Poster R10 - A Biohybrid Mechanosensor Integrated with a Soft Robot

Introduction: The ability to use biological systems as sensors in inorganic technologies has the potential to create new abilities where the advantages that have evolved in biology can be utilized to enhance technologies like robots. One direction that is particularly well suited for this is in the area of soft materials, where grippers can provide enhanced versatility and greater safety when working with humans or dealing with delicate subjects. The large degree of freedom provided by soft architectures and the non-linearities in both the materials and common actuation approaches make controlling the systems challenging. Improving tactile feedback and contact sensing systems is one avenue to alleviate this challenge. This is particularly true for robotic grasp and locomotion applications where contact is an implicit occurrence and sensing it is vital to ensuring that an appropriate dynamics model is being used by the onboard controller. Various inorganic approaches to sensing contact have been researched, but biological sensing remains less explored. Biological approaches provide access to flexible form factors, customizability via genetic engineering, and inherent biodegradability. In the biological space, dinoflagellates are a promising organism for usage as a biological mechanosensor due to their sub-second response time to excitation, straightforward culture protocol, longevity, and high sensitivity to shear stress. In this work, we explore development towards a flexible capsule design that contains dinoflagellate with embedded photosensing systems to provide onboard monitoring of the dinoflagellate luminescence (Figure 1).

Materials and

Methods: Sensor development requires a culture line to maintain a supply of algae throughout testing. The algae species being used for this work is Pyrocystis fusiformis, selected because it is one of the most sensitive and luminous dinoflagellate. To culture these cells, seed colonies were procured from Carolina Biological Supply Co. and PyroFarms along with each supplier’s respective growth medium. To explore optimizing growth in our lab, various lighting protocols have been implemented, all using a 6400k full daylight spectrum, T5 bulb, 10” above algae samples. The light intensity was modulated between an average PPFD of 16 - 105 μMol/(m^s*s) at the algae waterline.

Initial capsules have been fabricated with polydimethylsiloxane (PDMS) (Sylgard 184, Dow) due to its biocompatibility, manufacturability, and transparency easing its interfacing with photosensing hardware. Capsules have been fabricated with varied geometries, filled with fluid to simulate algae, and undergone compression testing to evaluate how design parameters may impact system response. With this data, capsules that are integrated with the algae can have their physical design tailored such that different stimuli thresholds are required to elicit luminescence.

To demonstrate a soft robot’s capacity to leverage data from this sensor, we have built shape memory alloy (SMA) actuated digits as a testing platform (Figure 2). This allows us to generate dynamics models for the system both when it moves freely and experiences contact. Photosensor readings from a capsule at the distal end can inform which dynamics model is appropriate for the onboard control system to use.

Results, Conclusions, and Discussions: To gage health of the algae colonies, cell counts were recorded for colonies from two lighting protocols. The 12 hour, unobscured grow light exposure yielded two colonies with visible luminescence while the 10 hour, occluded grow light exposure yielded four colonies with visible luminescence. Our improved cell densities for the latter protocol suggest it is more optimal for P. fusiformis in our lab setup. We desire a high density because it improves the luminosity signal.

To characterize the mechanical properties of prototype capsules, compression tests were conducted at varying strain rates, each to a compression of 50% the capsule height (2.5 mm). Across the tested strain rates, the max compressive force required for each geometry remained constant. A fluid filled polymer structure should exhibit some viscoelasticity so a wider range of strain rates should be tested on samples. Meanwhile the diameter of the capsule exhibits an inverse proportionality to the max compression force, which aligns with Euler-Bernoulli beam theory that larger spans are more susceptible to deflection. Understanding how physical parameters impact capsule response allows us to tune the capsule design and sensor readings to different contact excitation thresholds. This opens avenues to networks of capsules that can provide higher resolution tactile data.

Additionally, to demonstrate the algae signal being used by a robot, we need to elicit controlled contact with our SMA actuated digit. Figure 2 depicts our SMA actuated digit being commanded to various poses where we can place a rigid surface for the digit to contact. We model the digit dynamics with a time delay embedding approach. With a capsule inserted on the distal end, we can pick up luminosity readings from embedded photosensors.

Developing novel contact sensing approaches is vital to enhancing the ability of robots to navigate unstructured environments, both from a grasp and locomotion perspective as they are bound to experience unplanned contact. Exploring biological material as a sensing modality both improves the biodegradability of these systems and can tap into highly sensitive systems honed by evolution. This work may impact tactile systems, soft robotics, organic-inorganic hybrids, and mechanotransduction.

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