Shape memory alloys (SMAs) have been widely used in vascular stents, orthodontic archwires, and other fields due to their superelasticity and shape memory effect. However, existing applications predominantly exploit these properties in a single-use, domain-specific manner; no study has systematically abstracted the reversible transition between martensitic and austenitic phases into a universal design paradigm applicable across multiple engineering domains. This paper proposes, for the first time, the "Stiffness-Switching" paradigm: defining the controllable transition of SMAs between a martensitic state (flexible/compressible) and an austenitic state (rigid/high-energy-absorbing) as a unified design methodology encompassing four functional outputs: adaptive constant-force fitting, dynamic impact energy absorption, multi-stage programmable force control, and intracavitary adaptive support. Based on this paradigm, we further introduce and define ten original design principles, including "Suture-Without-Tightening", "Corner-Anchor Spatial Web", "Point-Actuation Perimeter-Absorption", "Variable Cross-Section Gradient Force Programming", and others. Four cross-domain case studies—a passive self-adaptive anti-snoring device, a modular quick-release fixation interface, a passive multi-level impact protection container, and a staged-force SMA adaptive fracture external fixation splint system—demonstrate the universality of the proposed paradigm. The paper also briefly discusses the potential of this paradigm in aerospace mechanical applications. The Stiffness-Switching paradigm provides a unified theoretical framework and design guide for SMA-based adaptive devices. The design methodology adopted in this paper is inspired by first-principles reasoning—abstracting the phase transformation behavior of SMA as a controllable switching between "soft" and "stiff" states, rather than making incremental improvements over existing adaptive solutions. Between soft and stiff, there is only a phase transformation.
hanbin zhao (Mon,) studied this question.