In this paper, the concept of scalability for actuators is introduced and explored, which is the capability to freely change the output characteristics on demand: displacement and force for a linear actuator, angular position and torque for a rotational actuator. This change can either be used to obtain power improvement (with a constant scale factor), or to improve the usability of a robotic system according to variable conditions (with a variable scale factor). Some advantages of a scalable design include the ability to adapt to changing environments, variable resolution of step size, ability to produce designs that are adequate for restricted spaces or that require strict energy efficiency, and intrinsically safe systems. Current approaches for scalability in actuators have shortcomings: the method to achieve scalability is complex, so obtaining a variable scaling factor is challenging, or they cannot scale both output characteristics simultaneously. Shape Memory Alloy (SMA) wire-based actuators can overcome these limitations, because its two output characteristics, displacement and force, are physically independent from each other. In this paper we present a novel design concept for linear scalable actuators that overcome SMA design and scalability limitations by using a variable number of SMA wires mechanically in parallel, immersed in a liquid that transmits heat from a separate heat source (wet activation). In this concept, more wires increase the maximum attainable force, and longer wires increase the maximum displacement. Prototypes with different number of SMA wires were constructed and tested in isometric experiments to determine force vs. temperature behavior and time response. The heat-transmitting liquid was either static or flowing using pumps. Scalability was achieved with a simple method in all tested prototypes with a linear correlation of maximum force to number of SMA wires. Flowing heat transmission achieved higher actuation bandwidth.

Dielectric elastomer actuators (DEAs) are a recent type of smart materials that show impressive performances as soft actuators, making them a promising technology for the development of implantable artificial muscles and soft robotic devices. Notably, they are explored as implants for the restoration of facial movements post paralysis. However, implementing DEAs that mimic natural muscles has been proven difficult, as DEAs provide in-plane expansion when actuated, while natural muscles contract upon stimulation. Multiple solutions can be found in literature, namely stack DEAs and fiber-reinforced DEAs. The fibers used for DEAs to achieve contractile motion rely on a fishnet design, where the angle between the fibers, the spacing, mechanical properties as well as the fiber dimensions can be set by establishing a fiber analytical model. Contraction has only been achieved with DEAs based on acrylic elastomer and pre-stretched with rigid frames, thus making them unsuitable as soft implants. This work introduces the first silicone-based, non-pre-stretched DEAs presenting in-plane contractile behavior by embedding such soft structured fiber sheets in the actuators. Fiber-reinforced DEAs were shown to achieve modest contractile strains, particularly with optimal fiber angles between 55^∘ and 65^∘, enhancing their ability to mimic muscle-like behavior. A peak occurs at approximately 60^∘, where the maximum contraction of −0.6% was achieved, resulting in a 3% error from the model. The low contractile strains of silicone-based DEAs indicate that further optimization is needed for real-world applications.

Diabetic foot ulcers (DFUs) remain a major complication of diabetes mellitus, with significant human and economic costs. Plantar pressure (PP) is the most widely used surrogate to assess DFU risk. Existing PP measurement insoles are primarily designed for in-clinic assessments using external instrumentation to guide passive offloading strategies. However, reliance on brief clinical measurements limits their ability to capture real-world loading patterns, motivating the development of sensing systems capable of continuous monitoring and integration within pressure-management footwear. Such systems require spatially discretized sensing elements fully embedded within the shoe architecture. This work presents the design and characterization of P-MAPS (Plantar-Monitoring Au-Plated Pressure System), a custom-topology pressure-sensing insole co-designed for precise spatial alignment with offloading elements. The sensor uses the piezoresistive response of Velostat combined with gold-plated electrodes to improve uniformity and repeatability. The device exhibits a measurement range of 45–350 kPa, a coefficient of variation below 2.5%, normalized hysteresis of 25.8%, and 6% drift after 5 min. Dynamic gait validation against a commercial reference (Pedar, Novel) demonstrated accurate peak-pressure estimation with 2.6% error under full spatial overlap, while enabling localized pressure measurement over actuated regions and revealing that sensing areas not co-located with actuator regions can misrepresent local pressures due to spatial averaging effects. These results establish P-MAPS as a robust and practical sensing solution for continuous, footwear-integrated plantar-pressure monitoring and adaptive offloading applications.

The non-linearity of shape memory alloy’s (SMA) input vs. output relationship has been a significant barrier to their broader application for actuators. In this article, we introduce a novel activation approach with the objective of improving the output nonlinearity in SMA actuators by using a liquid temperature gradient for activation. This gradient, with a narrow inflection point, selectively activates and deactivates different portions of the SMA, allowing control of the overall actuator’s output by adjusting the inflection point’s position. Two prototypes with the proposed concept were constructed and tested for force and displacement output. Their inner shape design facilitates the creation of a temperature gradient, whose inflection point is adjusted by varying the mix ratio of the inlet of hot and cold water. The performance of the prototypes was evaluated, including their ability to generate a temperature gradient, output linearity, activation dead time and time constant, and compatibility with a linear controller. The output linearity, activation dead time, and time constant were also compared with traditional wet activation. Our findings suggest that this technique significantly enhances the performance and predictability of SMA-based actuators and is compatible with PID controllers without additional feedforward or modeling techniques, offering a practical solution to a long-standing challenge in the field.