亲爱的景达，

我还想分享两篇同事的综述论文，特别阐述了水凝胶的应用。

水凝胶生物电子学 by Hyunwoo and Baoyang。

邵婷

亲爱的景达，

非常感谢您的好意。同时，衷心祝贺您在EASF的精彩演讲!你的问题很有挑战性，但也很有启发性。我将尝试从我的角度来回答这些问题。我也想听听你的想法。

你是对的。水凝胶黏附和水凝胶疲劳的研究虽然是同时开始的，但目前处于不同的发展阶段。水凝胶黏附作为转化科学的一项领先技术，已经显示出其广泛而直接的影响;而抗疲劳水凝胶的研究大多集中在基础研究或材料开发上。

抗疲劳水凝胶的一个应用，我也认为它具有巨大的转化潜力，那就是可摄取的水凝胶药丸作为胃保留装置，用于长期的心理信号监测、肥胖控制、药物输送和胃肠道成像。对于有能力进行这种体内测试的团队来说，我相信抗疲劳水凝胶具有巨大的潜力，可以实现以前无法实现的性能或功能。

In addition to the application of ingestible gastro retentive devices, fatigue-resistant hydrogels are crucial for many existing applications of soft materials towards soft machines. The majority of the efforts in soft machines are laboratory work. However, when targeting practical deployment, their long term reliability is the killing factor for their lifetime. If we look back to the development of metals, composites, and plastics, their fatigue studies have been a hot topic and fatigue evaluation has been a standard routine before the commercialization of a product. I feel the development of soft machines has not entered that stage yet, but will bring people's attention to more realistic factors such as lifetime, corrosion, aging.

I also need to point out that many applications of hydrogels actually do not need that high fatigue resistance. For example, in an epidermal wearable device, the deformation of the relevant materials is typically small. When used in human body, on-demand degrading is also more favorable. It really depends on the specific application conditions.

(2) As I mentioned in (1), ingestible hydrogel pills are the new application of PVA hydrogels. If we track literature of PVA hydrogels, mostly are focused on cartilage replacement. I had the experience of visiting Philips, where they used PVA as an ultrasound phantom with complex shapes. Robust coatings for existing biomedical devices seem to be also the interests of doctors.

If not limited to PVA hydrogels, I actually also see the potential of using hydrogels in applications beyond biomedical engineering. Hydrogels recently also bring interests to the area of sustainable water, energy harvesting, and agriculture. For example, the following paper used hydrogels for photovoltaic cooling (R. Li, Y. Shi, M. Wu, S. Hong, P. Wang, Photovoltaic panel cooling by atmospheric water sorption–evaporation cycle. Nature Sustainability, 1-8, 2020).

(3) We actually present two forms of PVA hydrogels: dry-annealed PVA hydrogel and nanofibrous PVA hydrogel. For the dry-annealed PVA hydrogel, it is isotropic, thereby it displays fatigue-resistance in any directions. However, for the nanofibrous PVA hydrogel, it is anisotropic. The fatigue resistance is high (i.e., 1250 J/m^2) when loading is along the direction of aligned fibrils, but relatively low (i.e., 233 J/m^2) when the loading is perpedicular to the aligned fibrils. Ruobing's work (//m.limpotrade.com/node/23079) also shows a similar anisotropic property of nanofibrous PVA hydrogels using a different fabrication technique.

To solve that, we print the nanofibrous hydrogels into mesh-like structures, so that the sample shows fatigue-resistance in both in-plane directions. I believe you show a much-advanced version of 3D printing to achieve fatigue-resistant elastomers.

Best,

Shaoting