Paper: Huang, H., Sakar, M.S., Petruska, A.J., Pané, S., & Nelson, B.J. (2016). Soft micromachines with programmable motility and morphology. Nature Communications, 7. https://doi.org/10.1038/ncomms12263
Referee Form:
1. Do you have any conflict of interest in reviewing this paper?
No
2. Expertise. Provide your expertise in the topic area of this paper.
1 – No Knowledge
3. Summary. Please summarize what you believe are the paper’s main contributions to the field of soft robotics.
This paper proposes a fabrication method for reconfigurable motile soft micromachines with compound bodies that respond to external control signals using magnetic control for mobility and spatiotemporally controlled heating for shape-shifting. The paper illustrates through various studies that the coupled effect of mobility modulation with the morphological transformation of the tail as well as the body determines swimming efficiency. The paper advances the field of soft robotics by introducing an origami-inspired rapid prototyping process for a novel microbot made of soft materials that can navigate complex and varied environments with its reconfigurable shape and motility. This ability can be leveraged in minimally invasive environmental and biomedical procedures which often comprise heterogeneous environments.
4. Strengths and Weaknesses. What are the main strengths and weaknesses of this work? Does the paper have strengths in originality and novelty?
The paper’s biggest strength is the novelty of the proposed mechanism and how it provides a holistic and comprehensive solution to various niche problems in bioinspired soft micro-robotics. It introduces a mechanism to fabricate a variety of bioinspired flagellated micromachines with varied forms such as the long slender form and stumpy form inspired by the Trypanosoma brucei as well as other forms such as a tubular body with a helical/spiral tail, and a helical body with a planar tail using morphological transformations actuated by magnetic and thermal signals. It lists in detail the methodology – from material specifications and patterning using lithographic techniques to machine architecture through particle alignment and morphological variations from thermal control. The figures illustrate the descriptions of motion and morphology well, and a lot of supplementary material is provided for reference. The paper also mentions unexpected findings that indicate the authenticity of the results. The one weakness of the experiments is that the different micro-swimmers were not tested in multiple environments, which the paper claims is one of the crucial contributions of the study. The different configurations are only tested in a viscous sucrose solution with a Reynolds number between 0.1 and 0.001.
5. Soundness. Are the ideas, algorithms, results, or studies technologically/methodologically sound?
I believe the paper is methodologically sound. To demonstrate the diverse capabilities of the composite material, the authors provide extensive analytical and empirical evidence and quantifiable metrics and data to evaluate performance. The authors also give a convincing argument supported by evidence for the independence in the effects of magnetization and thermally induced morphological manipulation, and how the coupled results affect folding along different axes and propulsion. The experiments also use sound methodology for collecting 3D trajectory data using 2 orthogonal cameras visualizing the workspace from the side and top views.
6. Related Work. Does the paper adequately describe related and prior work?
I believe the paper adequately describes previous/related work. The introduction mentions and analyses various works related to artificial micro-swimmers (typically inspired by flagellated structures like E. Coli) and various techniques for propulsion in a corkscrew-like motion. It also surveys and provides background on different techniques in the methodology for design and manufacturing, programming machine architecture through MNP alignment, programming triggered transformation of morphology using NIR heating, as well as propulsion.
7. Presentation. Is the paper well-organized, well-written, and clearly presented?
The paper is well-organized and presented. The methodology is broken down into multiple sections with a clear and concise logical flow. The paper begins with an introduction of the inspiration behind the design and a literature review, followed by results and methodology along with a discussion of the results.
8. Suggestions. Do you have suggestions for improving this paper? Please write several paragraphs detailing specific points of the paper that merit reconsideration. Be sure to address the text, figures and tables, mathematics, and grammar and spelling.
For the most part, the text, figures, and tables are clear in their labelling, context, and conclusions. Although I would have appreciated more contextual information in some areas, for example, in the usage of Timoshenko bimorph beam theory, I understand that researchers in the field are more familiar with such concepts than me.
Another suggestion would be to conduct studies in different fluid environments that closely resemble the use cases for the proposed micro-swimmers. For example, the paper mentions “transforming a conical head into a helical one may enable superior mobility in highly viscous fluids (as mastered by Spirochetes), while transforming back to the original shape may facilitate tissue penetration” as a use case, but the device itself does not demonstrate these capabilities as it was not tested in a heterogeneous environment.
I appreciated the organization of the paper but would suggest making supplementary materials/data publicly available instead of making it available on request, which could help with the verification of results.
9. Comments to Committee (Hidden from authors). Does the paper have enough originality and importance to merit publication? Is the paper relevant to the field?
I believe this paper has enough originality and importance to merit publication. Papers in the past have largely only considered the flagella (i.e. the tail) in actuation propulsion in flagellated artificial micro-swimmers. The paper proposes a novel mechanism that considers the coupled effects between the cell body and the flagella instead of just the flagellar propulsion mechanism. The paper also considers the motion of Trypanosoma brucei which can adapt its shape and respond to the environment by exploiting soft and stiff structures as opposed to controlled swimming motion, instead of the usual flagellated structures inspired by E. Coli. The novelty of this approach backed by technical soundness and empirical evidence warrants the publication of this paper.
10. Overall Rating. Provide your overall rating of the paper (5 is best)
5 – Definite accept: I would argue strongly for accepting this paper.