Department of Biological Sciences, Virginia Tech, Blacksburg, VA 24061, USA.
Department of Biological Sciences, Virginia Tech, Blacksburg, VA 24061, USA.
Acta Biomater. 2022 Oct 1;151:468-479. doi: 10.1016/j.actbio.2022.08.018. Epub 2022 Aug 13.
Tiny glue droplets along the viscous capture threads of spider orb webs prevent insects from escaping. Each droplet is formed of a protein core surrounded by a hygroscopic aqueous layer, which cause the droplet's adhesion to change with humidity. As an insect struggles to escape the web, a thread's viscoelastic core proteins extend, transferring adhesive forces to the thread's support fibers. Maximum adhesive force is achieved when absorbed atmospheric moisture allows a flattened droplet to establish sufficient adhesive contact while maintaining the core protein cohesion necessary for force transfer. We examined the relationship between these droplet properties and adhesive force and the work of extending droplets at five relative humidities in twelve species that occupy habitats which have different humidities. A regression analysis that included both flattened droplet area and core protein elastic modulus described droplet adhesion, but the model was degraded when core protein area was substituted for droplet. Species from low humidity habitats expressed greater adhesion at lower humidities, whereas species from high humidity habitats expressed greater adhesion at high humidities. Our results suggest a general model of droplet adhesion with two adhesion peaks, one for low humidity species, which occurs when increasing droplet area and decreasing protein cohesion intersect, and another for high humidity species, which occurs when area and cohesion have diverged maximally. These dual peaks in adhesive force explain why some species from intermediate and high humidity habitats express high adhesion at several humidities. STATEMENT OF SIGNIFICANCE: We characterized the effect of humidity on the adhesion of twelve orb weaving spider species' glue droplets and showed how humidity-mediated changes in the contact area of a droplet's outer, hygroscopic aqueous layer and the stiffness of its protein core affect droplet performance. This revealed how droplet adhesion has been tuned to the humidity of a species' habitat and allowed us to revise a model that describes the environmental determinants of droplet biomechanics.
微小的胶滴沿着蜘蛛蛛网粘性捕获丝排列，防止昆虫逃脱。每个胶滴由一个蛋白质核心组成，周围是一层吸湿的水层，这导致胶滴的附着力随湿度而变化。当昆虫挣扎着逃离蛛网时，一条线的粘弹性核心蛋白会延伸，将粘接力传递到线的支撑纤维上。当吸收的大气水分允许一个扁平的液滴在保持核心蛋白凝聚力以传递力的必要条件下建立足够的粘附接触时，最大的粘附力就会实现。我们研究了这些液滴特性与粘附力以及在 12 个栖息地湿度不同的物种中在五个相对湿度下扩展液滴的功之间的关系。包括压扁的液滴面积和核心蛋白弹性模量的回归分析描述了液滴的粘附，但当核心蛋白面积代替液滴时，模型就会退化。来自低湿度栖息地的物种在较低的湿度下表现出更大的粘附力，而来自高湿度栖息地的物种在较高的湿度下表现出更大的粘附力。我们的结果表明了一个液滴粘附的一般模型，有两个粘附峰值，一个是低湿度物种的，当增加液滴面积和减少蛋白质内聚力相交时出现，另一个是高湿度物种的，当面积和内聚力最大程度地发散时出现。这种粘附力的双峰现象解释了为什么一些来自中高湿度栖息地的物种在几个湿度下表现出高粘附力。
我们描述了湿度对 12 种轨道编织蜘蛛物种胶滴粘附的影响，并展示了湿度如何调节液滴外层吸湿水层的接触面积和其蛋白质核心的刚度变化如何影响液滴性能。这揭示了液滴粘附是如何根据物种栖息地的湿度进行调整的，并允许我们修改一个描述液滴生物力学环境决定因素的模型。
