My project brief remains largely the same as before–to create a robot capable of licking lollipops in an eerily biomimetic manner, count those licks, and, before finishing the lollipop, biting the candy with a hidden jaw mechanism.
The concept and background I discussed in the last post left a big technical hole: how to keep the tongue wet. The previous iteration of this project used a sponge for the tongue, but this presents both visual issues (it’s hard to make a sponge convincingly mimic a tongue), and mechanical issues (actuating complex movement within a sponge may not be feasible). Furthermore, the previous work I had looked at for this new version primarily used silicone, and I have some potential concerns regarding the ability of a material like silicone to hold onto water in a way that is useful.
Doing some research, I stumbled into hydrogel materials–complex structures of polymers capable of absorbing water, deformable, and biocompatible/biodegradable. They have wide applications in the medical and biotech fields, and are starting to get introduced into the field of soft robotics. Considering the tongue design discussed in the last blog post, which consists of 6 inflating/deflating chambers, a similar motion could be achieved using an electro-responsive hydrogel (1). By sending current through this material, it could swell, replacing the pneumatic air chambers in the silicone tongue. Switching to a hydrogel would allow the tongue to hold water in a manner hopefully similar to the sponge used in the previous iteration.
Furthermore, adding a microfiber layer on top of the hydrogel tongue could reduce the amount of water that evaporates from the tongue (2). Reducing evaporation may allow the tongue to perform more licks between rewettings and therefore allow for a more fluid, organic motion. This approach does increase friction, though, and may have an adverse effect on the tongue’s ability to wet the lollipop on consecutive licks.
There are some concerns about the structural integrity of hydrogels with regards to structural applications (1), and the extent to which this will be an issue for this particular project might require some testing. It is somewhat unclear to me where the line is drawn between structural and nonstructural applications.
With regards to manufacturing these materials, two possible approaches seem feasible. First, hydrogels can be directly 3D-printed using a Direct Ink Writing printer (similar in concept to a standard fused deposition modelling printer, but using liquid that dries rather than a melted solid that returns to room temperature). Mixing a carbomer solvent with the base materials for a hydrogel can create an ink capable of being 3D-printed without the use of complex, and sometimes problematic, support structures (3). Second, and likely even more feasible, is the ability to create viable hydrogels via casting in a rigid mold (2). Molds for this purpose can be produced in easily accessible materials such as PLA plastic (meaning molds can be 3D-printed). Constituent ingredients for a hydrogel can then be poured into the rigid mold removed after being properly cured.
- Hritwick Banerjee, Suhail Mohamed, Hongliang Ren. “Hydrogel Actuators and Sensors for Biomedical Soft Robots: Brief Overview with Impending Challenges.” Biomimetics, Volume 3, Issue 3. September 2018. doi: 10.3390/biomimetics3030015.
- Shuma Kanai, Yosuke Watanabe, MD Nahin Islam Shiblee, Ajit Khosla, Jun Ogawa, Masaru Kawakami, Hidemitsu Furukawa. “Skin-Mimic Hydrogel Materials with Water-Perspiration Control for Soft Robots Developed by 3D Printing.” ECS Transactions, Volume 98, Number 13, Pages 23-27. September 2020. doi: 10.1149/09813.0023ecst.
- Zhe Chen, Donghao Zhao, Binhong Liu, Guodong Nian, Xiaokeng Li, Jun Yin, Shaoxing Qu, Wei Yang. “3D Printing of Multifunctional Hydrogels.” Advanced Functional Materials, Volume 29, Issue 20, Pages 1900971. 2019. doi: 10.1002/adfm.201900971.
Additionally Consulted Sources:
- Hritweick Banerjee, Hongliang Ren. “Optimized Double-Network Hydrogel for Biomedical Soft Robots.” Soft Robotics, Volume 4, Number 3. 1 September 2017. doi: 10.1089/soro.2016.0059.
- Yin Cheng, Kwok Hoe Chan, Xiao-Qiao Wang, Tianpeng Ding, Tongtao Li, Xin Lu, Ghim Wei Ho. “Direct-Ink-Write 3D Printing of Hydrogels into Biomimetic Soft Robots.” ACS Nano, Volume 13, Issue 11, Pages 13176-13184. 26 November 2019. doi: 10.1021/acsnano.9b06144.
- Xuanming Lu, Weiliang Xu, Xiaoning Li. “PneuNet Based Control System for Soft Robotic Tongue.” IEEE 14th International Workshop on Advanced Motion Control, Pages 353-357. 2016. doi: 10.1109/AMC.2016.7496375.