In our research bioinspiration is a key concept that we exploit over and over to take advantage of systems engineered by nature in billions of years of evolution and this project is a clear example of it.
If we think about refocusing in optical systems for imaging or other applications, the paradigm is having rigid lenses being translated one with respect to the other like, for example, in a reflex camera:
That's not the way nature does it though (apart from some exceptions in cephalopods [1]) and we use a focusing system entirely different every day that doesn't move lenses around but "simply" reshape its lens in a controlled fashion:
The ciliary muscle and the zonule work together in the human to change the radius of curvature (and thus the focal length) of the crystalline lens in a process called accommodation [2]. Carpi et al. [3] took inspiration from this structural arrangement and functional behaviour of the pre-retinal components and developed a first proof of concept device that recreates the accommodation mechanism using Dielectric Elastomer Actuators (DEAs). In the figure below we see a morphological comparison between the human lens and the bioinspired lens with a schematic representations (frontal and sectional views) of the human crystalline lens surrounded by the annular ciliary muscle and connected to it via the annular zonule (a,b), and then drawings (frontal and sectional views) of the bioinspired device, consisting of a fluid-fi lled lens surrounded by an annular artificial muscle actuator.
Our work, in collaboration with Herbert Shea's group at EPFL (Neuchatel, Switzerland) was to start from this proof of concept device and optimise the device design, materials, optical properties and speed, and resulted in a fundamental publication in the field of DEAs, demonstrating the possibility of ultrafast soft devices. [4]
Dielectric elastomer actuators (DEA) are smart lightweight flexible materials integrating actuation, sensing, and structural functions. The field of DEAs has been progressing rapidly, with actuation strains of over 300% reported, and many application concepts demonstrated. However many DEAs are slow, exhibit large viscoelastic drift, and have short lifetimes, due principally to the use of acrylic elastomer membranes and carbon grease electrodes applied by hand. We've developed a DEA-driven tunable lens, the world’s fastest capable of holding a stable focal length. By using low-loss silicone elastomers rather than acrylics, a settling time shorter than 175 μs is obtained for a 20% change in focal length. The silicone-based lenses show a bandwidth 3 orders of magnitude higher compared to lenses of the same geometry fabricated from the acrylic elastomer. Stretchable electrodes, a carbon black and silicone composite, are precisely patterned by pad-printing and subsequently crosslinked, enabling strong adhesion to the elastomer and excellent resistance to abrasion. The lenses operate for over 400 million cycles without degradation, and show no change after more than two years of storage. This lens demonstrates the unmatched combination of strain, speed, and stability that DEAs can achieve, paving the way for complex fast soft machines.
Above is the cover of the journal issue that I designed
References: [1] Land MF, Nilsson DE. Animal eyes. Oxford University Press: Oxford; 2002 [2] F. M. Toates , Physiol. Rev. 1972 , 52 , 828 [3] F. Carpi, G. Frediani, S. Turco, D. De Rossi, “Bioinspired tunable lens with muscle-like electroactive elastomers”, Advanced Functional Materials, Vol. 21, pp. 4152–4158, 2011. DOI: 10.1002/adfm.201101253 [4] L. Maffli, S. Rosset, M. Ghilardi, F. Carpi, H. Shea, “Ultrafast all-polymer electrically tuneable silicone lenses”, Advanced Functional Materials, Vol. 25, pp. 1656–1665, 2015. DOI: 10.1002/adfm.201403942