Spider silk as a novel high performance biomimetic muscle driven by humidity

Abstract

SUMMARYThe abrupt halt of a bumble bee's flight when it impacts the almost invisible threads of an orb web provides an elegant example of the amazing strength and toughness of spider silk. Spiders depend upon these properties for survival, yet the impressive performance of silk is not limited solely to tensile mechanics. Here, we show that silk also exhibits powerful cyclic contractions, allowing it to act as a high performance mimic of biological muscles. These contractions are actuated by changes in humidity alone and repeatedly generate work 50 times greater than the equivalent mass of human muscle. Although we demonstrate that this response is general and occurs weakly in diverse hydrophilic materials, the high modulus of spider silk is such that it generates exceptional force. Furthermore, because this effect already operates at the level of single silk fibers, only 5 μm in diameter, it can easily be scaled across the entire size range at which biological muscles operate. By contrast, the most successful synthetic muscles developed so far are driven by electric voltage, such that they cannot scale easily across large ranges in cross-sectional areas. The potential applicability of silk muscles is further enhanced by our finding that silkworm fibers also exhibit cyclic contraction because they are already available in commercial quantities. The simplicity of using wet or dry air to drive the biomimetic silk muscle fibers and the incredible power generated by silk offer unique possibilities in designing lightweight and compact actuators for robots and micro-machines, new sensors, and green energy production.

How super is supercontraction? Persistent versus cyclic responses to humidity in spider dragline silk

“…3). This novel property of dragline silk can be exploited to do work and generate energy, offering potential for the development of biomimetic muscle fibers, sensors and other applications (Agnarsson et al, 2009b).…”

“…Thus, cyclic contraction can occur prior to supercontraction or after enough water has been absorbed to disrupt bonding within the glycine-rich linkers to cause supercontraction. Moreover, cyclic contraction is a phenomenon that occurs in other hydrophilic biological materials, with the magnitude of the response scaling directly with the stiffness of each material (Agnarsson et al, 2009b).…”

“…This cyclic response produces high forces that can be precisely controlled through humidity alone. Thus, spider silk emerges as an attractive model for biomimetic muscle fibers (Agnarsson et al, 2009b).…”

Hygromorphic actuator from a metal oxide film driven by a nano-capillary forest structure

“…Figure 4b compares the estimated energy densities and strain responses of the TNF film with typical values reported for materials frequently used or studied in the context of actuators and stimuli-responsive materials. [20][21][22][23][24] At this point, even though this TNF actuator has a much lower energy density than a shape memory alloy, it still shows a relatively high energy density compared to other actuators with sustainability and repeatability. Since this actuator uses a new hygromorphic actuating mechanism, it could be a useful actuator once the obstacles, such as response time and actuation speed, are solved.…”

“…In fact, onlỹ 15 mg of the titanium oxide nano-capillary films generated a larger force than most other actuating devices on a per mass basis. [20][21][22][23][24] Because the strain energy varies throughout a body, it is convenient to use the concept of strain energy density, which is a measure of how much energy is stored in small-volume elements throughout a material. To calculate the energy density of the free-standing TNF film, we assumed that the film is a uniform geometry in the form of an end-loaded cantilever of length L and calculated the strain energy for normal stresses acting in the elastic material upon deformation by a steadily increasing force F. The work done in extending the cantilever by a small amount, dx, is stored as elastic strain energy U, given as U = Fdx.…”