Smart Materials

For deep space travel, new materials are being explored to assist humans in dangerous environments, such as high radiation, extreme temperature, and extreme pressure. Synthetic Muscle™ is a class of electroactive polymer (EAP)-based materials and actuators that shape-morph at low voltage (1.5 V to 50 V), sense pressure (gentle touch to high impact), and attenuate force. These EAPs can survive and work in environments where humans cannot safely enter due to extreme environments or due to contagions that have no cure. From the Ras Labs-CASIS-ISS Experiment, the flown Synthetic Muscle™ samples compared well to the ground control samples, even after over a year on the International Space Station. Replicating human grasp has implications in robotics and prosthetics. EAP linkages can be actuated and EAP pressure sensors placed at the fingertip regions of robotic grippers for tactile feedback. With autonomy, artificial intelligence, machine learning, and EAP and other smart material technologies all coming together, there is an incredible fusion of mechanical and biological concepts to make truly innovative biomimetic motion. Smart materials will allow humanity to advance and survive on Earth and in space: on the ISS National Laboratory, the planned Moon base, the anticipated Mars settlements, and beyond.

Q&A with Astronaut Yvonne Cagle, MD, about Missions to Moon and Mars, and developments in amplified muscle restoration, plus the challenges for smart materials, robotics, and humanity as we explore the cosmos and consider colonizations in space.

Space-based astronomy and remote sensing systems would benefit from extremely large-aperture mirrors that can permit greater-resolution images. To be cost-effective and practical, such optical systems must be lightweight and capable of deployment from highly compacted stowed configurations. Such gossamer mirror structures are likely to be very flexible and therefore present challenges in achieving and maintaining the required optically precise shape. Active control based on dielectric elastomers was evaluated in order to address these challenges. Dielectric elastomers offer potential advantages over other candidate actuation technologies including high elastic strain, low power dissipation, tolerance of the space environment, and ease of commercial fabrication into large sheets. The basic functional element of dielectric elastomer actuation is a thin polymer film coated on both sides by a compliant electrode material. When voltage is applied between electrodes, a compressive force squeezes the film, causing it to expand in area. We have explored both material survivability issues and candidate designs of adaptive structures that incorporate dielectric elastomer actuation. Experimental testing has shown the operation of silicone-based actuator layers over a temperature range of –100 °C to 260 °C, suitable for most earth orbits. Analytical (finite element) and experimental methods suggested that dielectric elastomers can produce the necessary shape change when laminated to the back of a flexible mirror or incorporated into an inflatable mirror. Interferometric measurements verified the ability to effect controllable shape changes less than the wavelength of light. In an alternative design, discrete polymer actuators were shown to be able to control the position of a rigid mirror segment with a sensitivity of 1800 nm/V, suggesting that sub-wavelength position control is feasible. While initial results are promising, numerous technical challenges remain to be addressed, including the development of shape control algorithms, the fabrication of optically smooth reflective coatings, consideration of dynamic effects such as vibration, methods of addressing large numbers of active areas, and stowability and deployment schemes.

Hydro Muscle is fluid-actuated artificial muscle with top-rated strain and energy-efficient properties that can closely mimic biological muscle dynamics. In contrast, popular, 60-year-older McKibben muscle is not very efficient and cannot support a biologically realistic muscle strain. Currently, there are no low-cost commercially available fluid control valves that are suitable for a wide range of robotic applications. To address this market shortcoming, the Compact Robotic Flow Control (CRFC) Valve has been recently introduced. The IP-protected CRFC Valve can work in junction with the IP-protected Hydro Muscle. When integrated, Hydro Muscles and the CRFC Valve may be utilized as modular building blocks for robots that can be rapidly assembled and utilized as either perform-alone or wearable robotic systems. The synergy of the CRFC Valve with the cost-effective, energy-efficient, and excellent strain properties of the Hydro Muscle opens a door into a new age of fascinating, useful, and accessible/affordable fluid-operated wearable robotic solutions both in space and on Earth.

Pneumatic artificial muscles (PAMs) are simple mechanical actuators that consist of an elastomeric bladder within a braided mesh sleeve. Upon pressurization of the bladder, the actuator may either contract or extend axially, with the direction of motion dependent on the orientation of the braided sleeve fibers. PAMs that have been constructed for contractile motion are well known for their excellent actuator characteristics, including high specific power, specific work, and power density. Extensile PAMs, on the other hand, are unable to demonstrate comparable performance, in particular demonstrating much reduced blocked force, and are prone to buckling under axial compressive loading. For applications in which extensile motion and compressive force generation are desired, this chapter presents a novel actuator, known as the push-PAM, that harnesses the operational characteristics of a typical contraction/tension PAM, but changes the direction of motion and force with a simple conversion package that demands only a small increase in friction, weight, and cost. Quasi-static testing on this new actuator variant revealed the push-PAM’s full evolution of force with displacement for operating pressures ranging from 138 to 621 kPa (20–90 psi in 10 psi increments), in addition to the blocked force and free contraction capabilities at each pressure, confirming that the push-PAM was able to achieve forces and strokes comparable to a contractile PAM tested under the same conditions. Furthermore, it was shown that the performance of both types of actuators could be captured with the same modeling efforts. A refined force balance analysis was applied to these actuators that utilized additional modeling terms to account for three key nonlinear phenomena that were observed in both types of actuators: nonlinear PAM stiffness, hysteresis of the force vs. displacement response for a given pressure, and a pressure dead-band. This nonlinear model was shown to reconstruct the actuator response with minimal error for both the contractile PAM and the push-PAM.

Soft robots, robots that are constructed out of soft materials or using compliant actuation methods, can operate safely in complex environments without fear of damaging their surroundings or themselves. However, the soft materials and structures can be imprecise and difficult to control. This thesis seeks to remedy this problem for reverse pneumatic artificial muscle (rPAM) actuators, which can apply force by extending when pressurized. First, we discuss the modeling of simple linear rPAMs as well as the motion control of a linear rPAM-driven 1-degree-of-freedom (DoF) revolute joint. Control is done through an iterative sliding mode controller with and without a feedforward term. Next, we adapt this control approach to a soft planar bending segment and use a model reference adaptive controller to compensate for the variability of soft material mechanical properties. From there, we expanded the iterative sliding mode controller and used it to control 2-DoF joint modules using three linear rPAM actuators. We subsequently replaced this universal joint with the human wrist and developed a system to provide haptic feedback to the users, improving their performance computerized line-following task. We then developed a model to predict the behavior of bending actuators, both under load and while pressurized internally, which we used to perform inverse kinematic path following. These techniques represent a meaningful advancement in understanding and improving soft actuators, allowing them to move with speed and precision while resisting external forces.

In orbit, satellites are subjected to strong temperature variations depending on the environment they encounter (hot or cold phases of the mission). Thus, one of the challenges in thermal control of artificial satellites is to develop new coatings, with variable emissivity, light, inexpensive, and requiring little or no electricity consumption. In this context, several research groups have developed electroemissive devices based on organic or inorganic materials with tunable infrared (IR) optical properties. This chapter is also devoted to a bibliographical study addressing the concept of emissivity on the one hand and describing the state of the art of electroemissive devices (EED) on the other hand. The most advanced EED fully described in the literature is reported as well as the EED developed by CY Cergy Paris Université and Thales Alenia Space. The latter was evaluated in thermal-vacuum conditions close to the space environment for a proof-of-concept-level testing, and the thermal regulation performances were reported.