Characterization of Magnetorheological Impact Foams in Compression.

This study focuses on the development and compressive characteristics of magnetorheological elastomeric foam (MREF) as an adaptive cushioning material designed to protect payloads from a broader spectrum of impact loads. The MREF exhibits softness and flexibility under light compressive loads and low strains, yet it becomes rigid in response to higher impact loads and elevated strains. The synthesis of MREF involved suspending micron-sized carbonyl Fe particles in an uncured silicone elastomeric foam. A catalyzed addition crosslinking reaction, facilitated by platinum compounds, was employed to create the rapidly setting silicone foam at room temperature, simplifying the synthesis process. Isotropic MREF samples with varying Fe particle volume fractions (0%, 2.5%, 5%, 7.5%, and 10%) were prepared to assess the effect of particle concentrations. Quasi-static and dynamic compressive stress tests on the MREF samples placed between two multipole flexible strip magnets were conducted using an Instron servo-hydraulic test machine. The tests provided measurements of magnetic field-sensitive compressive properties, including compression stress, energy absorption capability, complex modulus, and equivalent viscous damping. Furthermore, the experimental investigation also explored the influence of magnet placement directions (0° and 90°) on the compressive properties of the MREFs.

Visco-Elastic Honeycomb Structures with Increased Energy Absorption and Shape Recovery Performance Using Buckling Initiators.

Energy-absorbing materials have extensive applications in aerospace and automotive applications. Research has shown buckling initiators, or triggers, in energy-absorbing tubular structures increase the energy absorbed by encouraging the side panels to fold when loaded out of plane in compression conditions. Additively manufactured TPE honeycombs were designed in this study to include these buckling initiators, which introduced a slight decrease in initial weight, as well as initial stress concentrations, while improving crashworthiness characteristics. The samples with buckling initiators (1BI) showed an increase in crush efficiency when directly compared to their no buckling initiator (0BI) counterparts. The 1BI samples maintained an increased crush efficiency regardless of the strain rate used. The samples with 1BI were able to better equilibrate the peak stress with the plateau stress. These honeycomb samples were found to maintain their crush efficiency, even after multiple rounds of compression testing. The quasi-static 0BI samples experienced a 23.4% decrease in the peak stress after multiple rounds of compression testing, while the 1BI samples saw approximately a 23.0% decrease. The 1BI samples averaged a decrease in crush efficiency of 0.5%, while the 0BI samples saw a decrease in crush efficiency of 5%. As the strain rate increased, the crush efficiency for the 1BI samples showed an increase in performance, with a smaller degradation in crush efficiency over multiple uses. Visco-elastic honeycomb with buckling initiators has a higher energy absorption than samples with no buckling initiators when exposed to multiple impact cycles.

Tunable Energy Absorbing Property of Bilayer Amorphous Glass Foam via Dry Powder Printing.

The research in this paper entails the design of material systems with tunable energy-absorbing properties. Hollow glass microspheres of different densities are layered using dry powder printing and subsequently sintered to form a cellular structure. The tunability of the bilayer foams is investigated using various combinations of hollow microspheres with different densities and different thickness ratios of the layers. The mechanical responses to quasi-static uniaxial compression of the bilayer foams are also investigated. These bilayer samples show different mechanical responses from uniform samples with a distinctive two-step stress-strain profile that includes a first and second plateau stress. The strain where the second plateau starts can be tuned by adjusting the thickness ratio of the two layers. The resulting tunable stress-strain profile demonstrates tunable energy absorption. The tunability is found to be more significant if the density values of each layer differ largely. For comparison, bilayer samples are fabricated using epoxy at the interface instead of a sintering process and a different mechanical response is shown from a sintered sample with the different stress-strain profile. Designing the layered foams allows tuning of the stress-strain profile, enabling desired energy-absorbing properties which are critical in diverse impact conditions.

Bending Properties of an Extensile Fluidic Artificial Muscle.

Low stiffness, large stroke, and axial force capabilities make Extensile Fluidic Artificial Muscles (EFAMs) a feasible soft actuator for continuum soft robots. EFAMs can be used to construct soft actuated structures that feature large deformation and enable soft robots to access large effective workspaces. Although FAM axial properties have been well studied, their bending behavior is not well characterized in the literature. Static and dynamic bending properties of a cantilevered EFAM specimen were investigated over a pressure range of 5-100 psi. The static properties were then estimated using an Euler-Bernoulli beam model and discrete elastic rod models. The experiments provided data for the determination of bending stiffness, damping ratio, and natural frequency of the tested specimen. The bending stiffness and the damping ratio were found to change fourfold over the pressure range. Experimentally validated bending properties of the EFAM presented insights into structural and control considerations of soft robots. Future work will utilize the data and models obtained in this study to predict the behavior of an EFAM-actuated continuum robot carrying payloads.

Vibration Suppression of a Composite Prosthetic Foot Using Piezoelectric Shunt Damping: Implications to Vibration-Induced Cumulative Trauma.

OBJECTIVE: Energy-storage-and-return (ESAR) prosthetic feet have improved amputee mobility due to their efficient conversion of strain energy to mechanical work. However, this efficiency is typically achieved using light-weight, high-stiffness materials, which generate high-frequency vibrations that are potentially injurious if transmitted to biological tissues. To reduce the vibration which may cause cumulative tissue trauma, high-frequency vibration suppression by piezoelectric shunt damping patches on a commercial ESAR foot was evaluated. METHODS: Two patches with either passive or active shunt circuits were placed on the foot to investigate vibration suppression during experimental tests where a plastic hammer was used to hit a clamped ESAR foot on the free end. Prosthesis bending moments at each modal frequency were obtained by finite element methods to identify piezoelectric patch placement. RESULTS: Both shunt circuits decreased vibration amplitudes at specific modes better than the no shunt case, but also increased the amplitude at specific frequencies. The vibration suppression performance of the active shunt circuit deteriorated at the second mode, while the vibration suppression performance of the passive shunt circuit deteriorated at all frequencies above the third mode. CONCLUSIONS: These results indicate piezoelectric shunt patches may be a viable strategy for decreasing vibrations of an ESAR foot, with active methods more efficient at suppressing high-frequency vibrations. Additional research is necessary to fine-tune the method for maximal vibration suppression. SIGNIFICANCE: Overall, this study indicates that high-frequency vibration suppression is possible using piezoelectric patches, possibly decreasing the cumulative tissue damage that may occur with repetitive exposure to vibration.

Successful Implementation of Unmanned Aircraft Use for Delivery of a Human Organ for Transplantation.

OBJECTIVE: To understand and overcome the challenges associated with moving life-urgent payloads using unmanned aircraft. BACKGROUND DATA: Organ transportation has not been substantially innovated in the last 60 years. Unmanned aircraft systems (UAS; ie, drones) have the potential to reduce system inefficiencies and improve access to transplantation. We sought to determine if UASs could successfully be integrated into the current system of organ delivery. METHODS: A multi-disciplinary team was convened to design and build an unmanned aircraft to autonomously carry a human organ. A kidney transplant recipient was enrolled to receive a drone-shipped kidney. RESULTS: A uniquely designed organ drone was built. The aircraft was flown 44 times (total of 7.38 hours). Three experimental missions were then flown in Baltimore City over 2.8 miles. For mission #1, no payload was carried. In mission #2, a payload of ice, saline, and blood tubes (3.8 kg, 8.4 lbs) was flown. In mission #3, a human kidney for transplant (4.4 kg, 9.7 lbs) was successfully flown by a UAS. The organ was transplanted into a 44-year-old female with a history of hypertensive nephrosclerosis and anuria on dialysis for 8 years. Between postoperative days (POD) 1 and 4, urine increased from 1.0 L to 3.6 L. Creatinine decreased starting on POD 3, to an inpatient nadir of 6.9 mg/dL. The patient was discharged on POD 4. CONCLUSIONS: Here, we completed the first successful delivery of a human organ using unmanned aircraft. This study brought together multidisciplinary resources to develop, build, and test the first organ drone system, through which we performed the first transplant of a drone transported kidney. These innovations could inform not just transplantation, but other areas of medicine requiring life-saving payload delivery as well.

Novel Bending and Helical Extensile/Contractile Pneumatic Artificial Muscles Inspired by Elephant Trunk.

Pneumatic artificial muscles (PAMs) are an extensively investigated type of soft actuator. However, the PAM motions have been limited somewhat to uniaxial contraction and extension, restraining the development of PAMs. Given the current strong interest in soft robotics, PAMs have been gaining renewed attention due to their excellent compliance and ease of fabrication. Herein, under the inspiration of the elephant trunk, a family of bending and helical extensile PAMs (HE-PAMs)/helical contractile PAMs (HC-PAMs) was proposed and analyzed. Through both experiment and analysis, a model of generalized bending behavior of PAMs was built and developed to investigate the properties of axial, bending, and helical PAMs in the same theoretical framework. The topological equivalence and bifurcation were found in the analysis and utilized to explain the behaviors of these different PAMs. Meanwhile, a coupled constant curvature and torsion kinematics model was proposed to depict the motion of PAMs more accurately and conveniently. Moreover, a soft tandem manipulator consisting of bending and helical PAMs was proposed to demonstrate their attractive potential.

An Initial Investigation of Unmanned Aircraft Systems (UAS) and Real-Time Organ Status Measurement for Transporting Human Organs.

Organ transportation has yet to be substantially innovated. If organs could be moved by drone, instead of ill-timed commercial aircraft or expensive charter flights, lifesaving organs could be transplanted more quickly. A modified, six-rotor UAS was used to model situations relevant to organ transportation. To monitor the organ, we developed novel technologies that provided the real-time organ status using a wireless biosensor combined with an organ global positioning system. Fourteen drone organ missions were performed. Temperatures remained stable and low (2.5 °C). Pressure changes (0.37-0.86 kPa) correlated with increased altitude. Drone travel was associated with less vibration (<0.5 G) than was observed with fixed-wing flight (>2.0 G). Peak velocity was 67.6 km/h (42 m/h). Biopsies of the kidney taken prior to and after organ shipment revealed no damage resulting from drone travel. The longest flight was 3.0 miles, modeling an organ flight between two inner city hospitals. Organ transportation may be an ideal use-case for drones. With the development of faster, larger drones, long-distance drone organ shipment may result in substantially reduced cold ischemia times, subsequently improved organ quality, and thousands of lives saved.

Variable recruitment in bundles of miniature pneumatic artificial muscles.

The natural compliance and force generation properties of pneumatic artificial muscles (PAMs) allow them to operate like human muscles in anthropomorphic robotic manipulators. Traditionally, manipulators use a single PAM or multiple PAMs actuated in unison in place of a human muscle. However, these standard manipulators can experience significant efficiency losses when operated outside their target performance ranges at low actuation pressures. This study considers the application of a variable recruitment control strategy to a parallel bundle of miniature PAMs as an attempt to mimic the selective recruitment of motor units in a human muscle. Bundles of miniature PAMs are experimentally characterized, their actuation behavior is modeled, and the efficiency gains and losses associated with the application of a variable recruitment control strategy are assessed. This bio-inspired control strategy allows muscle bundles to operate the fewest miniature PAMs necessary to achieve a desired performance objective, improving the muscle bundle's operating efficiency over larger ranges of force generation and displacement. The study also highlights the need for improved PAM fabrication techniques to facilitate the production of identical miniature PAMs for inclusion in muscle bundles.

Analytical model and stability analysis of the leading edge spar of a passively morphing ornithopter wing.

This paper presents the stability analysis of the leading edge spar of a flapping wing unmanned air vehicle with a compliant spine inserted in it. The compliant spine is a mechanism that was designed to be flexible during the upstroke and stiff during the downstroke. Inserting a variable stiffness mechanism into the leading edge spar affects its structural stability. The model for the spar-spine system was formulated in terms of the well-known Mathieu's equation, in which the compliant spine was modeled as a torsional spring with a sinusoidal stiffness function. Experimental data was used to validate the model and results show agreement within 11%. The structural stability of the leading edge spar-spine system was determined analytically and graphically using a phase plane plot and Strutt diagrams. Lastly, a torsional viscous damper was added to the leading edge spar-spine model to investigate the effect of damping on stability. Results show that for the un-damped case, the leading edge spar-spine response was stable and bounded; however, there were areas of instability that appear for a range of spine upstroke and downstroke stiffnesses. Results also show that there exist a damping ratio between 0.2 and 0.5, for which the leading edge spar-spine system was stable for all values of spine upstroke and downstroke stiffnesses.

Effect of bladder wall thickness on miniature pneumatic artificial muscle performance.

Pneumatic artificial muscles (PAMs) are actuators known for their high power to weight ratio, natural compliance and light weight. Due to these advantages, PAMs have been used for orthotic devices and robotic limbs. Small scale PAMs have the same advantages, as well as requiring greatly reduced volumes with potential application to prostheses and small scale robotics. The bladder of a PAM affects common actuator performance metrics, specifically: blocked force, free contraction, hysteresis, and dead-band pressure. This paper investigates the effect that bladder thickness has on static actuation performance of small scale PAMs. Miniature PAMs were fabricated with a range of bladder thicknesses to quantify the change in common actuator performance metrics specifically: blocked force, free contraction, and dead-band pressure. These PAMs were then experimentally characterized in quasi-static conditions, where results showed that increasing bladder wall thickness decreases blocked force and free contraction, while dead-band pressure increases. A nonlinear model was then applied to determine the structure of the stress-strain relationship that enables accurate modeling and the minimum number of terms. Two nonlinear models are compared and the identified parameters are analyzed to study the effect of the bladder thickness on the model.