic rigid chassis coupled with a high density of legs maximizes nanomotor speed and endurance. 

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2. The motor and the brake of the trailing leg in human walking: transtibial amputation limits ankle–knee torque covariation

   https://doi.org/10.1007/s00221-022-06513-5  

   Toney-Bolger, Megan E.; Chang, Young-Hui (January 2023, Experimental Brain Research)

   Lower-limb amputation limits inherent motor abundance in the locomotor system and impairs walking mechanics. Able- bodied walkers vary ankle torque to adjust step-to-step leg force production as measured by resultant ground reaction forces. Simultaneously, knee torque covaries with ankle torque to act as a brake, resulting in consistent peak leg power output meas- ured by external mechanical power generated on the center of mass. Our objective was to test how leg force control during gait is affected by joint torque variance structure in the amputated limb. Within the framework of the uncontrolled manifold analysis, we measured the Index of Motor Abundance (IMA) to quantify joint torque variance structure of amputated legs and its effect on leg force, where IMA>0 indicates a stabilizing structure. We further evaluated the extent to which IMA in amputated legs used individual (INV) and coordinated (COV) joint control strategies. Amputated legs produced IMA and INV values similar to intact legs, indicating that torque deviations of the prosthetic ankle can modulate leg force at the end of stance phase. However, we observed much lower COV values in the amputated leg relative to intact legs indicating that biological knee joint torque of the amputated leg does not covary with prosthetic ankle torque. This observation sug- gests inter-joint coordination during gait is significantly limited as a result of transtibial amputation and may help explain the higher rate of falls and impaired balance recovery in this population, pointing to a greater need to focus on inter-joint coordination within the amputated limb. 

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3. Direct Encoding of Tunable Stiffness Into an Origami-Inspired Jumping Robot Leg

   https://doi.org/10.1115/1.4056958  

   Chen, Fuchen; Aukes, Daniel M. (March 2024, Journal of Mechanisms and Robotics)

   Abstract The stiffness of robot legs greatly affects legged locomotion performance; tuning that stiffness, however, can be a costly and complex task. In this paper, we directly tune the stiffness of jumping robot legs using an origami-inspired laminate design and fabrication method. In addition to the stiffness coefficient described by Hooke’s law, the nonlinearity of the force-displacement curve can also be tuned by optimizing the geometry of the mechanism. Our method reduces the number of parts needed to realize legs with different stiffness while simplifying manual redesign effort, lowering the cost of legged robots while speeding up the design and optimization process. We have fabricated and tested the leg across six different stiffness profiles that vary both the nonlinearity and coefficient. Through a vertical jumping experiment actuated by a DC motor, we also show that proper tuning of the leg stiffness can result in an 18% improvement in lift-off speed and an increase of 19% in peak power output. 

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4. The nanocaterpillar's random walk: diffusion with ligand–receptor contacts

   https://doi.org/10.1039/D1SM01544C  

   Marbach, Sophie; Zheng, Jeana Aojie; Holmes-Cerfon, Miranda (April 2022, Soft Matter)

   Particles with ligand–receptor contacts bind and unbind fluctuating “legs” to surfaces, whose fluctuations cause the particle to diffuse. Quantifying the diffusion of such “nanoscale caterpillars” is a challenge, since binding events often occur on very short time and length scales. Here we derive an analytical formula, validated by simulations, for the long time translational diffusion coefficient of an overdamped nanocaterpillar, under a range of modeling assumptions. We demonstrate that the effective diffusion coefficient, which depends on the microscopic parameters governing the legs, can be orders of magnitude smaller than the background diffusion coefficient. Furthermore it varies rapidly with temperature, and reproduces the striking variations seen in existing data and our own measurements of the diffusion of DNA-coated colloids. Our model gives insight into the mechanism of motion, and allows us to ask: when does a nanocaterpillar prefer to move by sliding, where one leg is always linked to the surface, and when does it prefer to move by hopping, which requires all legs to unbind simultaneously? We compare a range of systems (viruses, molecular motors, white blood cells, protein cargos in the nuclear pore complex, bacteria such as Escherichia coli , and DNA-coated colloids) and present guidelines to control the mode of motion for materials design. 

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5. Motor guidance by long-range communication on the microtubule highway

   https://doi.org/10.1073/pnas.2120193119  

   Wijeratne, Sithara S.; Fiorenza, Shane A.; Neary, Alex E.; Subramanian, Radhika; Betterton, Meredith D. (July 2022, Proceedings of the National Academy of Sciences)

   Coupling of motor proteins within arrays drives muscle contraction, flagellar beating, chromosome segregation, and other biological processes. Current models of motor coupling invoke either direct mechanical linkage or protein crowding, which rely on short-range motor–motor interactions. In contrast, coupling mechanisms that act at longer length scales remain largely unexplored. Here we report that microtubules can physically couple motor movement in the absence of detectable short-range interactions. The human kinesin-4 Kif4A changes the run length and velocity of other motors on the same microtubule in the dilute binding limit, when approximately 10-nm–sized motors are much farther apart than the motor size. This effect does not depend on specific motor–motor interactions because similar changes in Kif4A motility are induced by kinesin-1 motors. A micrometer-scale attractive interaction potential between motors is sufficient to recreate the experimental results in a biophysical model. Unexpectedly, our theory suggests that long-range microtubule-mediated coupling affects not only binding kinetics but also motor mechanochemistry. Therefore, the model predicts that motors can sense and respond to motors bound several micrometers away on a microtubule. Our results are consistent with a paradigm in which long-range motor interactions along the microtubule enable additional forms of collective motor behavior, possibly due to changes in the microtubule lattice. 

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