Wire-form shape memory alloy actuators: modeling, design, and control

Wire-form shape memory alloy (WF-SMA) actuators have become integral components in numerous advanced applications ranging from robotics and aerospace to biomedicine thanks to their exceptional energy density, compact architectures and versatile actuation modes. Serving as a unique bridge between high-force actuation and material compliance, WF-SMAs enable the fabrication of intelligent soft materials and stretchable electronic systems. This work contributes a comprehensive and systematic assessment of WF-SMA actuators, including actuation modeling methodologies, typical actuator configurations, control strategies, and cutting edge applications in multiple fields. We firstly revisit the SMA actuation models with an emphasis on the theoretical foundation as well as current challenges in representing SMA's nonlinear, thermodynamic and actuation behaviors. Then, actuator design paradigms are classified according to the characteristic of the mechanical load (i.e. linear, nonlinear and differential) followed by briefly exploring the large-stroke strategies. Control approaches for manipulating WF-SMA systems are also surveyed covering a spectrum from conventional algorithms to smart strategies based on SMA-specific models, neural networks and integrated self-sensing methods. Drawing upon this assessment, we elucidate the key challenges that impede the widespread and practical application of SMA technologies, and suggest that future fabrication of WF-SMA actuators should increasingly rely on the integration of micro-nano fabrication techniques, flexible electronics, and multifunctional materials. Another promising direction for future research would be to prioritize the development of integrated modeling-design-control frameworks. Leveraging deep learning within the framework to navigate the complex nonlinearities of SMAs will directly improve operational performance and long-term reliability.