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Hybrid hydrogel sheets that undergo pre-programmed shape transformations

Zheng Jia's picture

Soft Matters(2014) Advance article

We present a novel strategy to achieve programmable shape transformation of hybrid hydrogel sheets by modulating both the in-plane and out-of-plane mismatches in mechanical properties. Both our experimental and computational results demonstrate that the shape transformation of hybrid hydrogel sheets shows rich features (e.g., rolling direction, axis, chirality, etc.) and versatile tunability (e.g., via various external stimuli, material properties, pattern geometry, etc.). This work can provide guidance for designing soft materials that are able to undergo more precise and complex shape transformation.

Zengjiang Wei, Zheng Jia, Jasmin Athas, Chaoyang Wang, Srinivasa Raghavan, Teng Li* and Zhihong Nie*. Hybrid hydrogel sheets that undergo pre-programmed shape transformations, Soft Matters, (2014) Advance article, DOI: 10.1039/C4SM01299B 

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Jinxiong Zhou's picture

Dear Zheng,

Thanks for sharing this very interesting paper, which is really an excellent work. I just take a glance and have two quick questions. 

1. The elastic modulus of BG depends on the concentration of crosslinker BIG as indicated in Fig. 4 of your paper. In your simulation model, how did you incorporate the variation of modulus with concentration of BIG?

2. I guess your simulation is an equilibrium state simulation, so you do not have time. In Fig3 (b) you show the time depedent width of BG or LG. How can you obtain this?

Thanks. 

Jinxiong

Zheng Jia's picture

Dear Prof. Zhou, thank you so much for your interest in our work. I would like to first briefly introduce the storyline of our paper and I think it could help me to better answer your questions. Basically, we did simulations with various nominal densities of polymer chains (the parameter N) of BG and the results indicate that, by tuning N of BG, different deformation modes (1-4) can be achieved. The simulation results are summarized in Fig.1. Inspired by our simulation findings, our collaborators carried out experiments with different crosslinker densities of BG (because crosslinker density is closely related to N) to get those deformation modes predicted by simulations. Modes 1-3 are detected in the experiments. The experimental findings are summarized in Fig.2-5. Now, let's get back to your questions:

1. I am sure you know our model very well. In the simulations, we prescribed N directly and N carries information about the modulus of BG (since NkT is the initial shear modulus of the dry state). The conclusion of the simulation is: different N's lead to different deformation modes. But we did not quantitatively correlate N with concentration of BIS crosslinker (shown in Fig. 4) because we do not have enough experimental measurements to correlate them. However, this does not affect the storyline of this paper.

2. You are absolutely right, our simulation is for equilibrium state. However, the time dependent data in Fig.3 was measured from experiments, not simulations. We just used simulation plots (intermediate states before the final equilibrium solution is reached) as illustration to schematically show how the hybrid sheet looks like for each data point in Fig.3. We did this because 1) for each data point, we compared the simulated intermediate shape with the real shape experimentally observed and they look quite similar. 2) the simulation plot has better qualities than the experimental photos in which gels are translucent. Therefore, those plots are used as illustration and they are not necessary to carry real physical meanings.

 

Please kindly let me know if these answer your questions. Please don't hesitate to contact me if you would like to discuss more. Many thanks!

 

Jinxiong Zhou's picture

Dear Zheng, Thank you for your detailed answering. Jinxiong

Jinxiong Zhou's picture

Dear Zheng,

Another two questions:

1. How did you enforce boundary conditions for the simulation in Figure 1? How did you know the boundary conditions is physical and realistic?

2. Did you use a temperature-dependent Flory-Huggins interaction coefficient? Have you fitted it to experiment?

Jinxiong

Zheng Jia's picture

Dear Prof. Zhou,

1. The mechanical boundary condition is traction-free because in the experiment the whole hybrid gel sheet is free-standing. But I guess maybe you were talking about how we impose temperature B.C.? We did not really control the temperature. Instead, we artificially assign a change of chemical potential to the hybrid sheet to mimic the volume change of BG and LG in response to temperature change. To ensure this is physical and realistic, we fine tune the chemical potential to make sure the size change of BG and LG in the simulation matches that measured in experiments (experimentally measured size change is shown in Fig.3)

2 No, we employed constant Flory-Huggins interaction coefficient. Because we did not really simulate the temperature-induced phase transformation, we just used chemical potential to control the volume change and thus mimic the deformation process of hybrid sheet, in that sense, we don't need temperature-dependent Chi

Best regards,

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