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1. Design of a Variable Stiffness Spring with Human-Selectable Stiffness

   https://doi.org/10.1109/ICRA48891.2023.10161305  

   Mathews, Chase W.; Braun, David J. (May 2023, 2023 IEEE International Conference on Robotics and Automation (ICRA))
2. Heterogeneous digital stiffness programming

   https://doi.org/10.1016/j.eml.2022.101832  

   Tao, Hongcheng; Danzi, Francesco; Silva, Christian E.; Gibert, James M. (August 2022, Extreme Mechanics Letters)
3. Materials with Electroprogrammable Stiffness

   https://doi.org/10.1002/adma.202007952  

   Levine, David J.; Turner, Kevin T.; Pikul, James H. (July 2021, Advanced Materials)

   Abstract Stiffness is a mechanical property of vital importance to any material system and is typically considered a static quantity. Recent work, however, has shown that novel materials with programmable stiffness can enhance the performance and simplify the design of engineered systems, such as morphing wings, robotic grippers, and wearable exoskeletons. For many of these applications, the ability to program stiffness with electrical activation is advantageous because of the natural compatibility with electrical sensing, control, and power networks ubiquitous in autonomous machines and robots. The numerous applications for materials with electrically driven stiffness modulation has driven a rapid increase in the number of publications in this field. Here, a comprehensive review of the available materials that realize electroprogrammable stiffness is provided, showing that all current approaches can be categorized as using electrostatics or electrically activated phase changes, and summarizing the advantages, limitations, and applications of these materials. Finally, a perspective identifies state‐of‐the‐art trends and an outlook of future opportunities for the development and use of materials with electroprogrammable stiffness. 

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4. Directional Stiffness Modulation of Parallel Robots with Kinematic Redundancy and Variable Stiffness Joints

   https://doi.org/10.1115/1.4043685  

   Orekhov, Andrew L.; Simaan, Nabil (July 2019, Journal of Mechanisms and Robotics)

   Parallel robots have been primarily investigated as po- tential mechanisms with stiffness modulation capabilities through the use of actuation redundancy to change internal preload. This paper investigates real-time stiffness modula- tion through the combined use of kinematic redundancy and variable stiffness actuators. A known notion of directional stiffness is used to guide the real-time geometric reconfig- uration of a parallel robot and command changes in joint- level stiffness. A weighted gradient-projection redundancy resolution approach is demonstrated for resolving kinematic redundancy while satisfying the desired directional stiffness and avoiding singularity and collision between the legs of a Gough/Stewart parallel robot with movable anchor points at its base and with variable stiffness actuators. A simulation study is carried out to delineate the effects of using kinematic redundancy with or without the use of variable stiffness ac- tuators. In addition, modulation of the entire stiffness matrix is demonstrated as an extension of the approach for modulating directional stiffness. 

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5. A Protein‐Adsorbent Hydrogel with Tunable Stiffness for Tissue Culture Demonstrates Matrix‐Dependent Stiffness Responses

   https://doi.org/10.1002/adfm.202309567  

   Li, Linqing; Griebel, Megan E.; Uroz, Marina; Bubli, Saniya Yesmin; Gagnon, Keith A.; Trappmann, Britta; Baker, Brendon M.; Eyckmans, Jeroen; Chen, Christopher S. (January 2024, Advanced Functional Materials)

   Abstract Although tissue culture plastic has been widely employed for cell culture, the rigidity of plastic is not physiologic. Softer hydrogels used to culture cells have not been widely adopted in part because coupling chemistries are required to covalently capture extracellular matrix (ECM) proteins and support cell adhesion. To create an in vitro system with tunable stiffnesses that readily adsorbs ECM proteins for cell culture, a novel hydrophobic hydrogel system is presented via chemically converting hydroxyl residues on the dextran backbone to methacrylate groups, thereby transforming non‐protein adhesive, hydrophilic dextran to highly protein adsorbent substrates. Increasing methacrylate functionality increases the hydrophobicity in the resulting hydrogels and enhances ECM protein adsorption without additional chemical reactions. These hydrophobic hydrogels permit facile and tunable modulation of substrate stiffness independent of hydrophobicity or ECM coatings. Using this approach, it is shown that substrate stiffness and ECM adsorption work together to affect cell morphology and proliferation, but the strengths of these effects vary in different cell types. Furthermore, it is revealed that stiffness‐mediated differentiation of dermal fibroblasts into myofibroblasts is modulated by the substrate ECM. The material system demonstrates remarkable simplicity and flexibility to tune ECM coatings and substrate stiffness and study their effects on cell function. 

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