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Journal Club: Magnetic Soft Materials and Robots: Design (Code), Applications and Future Directions

Xuanhe Zhao's picture

Magnetic Soft Materials and Robots: Design (Code), Applications and Future Directions

 

This post was adopted from a News & Views by Zhao and Kim recently published on Nature [1].

 

In science-fiction films, robots are often depicted as human-sized or larger machines made of rigid materials. However, robots made of soft materials or with flexible structures, and that can be much smaller than the human body, have attracted great interest in the past few years because they have the potential to interact with humans more safely than can rigid machines. Indeed, sufficiently small soft robots could even be used for biomedical applications in the human body. Various options are available to power these robots, but magnetic fields offer a safe and effective means of wireless operation in confined spaces in the body.

 

Design and model (code). The ability of minerals known as lodestones to align with Earth’s magnetic field was first reported in the ancient Chinese manuscripts Gui Gu Zi and Han Fei Zi, and was later used in early magnetic compasses [2]. A similar principle has been used in the past few years in magnetic soft robots [3–10], in which magnets of varying sizes (nanometres to millimetres) are integrated into flexible structures or soft materials. The tendency of the magnets to orient in externally applied magnetic fields provides a way of quickly moving or changing the shape of these untethered robots remotely. This actuation mechanism allows much flexibility in the design of the robots structures, magnetization patterns and strengths, and in when and where magnetic fields are applied to control the robots.

 

In addition, because the forces and torques exerted on magnets by external magnetic fields can be accurately calculated, models have been developed to quantitatively describe the actuation of specific robot designs. For example, Zhao et al developed a constitutive law for soft materials and robots with complex patterns of ferromagnetic domains, named Ideal Hard-Magnetic Soft Materials. Following the constitutive law, the effects of the forces and torques exerted by external magnetic fields can be characterized by an effective magnetic Cauchy stress [8, 11]. Zhao et al further implemented the constitutive law into an ABAQUS UEL subroutine (see the attachments for a paper on the theory, the UEL subroutine, and an example input file).

 

Applications and fabrications. Magnetic soft robots have been developed for various uses, especially in biomedical applications in which they interact closely with the human body. For example, self-foldingorigami robots have been reported that can crawl through the gut, patch wounds and dislodge swallowed objects [4]; and capsule-shaped robots have been made that roll along the inner surface of the stomach and can perform biopsies and deliver medicine [3]. Magnetically steerable robotic catheters have also been developed, which can perform minimally invasive surgery on the heart or inspect lung airways [5,7]. And much thinner, thread-like robots have been made that could potentially navigate the brain’s blood vessels to treat strokes or aneurysms [10]. These robots range in size from hundreds of micrometres to a few centimetres in diameter.

 

Existing methods for the construction of small magnetic soft robots have included the direct assembly of magnetic components [3-5,7], the magnetization of particle- loaded polymer sheets [6], the printing of soft composite materials that contain aligned magnetic particles [9,10], and more recently electron-beam lithography to make magnetically reconfigurable robots [12].

 

Future directions. Much work must still be done to achieve the full potential of magnetic soft robots for biomedical applications across various length scales. They must be designed using quantitative models to optimize their performance for specific tasks in relatively weak magnetic fields that is, to work out which reconfigurations are needed, the sizes of the forces that the robot must exert on its environment, and the speeds at which reconfigurations should occur and with which the forces should be applied. Advanced fabrication platforms will be crucial for implementing future designs. Methods for the real-time imaging and localization of robots deep in the human body are also needed, particularly in tight spaces, and must not interfere with the magnetic-actuation mechanisms. Artificial intelligence might be further developed to assist image analysis and robot control. Lastly, methods are needed for the safe retrieval or degradation of robots once they have performed their tasks. Degradation without toxicity or other adverse effects is particularly desirable.

 

Magnetic soft robots are also being extensively studied for applications beyond biomedicine [8], such as in flexible electronics, reconfigurable surfaces and active metamaterials (engineered materials consisting of subunits that take in energy locally, and then translate it into movement that can produce large-scale dynamic motion). A parallel set of platforms for the design, fabrication, imaging and control of magnetic soft robots across various length scales are therefore under development.

 

Reference

 

1. Zhao, X & Kim, Y. Nature  575, 58-59 (2019)

2. du Trémolet de Lacheisserie, é. in Magnetism:Fundamentals (eds du Trémolet de Lacheisserie, é., Gignoux, D. & Schlenker, M.) 3–6 (Springer, 2005).

3. Yim, S. & Sitti, M. IEEE Trans. Robot. 28, 183–194 (2012).

4. Miyashita, S. et al. 2016 IEEE Int. Conf. Robot. Automat. 909–916 (IEEE, 2016).

5. Edelmann, J., Petruska, A. J. & Nelson, B. J. J. Med. Robot. Res. 3, 1850002 (2018).

6. Hu, W., Lum, G. Z., Mastrangeli, M. & Sitti, M. Nature 554, 81–85 (2018).

7. Jeon, S. et al. Soft Robot. 6, 54–68 (2019).

8. Kim, Y., Yuk, H., Zhao, R., Chester, S. A. & Zhao, X. Nature 558, 274–279 (2018).

9. Xu, T., Zhang, J., Salehizadeh, M., Onaizah, O. & Diller, E. Sci. Robot. 4, eaav4494 (2019).

10. Kim, Y., Parada, G. A., Liu, S. & Zhao, X. Sci. Robot. 4, eaax7329 (2019).

11. Zhao, R., Kim, Y., Chester, S. A., Sharma, P. & Zhao, X. J. Mech. Phys. Solids 124, 244–263 (2019).

12.  Cui, J. et al. Nature 575, 164–168 (2019).

 

 

Comments

Xuanhe Zhao's picture

To facilitate the reading and discussion, I added two more papers on examples of magnetic soft materials and robots. I also posted the slides of my talk at 2019 Fall MRS on this topic. This nascent field will use mechanics and materials as a platform to synergize recent advances in AI, 5G and robotics, potentially making a significant impact on the society. For example, prototypes of magnetic soft robots controlled by Stereotaxis system have been approved for clinical applications by FDA.

Stephan Rudykh's picture

Dear Xuanhe,

thank you very much for introducing the magnetic soft materials & robots.  This is a really exciting avenue for soft materials!  I will need to read the papers in detail, but will shoot a quick question: are there non-zero stresses in the "reference state" (without any external field applied) because of the intercations of the magnets?

Sincerely,

Stephan

PS: On the occation, congratulations on the Thomas J. R. Hughes Young Investigator Award! Well deserved!!!

 

Xuanhe Zhao's picture

Dear Stephan,

This is an excellent question. The nominal magnetization of the hard-magnetic soft materials (HMSMM is much weaker than the applied magnetic field B . Therefore, to the first order approximation, we prescribed the magnetic potential energy as -FM.B by neglecting higer order terms of M. Experimentally, we did not observe significant deformation of the HMSM due to interactions of magnetized domains alone either. More discussions on this point, can be found on Page 275 of Reference [8].

We and a few other groups are working on formulating more sophisticated constitutive models for HMSM to include effects from higher order terms. Notably, if we directly embed magnets into structures, the interactions of closeby magnets may be not negligible. 

BTW, your contribution to this field is truly remarkable and nicely summarized in this post

https://imechanica.org/node/23791

Look forward to seeing more exciting works on theory, experiments and applications from your group!

Xuanhe

Teng zhang's picture

Dear Xuanhe,

Thanks a lot for the nice review and for highlighting the research opportunities. I have a question about the shape change design with or without a persistent magnetic field.

In the papers you cited, most of the magnetic composites will be deformed into different shapes under the external magnetic field and return to their original state after removing the magnetic field. In other words, a persistent magnetic field is required to maintain the actuated shapes. I can see this is great to design soft robots with dynamically controlled motion. For broader applications of the magnetic soft materials, one may want to maintain the new shape even after removing the external magnetic field to tune the structure properties associated with shapes (e.g., stiffness, roughness, friction, and light and sound propagation). The later design can be achieved by harnessing the bistable or multistable buckling, but also introduces additional complexity.

Could you please share your thoughts on the differences and similarities between the design of shape-changing magnetic structures with and without requiring a persistent magnetic field, in terms of applications and research challenges/opportunities for mechanics?

Thanks.

Teng

Xuanhe Zhao's picture

Dear Teng,

 

Thanks for the comments and insightful question. Maintaining the transformed shape without a persistent field can be very important in many applications. Your idea of bistable structures with magnetic-field trigger is a good one. Using another mechanism to maintain the transformed shape is another good idea.

 

To demonstrate how dynamic this field is, Ruike Zhao and Jerry Qi groups just published a paper on using shape-memory polymers to maintain the transformed shapes of ferromagnetic soft materials.

https://doi.org/10.1002/adma.201906657

 

This field is rapidly evolving, with new theory, experiments, computation and applications, towards both academic and societal impacts.

 

Best,

Xuanhe

 

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