3D Printed Bioinspired Surface Patterning for Soft Robotics

Designing Optimal Super-liquid-repellent Surfaces
7th August 2020

by Preeti Sharma, preeti.sharma@wur.nl, and Joshua A. Dijksman, joshua.dijksman@wur.nl, both Wageningen University & Research, the Netherlands

Patterned surfaces and interfaces are of paramount importance for various animal species to perform tasks for their survival. One such example can be found in the gecko, which demonstrates an exceptional ability to stick to and run on any vertical or inverted surface due to the presence of micron-to-nanometre-size features on their toes [1]. Also, in the plant world, 3D patterned surfaces with hooks can be found on the leaves of Galium aparine, commonly called catchweed, that can adhere to many surfaces [2, 3].

To understand the fundamental physical mechanisms that govern such adhesive behaviour, recreating bioinspired patterned surfaces is essential. Indeed, such studies can also inspire new, innovative designs of interfacial mechanics for new applications in robotics. However, it is still challenging to develop a simple, scalable methodology that allows a 3D structure to be realized, with overhanging features such as “mushroom” or “hook” shapes, essential for the “interlocking” mechanics so often observed in Nature.

Preeti Sharma (left) and Joshua A. Dijksman from Wageningen University & Research, the Netherlands. Copyright: authors.
Preeti Sharma (left) and Joshua A. Dijksman from Wageningen University & Research, the Netherlands. Copyright: authors.

Realization of Complex 3D Features

Currently, 3D printers are the easiest and most economic tool for the direct realization of complex 3D features. While submicron resolution3D printing can be achieved by highend technology, scientific work involving 3D printing in many labs is regularly done with normal stereolithography printers. These stereolithography or SLA 3D printers are a subset of 3D printers which use UV or visible light-curable resins as a 3D printing material. The liquid resin is cured layer by layer, exposing the resin selectively to light from a laser (laser-SLA), a beamer (digital light processing DLP) or a liquid crystal display (LCD). Among these, laser-SLA can fabricate the smallest of features, even those of 100 microns or less, as the minimum feature size is equal to the size of the laser beam at the focal point.

It is no surprise then, that we show that this kind of simple 3D printing method can be used for patterning surfaces with small and complex features such as 3D mushroom features (Figure a). We also print catchweed-like hooks and an array of sharp pillars standing at a 60 degree angle. The printed features show the desired mechanical functionality, for instance, the mushroom features adhere to rough substrates such as textiles via the mechanical interlocking of the micro-mushrooms into fabric asperities. Mechanical adhesives based on interlocking can induce damage and wear and tear on surfaces.

Our Approach

3D printed pattern with mushroom features on it. Copyright: Wageningen University & Research
3D printed pattern with mushroom features on it. Copyright: Wageningen University & Research

We therefore extended our 3D printing method to create complex 3D surface patterns in polydimethylsiloxane elastomer (PDMS) using a double moulding process [4]. The methodology can be easily applied: first, the positive 3D printed structure was replicated as negative in a highly stretchable elastomer Ecoflex 0030. Then standard PDMS Sylgard 184 was cast on the Ecoflex layer and cured in the negative Ecoflex for making the 3D printed replica. Note that an important aspect to take into account is the chemical composition of the different materials and the chemical reaction involved in the process. In fact, 3D printer resins are usually composed of methacrylates, which are reactive with the vinyl-terminated siloxane presented both in the Ecoflex and PDMS.

This means that if the 3D printed structure is not completely cured, it will react with the uncured Ecoflex and will chemically bond to it, making it impossible to peel off. Similar considerations are relevant when casting PDMS on Ecoflex. If the latter is not properly cured, the two silicone-based elastomers will react and seal together. We solved these issues using a chemical surface modification of the 3D printed and Ecoflex mould. Both surfaces were first activated by a plasma oven followed by a reaction with perfluorodecyltrichlorosilane (PFOTS) using a chemical vapour deposition (CVD) approach.

Novel Key Handles for Tuning the Adhesive Properties

Our work, which has been done in collaboration with Vittorio Saggiomo from the Bio-NanoTechnology group at Wageningen University, and Marleen Kamperman from Groningen University, shows that samples having 3D mushroom-shaped features have desirable mechanical functionality to adhere to soft and rough surfaces such as fabrics (Figure b) via mechanical interlocking. Although mechanical interlocking sounds like a simple process, and can occur annoyingly easily as a brief exposure to catchweed will surely confirm, there is a surprising level of microscopic dynamics occurring in the adhesion process itself. We have shown that attachment–detachment dynamics taking place at the interfaces can be experimentally accessed in great detail, revealing novel key handles for tuning the adhesive properties of soft patterned interfaces.

The main target of our work is indeed to reveal the fundamentals of patterned adhesion, with the overall aim of improving the adhesive strength of these interlocking-based adhesives. A fundamental understanding of mechanical adhesion and its scaling behaviour will perhaps allow us to understand other types of dry adhesives such as gecko adhesives, where the dynamics of an individual feature is hard to visualize due to the nanometric size of the features. Our work will certainly open up new routes towards active control and thus robotic optimization of mechanical adhesion, for example by embedding microfluidics or electromagnetic control features in the patterned adhesives. This work therefore naturally fits in the framework of the 4TU Soft Robotics initiative (https://dutchsoftrobotics.nl/), a collaborative Dutch research effort from the four technical universities in the Netherlands to develop enhanced soft robotics technology.

References

[1] M. D. Bartlett et al., Adv. Mater. 1078 (2012)
[2] A. J. Bowling et al., Protoplasma 233, 223 (2008)
[3] J. N. Burris et al., Plant Cell Rep. 37, 565 (2018)
[4] H. G. Andrews & J. P. S. Badyal, J. Adh. Sc. Tech. 28, 1243 (2014)

Surface pattered with 3D soft mushroom features showing adhesion via mechanical interlocking of mushroom features onto a textile fabric. Copyright: Wageningen University & Research
Surface pattered with 3D soft mushroom features showing adhesion via mechanical interlocking of mushroom features onto a textile fabric. Copyright: Wageningen University & Research
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