Yong-Lae Park

Design and Fabrication of Soft Artificial Skin Using Embedded Microchannels and Liquid Conductors

We describe the design, fabrication, and calibration of a highly compliant artificial skin sensor. The sensor consists of multilayered mircochannels in an elastomer matrix filled with a conductive liquid, capable of detecting multiaxis strains and contact pressure. A novel manufacturing method comprised of layered molding and casting processes is demonstrated to fabricate the multilayered soft sensor circuit. Silicone rubber layers with channel patterns, cast with 3-D printed molds, are bonded to create embedded microchannels, and a conductive liquid is injected into the microchannels. The channel dimensions are 200 μm (width) × 300 μm (height). The size of the sensor is 25 mm × 25 mm, and the thickness is approximately 3.5 mm. The prototype is tested with a materials tester and showed linearity in strain sensing and nonlinearity in pressure sensing. The sensor signal is repeatable in both cases. The characteristic modulus of the skin prototype is approximately 63 kPa. The sensor is functional up to strains of approximately 250%.

Hyperelastic pressure sensing with a liquid-embedded elastomer

A hyperelastic pressure transducer is fabricated by embedding silicone rubber with microchannels of conductive liquid eutectic gallium–indium. Pressing the surface of the elastomer with pressures in the range of 0–100 kPa will deform the cross-section of underlying channels and change their electric resistance by as much as 50%. Microchannels with dimensions as small as 25 µm are obtained with a maskless, soft lithography process that utilizes direct laser exposure. Change in electrical resistance is measured as a function of the magnitude and area of the surface pressure as well as the cross-sectional geometry, depth and relative lateral position of the embedded channel. These experimentally measured values closely match closed-form theoretical predictions derived from plane strain elasticity and contact mechanics.

Design and control of a bio-inspired soft wearable robotic device for ankle-foot rehabilitation.

We describe the design and control of a wearable robotic device powered by pneumatic artificial muscle actuators for use in ankle-foot rehabilitation. The design is inspired by the biological musculoskeletal system of the human foot and lower leg, mimicking the morphology and the functionality of the biological muscle-tendon-ligament structure. A key feature of the device is its soft structure that provides active assistance without restricting natural degrees of freedom at the ankle joint. Four pneumatic artificial muscles assist dorsiflexion and plantarflexion as well as inversion and eversion. The prototype is also equipped with various embedded sensors for gait pattern analysis. For the subject tested, the prototype is capable of generating an ankle range of motion of 27° (14° dorsiflexion and 13° plantarflexion). The controllability of the system is experimentally demonstrated using a linear time-invariant (LTI) controller. The controller is found using an identified LTI model of the system, resulting from the interaction of the soft orthotic device with a human leg, and model-based classical control design techniques. The suitability of the proposed control strategy is demonstrated with several angle-reference following experiments.

Real-Time Estimation of 3-D Needle Shape and Deflection for MRI-Guided Interventions

We describe a MRI-compatible biopsy needle instrumented with optical fiber Bragg gratings for measuring bending deflections of the needle as it is inserted into tissues. During procedures, such as diagnostic biopsies and localized treatments, it is useful to track any tool deviation from the planned trajectory to minimize positioning errors and procedural complications. The goal is to display tool deflections in real time, with greater bandwidth and accuracy than when viewing the tool in MR images. A standard 18 ga × 15 cm inner needle is prepared using a fixture, and 350-μm-deep grooves are created along its length. Optical fibers are embedded in the grooves. Two sets of sensors, located at different points along the needle, provide an estimate of the bent profile, as well as temperature compensation. Tests of the needle in a water bath showed that it produced no adverse imaging artifacts when used with the MR scanner.

Design and Characterization of a Soft Multi-Axis Force Sensor Using Embedded Microfluidic Channels

Thin, highly compliant sensing skins could provide valuable information for a host of grasping and locomotion tasks with minimal impact on the host system. We describe the design, fabrication, and characterization of a novel soft multi-axis force sensor made of highly deformable materials. The sensor is capable of measuring normal and in-plane shear forces. This soft sensor is composed of an elastomer (modulus: 69 kPa) with embedded microchannels filled with a conductive liquid. Depending on the magnitude and the direction of an applied force, all or part of the microchannels will be compressed, changing their electrical resistance. The two designs presented in this paper differ in their flexibility and channel configurations. The channel dimensions are approximately 200 × 200 μm and 300 × 700 μm for the two prototypes, respectively. The overall size of each sensor is 50 × 60 × 7 mm. The first prototype demonstrated force sensitivities along the two principal in-plane axes of 37.0 and -28.6 mV/N. The second prototype demonstrated the capability to detecting and differentiating normal and in-plane forces. In addition, this paper presents the results of a parameter study for different design configurations.

A Soft Strain Sensor Based on Ionic and Metal Liquids

A novel soft strain sensor capable of withstanding strains of up to 100% is described. The sensor is made of a hyperelastic silicone elastomer that contains embedded microchannels filled with conductive liquids. This is an effort of improving the previously reported soft sensors that uses a single liquid conductor. The proposed sensor employs a hybrid approach involving two liquid conductors: an ionic solution and an eutectic gallium-indium alloy. This hybrid method reduces the sensitivity to noise that may be caused by variations in electrical resistance of the wire interface and undesired stress applied to signal routing areas. The bridge between these two liquids is made conductive by doping the elastomer locally with nickel nanoparticles. The design, fabrication, and characterization of the sensor are presented.

Wearable soft sensing suit for human gait measurement

Wearable robots based on soft materials will augment mobility and performance of the host without restricting natural kinematics. Such wearable robots will need soft sensors to monitor the movement of the wearer and robot outside the lab. Until now wearable soft sensors have not demonstrated significant mechanical robustness nor been systematically characterized for human motion studies of walking and running. Here, we present the design and systematic characterization of a soft sensing suit for monitoring hip, knee, and ankle sagittal plane joint angles. We used hyper-elastic strain sensors based on microchannels of liquid metal embedded within elastomer, but refined their design with the use of discretized stiffness gradients to improve mechanical durability. We found that these robust sensors could stretch up to 396% of their original lengths, would restrict the wearer by less than 0.17% of any given joint’s torque, had gauge factor sensitivities of greater than 2.2, and exhibited less than 2% change in electromechanical specifications through 1500 cycles of loading–unloading. We also evaluated the accuracy and variability of the soft sensing suit by comparing it with joint angle data obtained through optical motion capture. The sensing suit had root mean square (RMS) errors of less than 5° for a walking speed of 0.89 m/s and reached a maximum RMS error of 15° for a running speed of 2.7 m/s. Despite the deviation of absolute measure, the relative repeatability of the sensing suit’s joint angle measurements were statistically equivalent to that of optical motion capture at all speeds. We anticipate that wearable soft sensing will also have applications beyond wearable robotics, such as in medical diagnostics and in human–computer interaction.

Bio-inspired active soft orthotic device for ankle foot pathologies

We describe the design of an active soft ankle-foot orthotic device powered by pneumatic artificial muscles for treating gait pathologies associated with neuromuscular disorders. The design is inspired by the biological musculoskeletal system of a human foot and a lower leg, and mimics the muscle-tendon-ligament structure. A key feature of the device is that it is fabricated with flexible and soft materials that provide assistance without restricting degrees of freedom at the ankle joint. Three pneumatic artificial muscles assist dorsiflexion as well as inversion and eversion. The prototype is also equipped with various embedded sensors for gait training and gait pattern analysis. The prototype is capable of 12° dorsiflexion from a resting position of an ankle joint and a 20° dorsiflexion from plantarflexion. Results of early feedback control experiments show controllability of ankle joint angles. Ultimately, we envision a system that not only can provide physical support to improve mobility but also can increase safety and stability during walking, while enhancing muscle usage and encouraging rehabilitation.

Design and Control of a Bio-inspired Human-friendly Robot

The increasing demand for physical interaction between humans and robots has led to an interest in robots that guarantee safe behavior when human contact occurs. However, attaining established levels of performance while ensuring safety creates formidable challenges in mechanical design, actuation, sensing and control. To promote safety without compromising performance, a human-friendly robotic arm has been developed using the concept of hybrid actuation. The new design employs high-power, low-impedance pneumatic artificial muscles augmented with small electrical actuators, distributed compact pressure regulators with proportional valves, and hollow plastic links. The experimental results show that significant performance improvement can be achieved with hybrid actuation over a system with pneumatic muscles alone. In this paper we evaluate the safety of the new robot arm through experiments and simulation, demonstrating that its inertia/power characteristics surpass those of previous human-friendly robots we have developed.

Force Sensing Robot Fingers using Embedded Fiber Bragg Grating Sensors and Shape Deposition Manufacturing

Force sensing is an essential requirement for dexterous robot manipulation. Although strain gages have been widely used, a new sensing approach is desirable for applications that require greater robustness, design flexibility and immunity to electromagnetic noise. An exoskeletal force sensing robot finger was developed by embedding fiber Bragg grating (FBG) sensors into a polymer-based structure. Multiple FBG sensors were embedded into the structure to allow the manipulator to sense and measure both contact forces and grasping forces. In order to fabricate a three-dimensional structure, a new shape deposition manufacturing (SDM) process was explored. The sensorized SDM-fabricated finger was then characterized using an FBG interrogator. A force localization scheme is also described