Hi Xueju,

Congratulations! Wonderful and inspring summary! I have two questions:

1. Similar to Lihua, it is interesting to see the mesogen alignment under a relatively small stretching strain of 9%-12% through 3D buckling, as opposed to the conventional mechanical stretching over 120% to align them. Any idea on how to understand the small-strain induced alignment phenomenon in the buckled structure?

2. I see you are using laser cutting to cut the first-step cured LCE samples to certain patterns. The localized heating during the cutting could dramatically change the properties of the cutting area, will this affect the final 3D buckled strucuture? Thanks.

Thank you for this sharing. -Jin

Teng, all great questions! Regarding the biomimetics/inspiration from nature, please also check Dr. Chris Yakacki's comment on mimicking soft tissues with LCEs. LCEs have been explored a lot for applications in artificial muscles. One recent work by Shengqiang on LCE microfiber actuators can be found from the following link.

https://www.science.org/doi/10.1126/scirobotics.abi9704

Hi Chris,

Thanks a lot for the great comments on the practical applications of LCEs and the biomimetics from nature! It is very helpful to share with the community, which I believe benefits a lot from many of your pioneering work in this field.

Xueju (Sophie)

Hi Jin,

Good question! LCEs have many exceptional properties, including large, reversible shape changes that have great potential as actuators, high energy dissipation, soft elasticity, etc. When LCEs are used for actuation purposes, thin-film geometries are usually desired because of the ease of programming with existing techniques (mechanical alignment, surface patterning, etc.) and easy actuation. For example, LCEs are typically light- or heat-responsive, but neither light nor heat works well for very thick (bulky) LCEs due to the relatively shallow light penetration depth or the thermal gradient in thick LCEs during actuation. When LCEs are utilized for other properties like energy dissipation, usually bulky LCE structures are fabricated using techniques like molding or digital light processing (DLP) 3D printing, where mesogens do not need to be programmed because no actuation is needed (https://onlinelibrary.wiley.com/doi/full/10.1002/adma.202000797).

Regarding programming 3D (bulky/volumetric) LCEs, it is possible through techniques like the direct ink writing (DIW) 3D printing, where mesogens are aligned along the printing path during the fabrication of the 3D LCE structure. For 3D bulky LCEs prepared via other techniques like molding, mechanical programming may work but the alignment may not be uniform.

Xueju (Sophie)

Hi Pradeep, great question! So far our study on magnetic LCEs is relatively qualative, focusing on the dual actuation, so I do not know if there is any hysteretic effect in the material after repeated actuations either purely from the magnetic part or from the interaction (e.g., magnetic forces can affect the alignment of mesogens) in the magnetic LCE composite. But it is a very interesting question to explore. I will keep you posted if we find something out.

Xueju (Sophie)

Ruobing, thanks for sharing your thoughts and the works!

Lihua,

Thanks a lot for sharing your great works and openly accessibly code! I read both papers before but did not read the modeling part in details. I will take a closer look and will let you know if I have any questions.

Xueju

Great work and summary, Sophie! To add to the conversation about practical applications and nature, LCEs mimic human tissues quite remarkably. Beyond the reversible actuation like a muscle, LCEs mimic soft tissues as they have low modulus (~1 MPa), high dissipation, hierarchical order, and anisotropy.

Thanks Ruobing for the detailed explanations and sharing the nice work! The two papers are indeed very examples of coupled and decoupled light and heat effects.

Looking forward to more exciting work from you.

Best,

Teng

Hi Teng,

For LCE-like materials in nature: many biological molecules essentially form liquid crystalline phase, such as those in a biofilm. I could imagine as they are "crosslinked" in their nature form, they might be treated as liquid crystal networks. However, I am not in the field and am not sure if their mechanical behaviors show analogy to synthetic LC elastomers.

Light actuation of a LCE can be based on photothermal or photochemical. For photochemical mechanism, there are rich examples in nature that convert light to other forms of energy or signal using the same mechanism. Examples include the cis-trans photoisomerization for vision, photochemistry-induced DNA damage and repair, and photosynthesis.

The coupling between light and heat during an actuation is intriguing. If it is photothermal actuation, then the coupling is basically from photo-induced heating and the subsequent thermomechanics of LCE. If it is photochemistry, the light-temperature coupling might be more complex. They indeed couple in a nonlinear way. In both cases, kinetics may play important roles. I attach a recent theoretical work by my student on the light-temperature coupling in LCEs: https://www.sciencedirect.com/science/article/abs/pii/S2352431622000062

Also, in certain scenario, light and heat can be decoupled by properly designing the experiment. This was an important problem in light actuation of LCEs when it initially emerged: people were wondering whether the actuation is from photochemistry, or merely photochemistry-induced heating. A pioneering work by Yu and Ikeda et al answered this question by looking into light with various polarization and LCEs with polydomain (https://www.nature.com/articles/425145a).

Best,

Ruobing

Xueju, thank you for your response. This is interesting. Recently, we published the following papers on LCE shape morphing using a combined method of experiments, analytical differential geometry modeling, and finite element analysis. We have written an ABAQUS UMAT for LCEs based on the neo-classical model. It can be openly accessed as a supplementary material in the second paper, if you are interested in using it. Please feel free to let us know if you have any questions about it.

https://onlinelibrary.wiley.com/doi/abs/10.1002/adma.202000609

https://pubs.rsc.org/en/content/articlelanding/2022/sm/d1sm01830b/unauth

Hi Sophie,

Thank you for summarizing all these fantastic research progress about LCE! Most of them have 2D geometries or small thicknesses in applications. I am wondering whether bulk LCE materials are also applied to 3D (volumetric) applications or can be programmed in 3D?

Best regards,

Jin

Hi Lihua,

Great question! Yes. The maximum principal strain within the buckled 3D LCE structures we have so far varies from 2.2% to 23%. Regarding the order parameter, we were not able to directly measure it because the strain distributions and mesogen orientations in the 3D LCE structures are very complicated and current techniques for measuring mesogen alignment (WAXS and FTIR) require 2D thin film samples. To characterize the alignment within buckled 3D LCE structures, we performed FEA modeling on maximum principal strain distributions within the 3D structure and then conducted polarized FTIR measurements of stretched LCE thin film samples with strain levels corresponding to those from FEA modeling, since the mesogens and polymer chains tend to align in the direction of maximum principal strains. Here is the order parameter as a function of strains levels corresponding to those in a 3D LCE structure (also shown in Figure 8D in the post). It is observed that the order parameter increases with the strain level, achieving a magnitude of 0.37 at a strain of 12%, which shows a notable alignment of polymer chains within the LCE sample.

More information could be found from the following two papers.

https://onlinelibrary.wiley.com/doi/10.1002/adfm.202100338

https://pubs.acs.org/doi/10.1021/acsami.0c21371

Interstingly, we found that all the 3D buckled LCE structures have very good reversible shape-morphing capabilities even for those with very small strain levels (a few percent). There are still a lot of interesting questions we would like to explore here. For example, in our works, we only used a simple neo-Hookean model to simulate the buckling process. A model that incorporates the complicated constitutive laws of LCEs to simulate the buckling of LCE structures would be expected to improve the accuracy of the results. Also, in situ/ex situ measurement of the complicated spatial mesogen alignment in 3D LCE structures would provide important input for the modeling and design of shape-morphing 3D LCE structures. Collaborations are always welcome.

Best,

Xueju

Hi Sophie, this is an excellent overview of the subject which I really enjoyed reading. I have an intrest in the magnetically responsive system you created. Do you happen to know if any hysteretic effect sets in after repeated actuations of the system? The hysteresis need to be just magnetic but perhaps an overall one pertaining to the interaction of the mesogen and magnetic patterns.

Hi Xueju,

Thanks a lot for this excellent review! You give a very nice example of how mechanics can be tightly integrated with material and structure advancement to acheive new functions.

I have a very general question about LCE, which responds to heat and light. Are there LCE like materials/structures in nature that work in a similar way? We often seek inspiration from nature, such as hydrogels and composites. I do not find too much discussion about the nature counterpart of LCE.

Also, light and heat are generally coupled if the stimuli are lights. Is it possible to quntify the effects of light and heat indivually? Or they will likely be coupled in a nonlinear way.

Thanks.

Best,

Teng

Hi Zhijian and Xueju,

Thank you for the sharing the methods especially the imprint one. That is very interesting! I think there is a lot to explore in mechanics in this aspect of method.

Regards,

Ruobing

Xueju, wonderful article! I have a question about mesogens aligned by buckling. In a two-step crosslinking, people typically apply relatively high strain to align mesogens. However, in your designs, you use compressive strain due to buckling to align mesogens. I expect the compressive strain is pretty low. What's the typical order parameter and spontaneous strain that you can achieve?

Thanks, Xueju. Your elaboration makes lots of sense.

Hi Shengqiang,

Thanks for your kind note. This is a very good question! Yes. Shape memory polymers (SMPs) can be classified into one-way, two-way, and multiple (multiple temporary shapes) SMPs. One major category of two-way SMPs is based on semicrystalline polymers that can exhibit a reversible shape memory property under constant stress, such as crystallization-induced elogntation during cooling and melting-induced contraction upon heating. Actually people also consider LCEs as another major category of two-way SMPs, like in this review paper on SMPs (https://onlinelibrary.wiley.com/doi/full/10.1002/adma.202000713#adma202000713-bib-0056). Compared to semicrystalline-polymer-based two-way SMPs, I think the major advantages of LCEs are still the soft elasticity, large, reversible shape changes, and softness due to the combination of polymer networks and liquid crystals (mesogens). The modulus of typical SMPs is usually on the order of GPa, while LCEs can be much softer (modulus less than 1 MPa) and therefore would be better for applications where very soft materials are needed like in soft robotics and tissue scaffolds. In addition, we can also incoporate polymer crystalinity into LCEs for load-bearing, shape-morphing capabilities like in the works of Taylor (https://pubs.acs.org/doi/10.1021/acs.macromol.7b00567) that Ruobing mentioned and Yakacki (https://pubs.rsc.org/en/content/articlelanding/2017/sm/c7sm01380a). Compared to one-way and multiple (temporary shapes) SMPs, I think the major advantage of LCEs is the reversible shape changes. To me, LCEs offer more functionality and flexibility compared to (other) SMPs. Please feel free to share your thoughts!

Best,

Xueju

Hi Zhijian,

Good question! The strain energy (W) required to buckle the structure is related to the elastic modulus (E), thickness (t), and lateral dimension (w) of the structure like ribbons via a simple scaling law: W ∝ Ewt^3. So we can tailor the thickness of the soft lightly crosslinked LCE to enable successful buckling. For structures shown in our work (https://onlinelibrary.wiley.com/doi/full/10.1002/adfm.202100338), we typically use 200-600 micronmeter thick LCE films. In addition, we can also enhance the modulus of pure LCEs by incorporating hard elements like magnetic particles (Figure 5A-5B in this journal club) or by modulating the space length during the synthesis of LCEs (introducing polymer crystalinity). We have not tried tailoring the crosslinking density in the lightly crosslinked stage of LCEs before buckling to increase its modulus, but we believe that will also help!

Hi Ruobing,

Thanks for your kind note and good question! Well-controlled spatial programming is important to create complicated shape-morphing LCE structures, but is usually difficult to achieve in a facile manner. We have tried a few different techniques, inlcuding buckling (compressive and tensile buckling), folding, and also imprinting that Zhijian mentioned. All of them actually worked pretty well regarding creating spatially aligned LCE structures with reversible shape-morphing capabilities. Designing the mechanics in the material/structure definitely plays an important role in such programming. One thing that would help with such design is the quantitative characterization of the spatial alignment as a function of strain/stress in the material/structure to serve as an input. But this type of characterization is also a challenge as I mentioned in this journal club, because most current techniques including WAXS and FTIR require 2D thin film samples for testing.

Best,

Xueju

Hi Shaoting,

I probably should have clarified that the strain-induced alignment of polymer chains is observered but may be different from the SIC you mentioned. Please refer to the followed dicussion on semi-crystaline LC networks by integrating polymer cyrstalinity and liquid crystalinity within LCEs.

Hi Ruobing,

Thank you for sharing the great works of combining polymer crystallinity with liquid crystalinity in LCEs to offer both load-bearing and shape-morphing capabilities. It is indeed a very interesting material system that offers many new opportunities. The phase field model also looks quite interesting. It also reminds me a work along this line by Yakacki https://pubs.rsc.org/en/content/articlelanding/2017/sm/c7sm01380a, which discusses integrating semi-crystalinity with LCEs as well as tuning the rate of polymer crystalization (from 5 min to 2-3 hours) by varying the spacer length while maintaining the same mesogen (RM257) during the synthesis of LCEs.

Hi Xuejue,

Thanks for the informative review. Using the poping structure to program LCE is very creative. Congratulations!

I have one general question regarding, which I also often ask myself and the students. What are the advantage/uniqueness of LCE as compared to shape memory polymers (in particular, two way shape memory polymer), for various practical applications?

shengqiang

Hi Xueju,

Thanks for the timely review. I am interested in the spatial programming via buckling. The lightly crosslinked LCE is usually very soft. Is it needed to tailor the crosslinking density and thickness of the lightly crosslinked LCE to make the buckling structure?

Best,

Zhijian

Hi Ruobing,

In addtion to the 3D printing and surface alignment, people also used the imprint lithography to make the LCE 3D structure. In the printing and surface alignment techniques, the 2D film is in liquid crystal state and after heating, the 2D film transit to 3D structure. While with the imprint method, the LCE would change from the 3D shape to a 2D film after heating. The mesogen direction and alignment is determined by the localized stretching in the imprint process. In Xueju's work, the shape in the isotropic state is also a flat film. Very interesting design.

Best,

Zhijian

Hi Xueju,

Thank you for putting together this nice and timely topic of LCE for the mechanics community. I am particularly interested in your methods of controlled local programming in 2.3. In addition to the crosslink density, it is interesting and also very challenging to achieve spatially patterned mesogen direction programming. Some existing methods include printing and surface alignment, which are relatively difficult to implement without certain expertise. Are there any other potential methods to achieve complex direction programming? In particular, I am wondering if designing the mechanics in the material/structure can play an important role in such programming.

Best regards,

Ruobing

Hi Shaoting,

I am not aware of SIC in rubbery LCEs. But semicrystalline LC polymer networks can be made, where strain-induced (re)crystallization is expected. This is a new kind of material developed by a few groups such as Ware (https://pubs.acs.org/doi/10.1021/acs.macromol.7b00567) and Hayward (https://pubs.acs.org/doi/10.1021/acsmacrolett.0c00328). It is a very interesting material system with a lot of unknowns to explore. We had a recent paper using phase field modeling to study its phase transition under various light and temperature (https://journals.aps.org/pre/abstract/10.1103/PhysRevE.103.033003).

Hi Shaoting,

Thank you for your kind note and very good question! Actually, we did observe strain-induced alignment of polymer chains in LCEs. For example, in our work (ACS Applied Materials & Interfaces 13.7 (2021): 8929-8939 https://pubs.acs.org/doi/abs/10.1021/acsami.0c21371), which is also shown in Figure 8B-8D of this journal club, we characterized the alignment degree of polymer chains in uniaxially stretched LCEs as a function of strain using Polarized FTIR. We showed that the alignment degree (order parameter) of polymer chains increases with the strain.

Regarding your second question, the alignment of the polymer chains and the alignment of mesogen units in LCEs are inherently linked, especially for main-chain LCEs where mesogens are incorporated into the polymer backbone. Therefore, people usually use the polymer chain alignment to estimate the mesogen alignment degree including in our work. Given this strong coupling/linking between the polymer chain and mesogen alignment, during the nematic-to-isotropic state transition in LCEs, the aligned polymer networks are also disrupted into an isotropic state along with mesogens. Recently, Dr. Kai Yu also performed real-time characterizations of alignment and reorientation of polymer chains in LCEs using in situ optical measurements https://pubs.acs.org/doi/full/10.1021/acsami.1c20082.

Hopefully this is helpful.

Best

Xueju