# Soft robotics

Soft-Legged Wheel-Based Robot with Terrestrial Locomotion Abilities.

Soft Robotics is the specific subfield of robotics dealing with constructing robots from highly compliant materials, similar to those found in living organisms. [1]

Soft robotics draws heavily from the way in which living organisms move and adapt to their surroundings. In contrast to robots built from rigid materials, soft robots allow for increased flexibility and adaptability for accomplishing tasks, as well as improved safety when working around humans.[2] These characteristics allow for its potential use in the fields of medicine and manufacturing.

## Types and designs

The bulk of the field of soft robotics is based upon the design and construction of robots made completely from compliant materials, with the end result being similar to invertebrates like worms and octopi. The motion of these robots is difficult to model, as continuum mechanics apply to them, and they are sometimes referred to as continuum robots. Soft Robotics is the specific sub-field of robotics dealing with constructing robots from highly compliant materials, similar to those found in living organisms. Similarly, soft robotics also draws heavily from the way in which these living organisms move and adapt to their surroundings. In contrast to robots built from rigid materials, soft robots allow for increased flexibility and adaptability for accomplishing tasks, as well as improved safety when working around humans.[2] These characteristics allow for its potential use in the fields of medicine and manufacturing. However, there exist rigid robots that are also capable of continuum deformations, most notably the snake-arm robot.

Also, certain soft robotic mechanics may be used as a piece in a larger, potentially rigid robot. Soft robotic end effectors exist for grabbing and manipulating objects, and they have the advantage of producing a low force that is good for holding delicate objects without breaking them.

In addition, hybrid soft-rigid robots may be built using an internal rigid framework with soft exteriors for safety. The soft exterior may be multifunctional, as it can act as both the actuators for the robot, similar to muscles in vertebrates, and as padding in case of a collision with a person.

## Biomimicry

Plant cells can inherently produce hydrostatic pressure due to a solute concentration gradient between the cytoplasm and external surroundings (osmotic potential). Further, plants can adjust this concentration through the movement of ions across the cell membrane. This then changes the shape and volume of the plant as it responds to this change in hydrostatic pressure. This pressure derived shape evolution is desirable for soft robotics and can be emulated to create pressure adaptive materials through the use of fluid flow.[3] The following equation[4] models the cell volume change rate:

${\displaystyle {\dot {V}}=AL_{p}(-\Delta P+\Delta \pi )}$
${\displaystyle {\dot {V}}}$ is the rate of volume change.
${\displaystyle A}$ is the cell membrane.
${\displaystyle L_{p}}$ is the hydraulic conductivity of the material.
${\displaystyle \Delta P}$ is the change in hydrostatic pressure.
${\displaystyle \Delta \pi }$ is the change in osmotic potential.

This principle has been leveraged in the creation of pressure systems for soft robotics. These systems are composed of soft resins and contain multiple fluid sacs with semi-permeable membranes. The semi-permeability allows for fluid transport that then leads to pressure generation. This combination of fluid transport and pressure generation then leads to shape and volume change.[3]

Another biologically inherent shape changing mechanism is that of hygroscopic shape change. In this mechanism, plant cells react to changes in humidity. When the surrounding atmosphere has a high humidity, the plant cells swell, but when the surrounding atmosphere has a low humidity, the plant cells shrink. This volume change has been observed in pollen grains[5] and pine cone scales.[3][6]

## Manufacturing

Conventional manufacturing techniques, such as subtractive techniques like drilling and milling, are unhelpful when it comes to constructing soft robots as these robots have complex shapes with deformable bodies. Therefore, more advanced manufacturing techniques have been developed. Those include Shape Deposition Manufacturing (SDM), the Smart Composite Microstructure (SCM) process, and 3D multimaterial printing.[2][7]

SDM is a type of rapid prototyping whereby deposition and machining occur cyclically. Essentially, one deposits a material, machines it, embeds a desired structure, deposits a support for said structure, and then further machines the product to a final shape that includes the deposited material and the embedded part.[7] Embedded hardware includes circuits, sensors, and actuators, and scientists have successfully embedded controls inside of polymeric materials to create soft robots, such as the Stickybot[8] and the iSprawl.[9]

SCM is a process whereby one combines rigid bodies of carbon fiber reinforced polymer (CFRP) with flexible polymer ligaments. The flexible polymer act as joints for the skeleton. With this process, an integrated structure of the CFRP and polymer ligaments is created through the use of laser machining followed by lamination. This SCM process is utilized in the production of mesoscale robots as the polymer connectors serve as low friction alternatives to pin joints.[7]

3D printing can now produce shape morphing materials whose shape is photosensitive, thermally activated, or water responsive. Essentially, these polymers can automatically change shape upon interaction with water, light, or heat. One such example of a shape morphing material was created through the use of light reactive ink-jet printing onto a polystyrene target.[10] Additionally, shape memory polymers have been rapid prototyped that comprise two different components: a skeleton and a hinge material. Upon printing, the material is heated to a temperature higher than the glass transition temperature of the hinge material. This allows for deformation of the hinge material, while not affecting the skeleton material. Further, this polymer can be continually reformed through heating.[10]

## Control

All soft robots require some system to generate reaction forces, to allow the robot to move in and interact with its environment. Due to the compliant nature of these robots, this system must be able to move the robot without the use of rigid materials to act as the bones in organisms, or the metal frame in rigid robots. However, several solutions to this engineering problem exist and have found use, each possessing advantages and disadvantages.

One of these systems uses Dielectric Elastomeric Actuators (DEAs), materials that change shape through the application of a high-voltage electric field. These materials can produce high forces, and have high specific power (W/kg). However, these materials are best suited for applications in rigids robots, as they become inefficient when they do not act upon a rigid skeleton. Additionally, the high-voltages required can become a limiting factor in the potential practical applications for these robots.[11]

Another system uses springs made of shape-memory alloy. Although made of metal, a traditionally rigid material, the springs are made from very thin wires and are just as compliant as other soft materials. These springs have a very high force-to-mass ratio, but stretch through the application of heat, which is inefficient energy-wise.[11]

Pneumatic artificial muscles are yet another method used for controlling soft robots. By changing the pressure inside a flexible tube, it will act as a muscle, contracting and extending, and applying force to what it’s attached to. Through the use of valves, the robot may maintain a given shape using these muscles with no additional energy input. However, this method generally requires an external source of compressed air to function.[2]

## Uses and applications

Soft robots can be implemented in the medical profession, specifically for invasive surgery. Soft robots can be made to assist surgeries due to their shape changing properties. Shape change is important as a soft robot could navigate around different structures in the human body by adjusting its form. This could be accomplished through the use of fluidic actuation.[12]

Soft robots may also be used for the creation of flexible exosuits, for rehabilitation of patients, assisting the elderly, or simply enhancing the user’s strength. A team from Harvard created an exosuit using these materials in order to give the advantages of the additional strength provided by an exosuit, without the disadvantages that come with how rigid materials restrict a person’s natural movement.[13]

Traditionally, manufacturing robots have been isolated from human workers due to safety concerns, as a rigid robot colliding with a human could easily lead to injury due to the fast paced motion of the robot. However, soft robots could work alongside humans safely, as in a collision the compliant nature of the robot would prevent or minimize any potential injury.

There is a company, Soft Robotics Inc., www.softroboticsinc.com[14], in Cambridge, MA that has commercialized soft robotics systems for industrial and collaborative robotics applications. Theses systems are in use in food packaging, consumer goods manufacturing, and retail logistics applications.

## In popular culture

The 2014 Disney film Big Hero 6 revolved around a soft robot, Baymax, originally designed for use in the healthcare industry. In the film, Baymax is portrayed as a large yet unintimidating robot with an inflated vinyl exterior surrounding a mechanical skeleton. The basis of the Baymax concept come from real life research on applications of soft robotics in the healthcare field, such as roboticist Chris Atkeson's work at Carnegie Mellon's Robotics Institute.[15]

## References

1. ^ Trivedi, D., Rahn, C. D., Kier, W. M., & Walker, I. D. (2008). Soft robotics: Biological inspiration, state of the art, and future research. Applied Bionics and Biomechanics, 5(3), 99-117.
2. ^ a b c d Rus, Daniela; Tolley, Michael T. (27 May 2015). "Design, fabrication and control of soft robots". Nature. 521 (7553): 467–475. doi:10.1038/nature14543.
3. ^ a b c Li, Suyi; Wang, K. W. (1 January 2017). "Plant-inspired adaptive structures and materials for morphing and actuation: a review". Bioinspiration & Biomimetics. 12 (1). ISSN 1748-3190. doi:10.1088/1748-3190/12/1/011001. Retrieved 27 April 2017.
4. ^ Dumais, Jacques; Forterre, Yoël (21 January 2012). ""Vegetable Dynamicks": The Role of Water in Plant Movements". Annual Review of Fluid Mechanics. 44 (1): 453–478. doi:10.1146/annurev-fluid-120710-101200.
5. ^ Katifori, Eleni; Alben, Silas; Cerda, Enrique; Nelson, David R.; Dumais, Jacques (27 April 2010). "Foldable structures and the natural design of pollen grains". Proceedings of the National Academy of Sciences. 107 (17): 7635–7639. doi:10.1073/pnas.0911223107.
6. ^ Dawson, Colin; Vincent, Julian F. V.; Rocca, Anne-Marie (18 December 1997). "How pine cones open". Nature. 390 (6661): 668–668. doi:10.1038/37745.
7. ^ a b c Cho, Kyu-Jin; Koh, Je-Sung; Kim, Sangwoo; Chu, Won-Shik; Hong, Yongtaek; Ahn, Sung-Hoon (11 October 2009). "Review of manufacturing processes for soft biomimetic robots". International Journal of Precision Engineering and Manufacturing. 10 (3): 171–181. doi:10.1007/s12541-009-0064-6.
8. ^ Kim, S.; Spenko, M.; Trujillo, S.; Heyneman, B.; Mattoli, V.; Cutkosky, M. R. (1 April 2007). "Whole body adhesion: hierarchical, directional and distributed control of adhesive forces for a climbing robot". Proceedings 2007 IEEE International Conference on Robotics and Automation: 1268–1273. doi:10.1109/ROBOT.2007.363159. Retrieved 27 April 2017.
9. ^ Cham, Jorge G.; Bailey, Sean A.; Clark, Jonathan E.; Full, Robert J.; Cutkosky, Mark R. (1 October 2002). "Fast and Robust: Hexapedal Robots via Shape Deposition Manufacturing". The International Journal of Robotics Research. 21 (10-11): 869–882. ISSN 0278-3649. doi:10.1177/0278364902021010837. Retrieved 27 April 2017.
10. ^ a b Truby, Ryan L.; Lewis, Jennifer A. (14 December 2016). "Printing soft matter in three dimensions". Nature. 540 (7633): 371–378. doi:10.1038/nature21003.
11. ^ a b Kim, Sangbae; Laschi, Cecilia; Trimmer, Barry (May 2013). "Soft robotics: a bioinspired evolution in robotics". Trends in Biotechnology. 31 (5): 287–294. doi:10.1016/j.tibtech.2013.03.002.
12. ^ Cianchetti, Matteo; Ranzani, Tommaso; Gerboni, Giada; Nanayakkara, Thrishantha; Althoefer, Kaspar; Dasgupta, Prokar; Menciassi, Arianna (1 June 2014). "Soft Robotics Technologies to Address Shortcomings in Today's Minimally Invasive Surgery: The STIFF-FLOP Approach". Soft Robotics. 1 (2): 122–131. ISSN 2169-5172. doi:10.1089/soro.2014.0001. Retrieved 27 April 2017.
13. ^ Walsh, Conor; Wood, Robert (5 August 2016). "Soft Exosuits". Wyss Institute. Retrieved 27 April 2017.
14. ^ "Home". Soft Robotics. Retrieved 2017-08-18.
15. ^ Trimboli, Brian (Nov 9, 2014). "CMU’s soft robotics inspire Disney’s movie Big Hero 6 - The Tartan". The Tartan. Carnegie Mellon University. Retrieved 2016-08-15.