Statistical Modeling of Photo-Bending Actuation of Hybrid Silicones Mixed with Azobenzene Powder
Round 1
Reviewer 1 Report
This work describes design and development of mechanically responsive organic materials based upon a combination of azo motif and silicones. The hybrid materials can be applied as photochemical actuators. In this scheme, azo groups are photoresponsive and silicones provide a scaffold. The authors provide statistical modeling based on experimental results, which will be useful in optimization of photo-actuation behaviors of such actuators. Due to a fundamental nature of the work, I recommend publication. However, I recommend revision of the manuscript.
Is azobenzene an ideal photoactuator? Does the thermal back reaction not a problem in actuator applications? Sensitivity: Is the photosensitivity presented good enough for photoactuators? Is the degree or extent of photo-bending response enough for real life applications? Can the direction of bending be controlled in the present actuator system? In the supporting information, the authors could provide a detailed experimental procedure for the fabrication (amounts of chemicals etc).
A discussion of these aspects in the revised manuscript would help the reader.
Author Response
Thank you for the good evaluation and important comments for our paper. We have revised the manuscript according to the comments.
Reviewer’s Comment 1:
Is azobenzene an ideal photoactuator? Does the thermal back reaction not a problem in actuator applications?
Author’s Response 1:
Azobenzene proceeds photochemical isomerization and/or heat generation when absorbing light. The actuation of this work is based on heat generation (photothermal effect) of azobenzene, as the inserted description on page 3–4. The actuation mechanism may limit some environments, but for soft robotics, the temperature should have a lower impact as the contact with human tissues is not always present. Furthermore, it will be possible for azobenzene materials to actuate photochemically when designed in other processes, diversifying the photo-actuation responses.
Reviewer’s Comment 2:
Is the photosensitivity presented good enough for photoactuators? Is the degree or extent of photo-bending response enough for real life applications?
Author’s Response 2:
As to applications, the use and environment depend on the material properties. In order to use the presented solutions in real-life applications, a specific design has to be performed. Preferable use of this photo-bending would be very gentle touch and grip. Another challenge for this study would be to characterize the actuator forces obtained by photo-bending. The actuation force may be an interesting parameter for designing and developing innovative actuators. To clarify this, we added the above description on page 11.
Reviewer’s Comment 3:
Can the direction of bending be controlled in the present actuator system?
Author’s Response 3:
The bending depends on the stimulus (light) direction, the position of the photo-initiators in the structure and the strip (paper). Therefore, modifying these parameters can be used to adjust the bending direction. This description was added on page 3.
Reviewer’s Comment 4:
In the supporting information, the authors could provide a detailed experimental procedure for the fabrication (amounts of chemicals etc).
Author’s Response 4:
According to the comment, we added more detailed fabrication procedure in the supplementary file.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
This article is overall a nice paper on using a new statistical model to describe the photoinduced bending and unbending motions of actuators based on silicones and azobenzene. The photoinduced actuators are attracting more and more attention recently but usually lack of vigorous theoretic work to illustrate the underlying mechanism. This article would be a good example to combine both experimental work and computational prediction together. I will recommend it to be accepted and published in the Actuators after some minor revision and I also have a few questions which may help us to get a better understanding of this article.
I wonder if there is any photochemistry undergoing inside the silicone and azobenzene film when the film is irradiated by UV light. Do the azobenzene molecules undergo cis-trans photoisomerization when the polymer films under light irradiation? The bending and unbending motions seem to be reversible but the bending is controlled by UV light while the unbending is due to the thermal relaxation of the silicone film. In most cases of azobenzene, the unbending motion can also be initiated by visible light. Based on the description of the article, the pure silicone film with 0 % of azobenzene can also bend under UV light and unbend after light is switched off. So another question that follows the 1st question will be, what is the role of azobenzene inside the silicone film? According to the experimental data and computational prediction, it seems that the higher loading of azobenzene may help to achieve larger deflection and reach the maximum deflection faster. But I wonder the improvement is caused by the photochemistry of azobenzene or other effects. Also, as the pure silicone film can bend, I wonder if the authors have ever tried the polymer film with only the 2nd That is, preparing some polymer films with one homogenous mixture of silicone and azobenzene with various loadings (1 wt%, 5 wt% and 10 wt%) but without the 1st layer (pure silicone). If the pure silicone film can bend under light due to photothermal effects, is it likely that the other homogenous films with different azobenzene loadings may bend as well?
Comments for author File: Comments.pdf
Author Response
Thank you for the good evaluation and important comments for our paper. We have revised the manuscript according to the comments.
Reviewer’s Comment 1:
I wonder if there is any photochemistry undergoing inside the silicone and azobenzene film when the film is irradiated by UV light. Do the azobenzene molecules undergo cis-trans photoisomerization when the polymer films under light irradiation? The bending and unbending motions seem to be reversible but the bending is controlled by UV light while the unbending is due to the thermal relaxation of the silicone film. In most cases of azobenzene, the unbending motion can also be initiated by visible light. Based on the description of the article, the pure silicone film with 0 % of azobenzene can also bend under UV light and unbend after light is switched off.
Author’s Response 1:
In order to check the influence of photochemical isomerization on the bending, we observed unbending behavior under visible light irradiation. If photochemical isomerization is a dominant effect of actuation, the unbending under visible light should be faster than that without visible light, because visible light will accelerate cis-to-trans photochemical back-isomerization, as reported in the photomechanical crystal of the same compound, 4-aminoazobenzene. The result was that the unbending under visible light was slower than that without visible light. This means that the photochemical isomerization should undergo, but that is not the dominant effect on actuation. Rather, the photothermal effect mainly contributes to actuation, and the slower unbending under visible light should be also due to the photothermal effect of azobenzene, because azobenzene absorbs visible light and generates heat. To clarify this, sentences were added on page 3–4.
Reviewer’s Comment 2:
So another question that follows the 1st question will be, what is the role of azobenzene inside the silicone film? According to the experimental data and computational prediction, it seems that the higher loading of azobenzene may help to achieve larger deflection and reach the maximum deflection faster. But I wonder the improvement is caused by the photochemistry of azobenzene or other effects.
Author’s Response 2:
As mentioned to the 1st comment, the effect of azobenzene contents on the bending behavior is photothermal effect rather than photochemical isomerization. Thus, higher loading of azobenzene powder contributes to heat generation though light absorption.
Reviewer’s Comment 3:
Also, as the pure silicone film can bend, I wonder if the authors have ever tried the polymer film with only the 2nd That is, preparing some polymer films with one homogenous mixture of silicone and azobenzene with various loadings (1 wt%, 5 wt% and 10 wt%) but without the 1st layer (pure silicone). If the pure silicone film can bend under light due to photothermal effects, is it likely that the other homogenous films with different azobenzene loadings may bend as well?
Author’s Response 3:
We have observed the actuation behaviors of the 2nd layer only, that is, silicone monolayer homogenously mixed with azobenzene powder. The monolayer also bent when constrained by paper, and the result was added as Supplementary Figure 1.
Author Response File: Author Response.pdf
