Individualization of exosuit assistance based on measured muscle dynamics during versatile walking.

Variability in human walking depends on individual physiology, environment, and walking task. Consequently, in the field of wearable robotics, there is a clear need for customizing assistance to the user and task. Here, we developed a muscle-based assistance (MBA) strategy wherein exosuit assistance was derived from direct measurements of individuals' muscle dynamics during specific tasks. We recorded individuals' soleus muscle dynamics using ultrasonographic imaging during multiple walking speeds and inclines. From these prerecorded images, we estimated the force produced by the soleus through inefficient concentric contraction and designed the exosuit assistance profile to be proportional to that estimated force. We evaluated this approach with a bilateral ankle exosuit at each measured walking task. Compared with not wearing a device, the MBA ankle exosuit significantly reduced metabolic demand by an average of 15.9, 9.7, and 8.9% for level walking at 1.25, 1.5, and 1.75 meters second−1, respectively, and 7.8% at 1.25 meters second−1 at 5.71° incline while applying lower assistance levels than in existing literature. In an additional study (n = 2), we showed for multiple walking tasks that the MBA profile outperforms other bioinspired strategies and the average profile from a previous optimization study. Last, we show the feasibility of online assistance generation in a mobile version for overground outdoor walking. This muscle-based approach enables relatively rapid (~10 seconds) generation of individualized low-force assistance profiles that provide metabolic benefit. This approach may help support the adoption of wearable robotics in real-world, dynamic locomotor tasks by enabling comfortable, tailored, and adaptive assistance.

Constitutive modeling of porcine liver in indentation using 3D ultrasound imaging.

In this work we present an inverse finite-element modeling framework for constitutive modeling and parameter estimation of soft tissues using full-field volumetric deformation data obtained from 3D ultrasound. The finite-element model is coupled to full-field visual measurements by regularization springs attached at nodal locations. The free ends of the springs are displaced according to the locally estimated tissue motion, and the normalized potential energy stored in all springs serves as a measure of model-experiment agreement for material parameter optimization. We demonstrate good accuracy of estimated parameters and consistent convergence properties on synthetically generated data. We present constitutive model selection and parameter estimation for perfused porcine liver in indentation, and demonstrate that a quasilinear viscoelastic model with shear modulus relaxation offers good model-experiment agreement in terms of indenter displacement (0.19 mm RMS error) and tissue displacement field (0.97 mm RMS error).

Thomas McMahon: a dedication in memoriam.

Thomas A. McMahon (1943-1999) was a pioneer in the field of biomechanics. He made primary contributions to our understanding of terrestrial locomotion, allometry and scaling, cardiac assist devices, orthopedic biomechanics, and a number of other areas. His work was frequently characterized by the use of simple mathematical models to explain seemingly complex phenomena. He also validated these models through creative experimentation. McMahon was a successful inventor and also published three well-received novels. He was raised in Lexington, Massachussetts, attended Cornell University as an undergraduate, and earned a PhD at MIT. From 1970 until his death, he was a member of the faculty of Harvard University, where he taught biomedical engineering. He is fondly remembered as a warm and gentle colleague and an exemplary mentor to his students.

Tactile imaging of breast masses: first clinical report.

HYPOTHESIS: Tactile imaging can accurately document the palpable extent of breast masses. DESIGN: Prospective nonrandomized interventional trial, comparing mass size estimates from preoperative physical examination, ultrasound, and tactile imaging with postoperative measurements of the resected masses. SETTING: A community ambulatory surgical center and a university hospital tertiary care center. PATIENTS: Twenty-three women undergoing surgical excision of breast masses. All subjects had a single, palpable, dominant mass, 0.5 to 3 cm in diameter. INTERVENTION: Prior to surgery, the size of each mass was estimated from tactile imaging using an array of pressure sensors that is stroked over the mass. Size was also estimated by ultrasound and physical examination. Immediately following resection of the mass, it was bisected, and the palpable extent was measured with a caliper. MAIN OUTCOME MEASURE: Maximum mass diameter estimates from ultrasound, physical examination, and tactile imaging, compared with the resected measurement. RESULTS: Tactile imaging estimates were repeatable (7.5% mean SD for multiple estimates of the same mass) and show good agreement with the resected measurements. Mean absolute error was 13%, and linear regression with zero intercept had a slope of 0.94, r(2) = 0.51. Physical examination and ultrasound estimates had respective mean absolute errors of 46% and 34%, regression slopes of 1.27 and 0.89, and r(2) = 0.28 and 0.37. CONCLUSIONS: Tactile imaging can provide accurate and reproducible estimates of the size of breast masses. This capability can enhance cancer surveillance for patients with benign masses (eg, due to scarring or fibrocystic changes) because previous work suggests that reliable detection of a difference in mass size by physical examination requires a 40% change in diameter. In contrast, this study suggests tactile imaging requires only a 15% change (95% confidence interval).

Dynamic lumped element response of the human fingerpad.

The dynamic response of the fingerpad plays an important role in the tactile sensory response and precision manipulation, as well as in ergonomic design. This paper investigates the dynamic lumped element response of the human fingerpad in vivo to a compressive load. A flat probe indented the fingerpad at a constant velocity, then held a constant position. The resulting force (0-2 N) increased rapidly with indentation then relaxed during the hold phase. A quasilinear viscoelastic model successfully explained the experimental data. The instantaneous elastic response increased exponentially with position, and the reduced relaxation function included three decaying exponentials (with time constants of approximately 4 ms, 70 ms, and 1.4 s) plus a constant. The model was confirmed with data from sinusoidal displacement trajectories.

Dynamic contact of the human fingerpad against a flat surface.

This paper investigates the dynamic, distributed pressure response of the human fingerpad in vivo when it first makes contact with an object. A flat probe was indented against the fingerpad at a 20 to 40 degree angle. Ramp-and-hold and sinusoidal displacement trajectories were applied to the fingerpad within a force range of 0-2 N. The dynamic spatial distribution of the pressure response was measured using a tactile array sensor. Both the local pressure variation and the total force exhibited nonlinear stiffness (exponential with displacement) and significant temporal relaxation. The shape of the contact pressure distribution could plausibly be described by an inverted paraboloid. A model based on the contact of a rigid plane (the object) and a linear viscoelastic sphere (the fingerpad), modified to include a nonlinear modulus of elasticity, can account for the principal features of the distributed pressure response.

Robotics for surgery.

Robotic technology is enhancing surgery through improved precision, stability, and dexterity. In image-guided procedures, robots use magnetic resonance and computed tomography image data to guide instruments to the treatment site. This requires new algorithms and user interfaces for planning procedures; it also requires sensors for registering the patient's anatomy with the preoperative image data. Minimally invasive procedures use remotely controlled robots that allow the surgeon to work inside the patient's body without making large incisions. Specialized mechanical designs and sensing technologies are needed to maximize dexterity under these access constraints. Robots have applications in many surgical specialties. In neurosurgery, image-guided robots can biopsy brain lesions with minimal damage to adjacent tissue. In orthopedic surgery, robots are routinely used to shape the femur to precisely fit prosthetic hip joint replacements. Robotic systems are also under development for closed-chest heart bypass, for microsurgical procedures in ophthalmology, and for surgical training and simulation. Although results from initial clinical experience is positive, issues of clinician acceptance, high capital costs, performance validation, and safety remain to be addressed.

Identification of the mechanical impedance at the human finger tip.

Rapid transients were applied to the outstretched human index finger tip, which resulted in motion primarily at the metacarpophalangeal (MCP) joint in extension and in abduction. A second-order linear model was fit to approximately 20 milliseconds of the force and displacement data to determine the effective mechanical impedance at the finger tip. Ranges of mass, damping, and stiffness parameters were estimated over a range of mean finger tip force (2-20 N for extension, 2-8 N for abduction). Effective translational finger tip mass for each subject was relatively constant for force levels greater than 6 N for extension, and constant throughout the abduction trials. Stiffness increased linearly with muscle activation. The estimated damping ratio for extension trials was about 1.7 times the ratio for abduction.

Visualization of turbulent flame fronts with planar laser-induced fluorescence.

This report concerns the quantitative time-resolved visualization of reaction zones in laminar, transitional, and turbulent nonpremixed flames. Two-dimensional OH molecular concentrations were measured with planar laser-induced fluorescence excited by a sheet of light (formed from a single tunable ultraviolet laser pulse) and detected with a two-dimensional, image-intensified photodiode array camera. From the resulting data details of instantaneous flame front structures (including positions, shapes, and widths) were obtained.