Different Levels Of Stiffness Research Articles

Stretchable printed circuit boards using a silicone substrate of variable stiffness and conventional PCB fabrication methods

Abstract Stretchable printed circuit boards (S-PCBs) offer unique advantages over rigid PCBs such as enabling conformability to changing environments and ergonomic designs with increased lifetime under dynamic loads. This study introduces an innovative fabrication method and a cost-effective solution for rapid prototyping S-PCBs using commercially available materials. By utilizing silicone substrates with different levels of stiffness and structured copper sheets for electrical connections, the S-PCBs feature ‘stiff islands’ embedded in a flexible base material. This fabrication method helps alleviate mechanical strain on strain-sensitive components while allowing local deformation of the S-PCB. The proposed fabrication method does therefore enable to integrate surface mount device components into S-PCBs and facilitates complex circuit designs while maintaining stretchability and fatigue resistance. Through material characterization, video strain analysis, as well as quasi-static and cyclic loading tests, this article demonstrates the efficacy of our approach. Based on the experimental results, we provide insights into failure modes and suggest design principles to further enhance the durability of S-PCBs fabricated with our method. We then conclude by presenting a soft wearable S-PCB demonstrator. The S-PCBs fabricated with this method withstood mechanical strains up to 100% and cyclic loads with 30% strain up to 625 cycles. The results are very promising for applications in soft robotics, wearable devices, and soft sensors and actuators. Overall, our study offers a comprehensive toolkit for fast S-PCB prototyping, paving the way for advancements in stretchable electronics with a high degree of complexity and stretchability.

Soft Sensory-Motor System Based on Ionic Solution for Robotic Applications.

Soft robots claim the architecture of actuators, sensors, and computation demands with their soft bodies by obtaining fast responses and adapting to the environment. Sensory-motor coordination is one of the main design principles utilized for soft robots because it allows the capability to sense and actuate mutually in the environment, thereby achieving rapid response performance. This work intends to study the response for a system that presents coupled actuation and sensing functions simultaneously and is integrated in an arbitrary elastic structure with ionic conduction elements, called as soft sensory-motor system based on ionic solution (SSMS-IS). This study provides a comparative analysis of the performance of SSMS-IS prototypes with three diverse designs: toroidal, semi-toroidal, and rectangular geometries, based on a series of performance experiments, such as sensitivity, drift, and durability. The design with the best performance was the rectangular SSMS-IS using silicon rubber RPRO20 for both internal and external pressures applied in the system. Moreover, this work explores the performance of a bioinspired soft robot using rectangular SSMS-IS elements integrated in its body. Further, it investigated the feasibility of the robot to adapt its morphology online for environment variability, responding to external stimuli from the environment with different levels of stiffness and damping.

The Three-dimensional Environment of Type Ⅰ Collagen Gels With Varying Stiffness Modulates the Immunological Functions of NK Cells

To construct type Ⅰ collagen gels with different stiffness and to investigate the effects of three-dimensional (3D) culture environments of the gels on the morphology, free migration ability, and cell killing function of natural killer (NK) cells. Type Ⅰ collagen was isolated from the tails of Sprague Dawley (SD) rats and collagen gels with different levels of stiffnesses were prepared accordingly. The microstructure of the collagen gels was observed by laser confocal microscopy. The stiffness of the collagen gels was assessed by measuring the plateau modulus with a rheometer. NK-92MI cells were cultured in collagen gels with different levels of stiffness. The morphology of NK-92MI cells was observed by inverted microscope. High content imaging system was used to record the free migration process of NK-92MI cells and analyze the migration speed and distance. NK-92MI cells were cultured with type Ⅰ collagen gels with different levels of stiffness for 24 h and 48 h and, then, co-cultured with human colorectal DLD-1, a human adenocarcinoma epithelial cell line. CCK8 assay was performed to determine the proliferation rate of DLD-1 cells and analyze the cell killing ability of NK-92MI cells. Low-stiffness type Ⅰ collagen gel and high-stiffness type Ⅰ collagen gel with the respective stiffness of (10.970±2.10) Pa and (114.50±3.40) Pa were successfully prepared. Compared with those cultured with the low-stiffness type Ⅰ collagen gel, the NK-92MI cells in the high-stiffness type Ⅰ collagen gel showed a more elongated shape (P<0.05), the mean area of the cells was reduced ([69.88±26.97] μm2 vs. [46.59±21.62] μm2, P<0.05), the roundness of the cells decreased (0.82±0.12 vs. 0.78±0.18, P<0.05), cell migration speed decreased ([2.50±0.91] μm/min vs. [1.70±0.72] μm/min, P<0.001) and the migration distance was shortened ([147.10±53.74] μm vs. [98.03± 40.95] μm, P<0.0001), with all the differences being statistically significant. Compared with those cultured with the low-stiffness type Ⅰ collagen gel, NK-92MI cells cultured with high-stiffness type Ⅰ collagen gel for 24 h could promote DLD-1 cell proliferation, with the proliferation rate being (46.39±12.79)% vs. (65.87±4.45)% (P<0.05) and reduce the cell killing ability. Comparison of the cells cultured for 48 h led to similar results, with the proliferation rates being (31.36±2.88)% vs. (74.57±2.16)% (P<0.05), and the differences were all statistically significant. The 3D culture environment of type Ⅰ collagen gels with different levels of stiffness alters the morphology, migration ability, and killing function of NK-92MI cells. This study provides the research basis for exploring and understanding the mechanisms by which the biomechanical microenvironment affects the immune response of NK cells, as well as laying the theoretical foundation for optimizing immunotherapy protocols.

Piezo1 Mediates the Regulation of Substrate Stiffness on Primary Cilia in Chondrocytes

To investigate how substrate stiffness regulates the morphology of primary cilia in chondrocytes and to illustrate how Piezo1 mediates the morphology regulation of primary cilia by substrate stiffness. Polydimethylsiloxane (PDMS) curing agent and the main agent (Dow Corning, Beijing, China) were mixed at the ratio of 1∶10 (stiff), 1∶50 (medium stiffness), and 1∶70 (soft), respectively, to prepare substrate films with the thickness of 1 mm at different levels of stiffness, including stiff substrate of (2.21±0.12) MPa, medium-stiffness substrate of (54.47±6.06) kPa, and soft substrate of (2.13±0.10) kPa. Chondrocytes were cultured with the substrates of three different levels of stiffness. Then, the cells were treated with Tubastatin A (Tub A) to inhibit histone deacetylase 6 (HDAC6), Piezo1 activator Yoda1, and inhibitor GsMTx4, respectively. The effects of HDAC6, Yoda1, and GsMTx4 on chondrocyte morphology and the length of primary cilia were analyzed through immunofluorescence staining. The stiff substrate increased the spread area of the chondrocytes. Immunofluorescence assays showed that the cytoskeleton and the nuclear area of the cells on the stiff substrate were significantly increased (P<0.05) and the primary cilia were significantly extended (P<0.05) compared with those on the medium-stiffness and soft substrates. However, the presence rate of primary cilia was not affected. The HDAC6 activity of chondrocytes increased with the decrease in substrate stiffness. When the activity of HDAC6 was inhibited, the cytoskeletal area, the nuclei area, and the primary cilium length were increased more significantly on the stiff substrate (P<0.05). Further testing showed that Piezo1 activator and inhibitor could regulate the activity of HDAC6 in chondrocytes, and that the length of primary cilia was significantly increased after treatment with the activator Yoda1 (P<0.05). On the other hand, the length of primary cilia was significantly shortened on the stiff substrate after treatment with the inhibitor GsMTx4 (P<0.05). Both substrate stiffness and Piezo1 may affect the morphology of chondrocyte primary cilia by regulating HDAC6 activity.

Endothelial glycocalyx sensitivity to chemical and mechanical sub-endothelial substrate properties.

Glycocalyx (GCX) is a carbohydrate-rich structure that coats the surface of endothelial cells (ECs) and lines the blood vessel lumen. Mechanical perturbations in the vascular environment, such as blood vessel stiffness, can be transduced and sent to ECs through mechanosensors such as GCX. Adverse stiffness alters GCX-mediated mechanotransduction and leads to EC dysfunction and eventually atherosclerotic cardiovascular diseases. To understand GCX-regulated mechanotransduction events, an in vitro model emulating in vivo vessel conditions is needed. To this end, we investigated the impact of matrix chemical and mechanical properties on GCX expression via fabricating a tunable non-swelling matrix based on the collagen-derived polypeptide, gelatin. To study the effect of matrix composition, we conducted a comparative analysis of GCX expression using different concentrations (60-25,000μg/mL) of gelatin and gelatin methacrylate (GelMA) in comparison to fibronectin (60μg/mL), a standard coating material for GCX-related studies. Using immunocytochemistry analysis, we showed for the first time that different substrate compositions and concentrations altered the overall GCX expression on human umbilical vein ECs (HUVECs). Subsequently, GelMA hydrogels were fabricated with stiffnesses of 2.5 and 5kPa, representing healthy vessel tissues, and 10kPa, corresponding to diseased vessel tissues. Immunocytochemistry analysis showed that on hydrogels with different levels of stiffness, the GCX expression in HUVECs remained unchanged, while its major polysaccharide components exhibited dysregulation in distinct patterns. For example, there was a significant decrease in heparan sulfate expression on pathological substrates (10kPa), while sialic acid expression increased with increased matrix stiffness. This study suggests the specific mechanisms through which GCX may influence ECs in modulating barrier function, immune cell adhesion, and mechanotransduction function under distinct chemical and mechanical conditions of both healthy and diseased substrates.

Extracellular matrix stiffness aggravates urethral stricture through Igfbp3/Smad pathway

Urethral stricture refers to the narrowing of the urethral lumen. While previous studies have hinted at inflammation as the initial driver of this condition, the reasons and mechanisms behind its progression remain largely unknown. By Atomic force microscope (AFM), researchers measured the matrix stiffness of urethra to be 5.23 ± 0.37 kPa for normal tissue and 41.59 ± 2.48 kPa for stricture urethral scar. Similar results were observed in rat urethral stricture models, where the matrix stiffness of normal urethra was 4.29 ± 0.82 kPa, while 32.94 ± 7.12 kPa for urethral stricture scar. Notably, the matrix stiffness increased in rat models over time. To further investigate, polyacrylamide hydrogels were employed to mimic different levels of stiffness for normal and stricture condition. Interestingly, higher matrix stiffness led to an increased fibroblast-to-myofibroblast transition (FMT) in rat urethral fibroblasts, indicated by enhanced expression of α-SMA and Collagen I, as well as changing in the morphology of fibroblast. RNA-seq analysis suggested that Igfbp3/Smads might regulate the progressive FMT in urethral stricture. In the experiment where the expression of Igfbp3 was inhibited, increasing matrix stiffness lose the potential to stimulate FMT progression and the expression of p-Smad2/3 decreased. On the contrary, overexpression of Igfbp3 promoted the process of FMT in urethral fibroblasts. In conclusion, Igfbp3/Smad pathway appeared to be involved in the progression of urethral fibrosis. This finding suggested that Igfbp3/Smad might be an promising target for future research and treatment in this filed.

FEMA P695 seismic performance evaluation of Cold-formed steel buildings with different shear wall strengths and stiffnesses

Lateral load capacity and stiffness of CFS framed OSB sheathed shear walls can be improved by increasing the number of fasteners between sheathing panel and framing members or by providing sheathing on both sides of panel. Both of these methods could be a viable option to meet high seismic demands when the number of walls is limited by architectural or other concerns. With the increasing use of CFS building systems in high seismic regions, such applications are expected to be more widespread in the future. The present study aims to investigate the seismic response of CFS framed archetype buildings utilizing OSB sheathed wall panels with different levels of stiffness and shear resistance. Nonlinear time history analyses were conducted on 28 archetype buildings and collapse performance evaluation was performed according to FEMA P695 methodology. Load-displacement hysteresis behaviors of wall panels measured in a recent experimental study were adopted to simulate the shear wall response in the numerical models. Buildings utilizing shear walls that are sheathed with OSB panels on one side with 300 mm fastener spacing possess limited overstrength and fail to satisfy the performance criteria regardless of the response modification coefficient used for design. With the use of shear walls that are constructed with 50 mm fastener spacing and single or double sided sheathing configurations, the building design is governed by drift limitation instead of strength requirement. Irregular drift profiles occur with lateral displacements localizing at the first floor when shear walls with single sided sheathing and loose fastener layout is utilized. Using shear walls with a dense fastener layout results in a uniform distribution of lateral displacements among floors.

Stiff Extracellular Matrix Promotes Invasive Behaviors of Trophoblast Cells.

The effect of extracellular matrix (ECM) stiffness on embryonic trophoblast cells invasion during mammalian embryo implantation remains largely unknown. In this study, we investigated the effects of ECM stiffness on various aspects of human trophoblast cell behaviors during cell-ECM interactions. The mechanical microenvironment of the uterus was simulated by fabricating polyacrylamide (PA) hydrogels with different levels of stiffness. The human choriocarcinoma (JAR) cell lineage was used as the trophoblast model. We found that the spreading area of JAR cells, the formation of focal adhesions, and the polymerization of the F-actin cytoskeleton were all facilitated with increased ECM stiffness. Significantly, JAR cells also exhibited durotactic behavior on ECM with a gradient stiffness. Meanwhile, stiffness of the ECM affects the invasion of multicellular JAR spheroids. These results demonstrated that human trophoblast cells are mechanically sensitive, while the mechanical properties of the uterine microenvironment could play an important role in the implantation process.

Simultaneous Evaluation of Stiffness and Histology in Orthopedic Diseases Using Scanning Acoustic Microscope

Organs with different levels of stiffness support the musculoskeletal system. Light microscopy cannot evaluate organ stiffness, whereas scanning acoustic microscopy (SAM) discriminates stiffness based on speed-of-sound (SOS) because sound waves pass faster in stiffer tissues. This study aimed to evaluate SOS imaging for orthopedic diseases using formalin-fixed paraffin-embedded sections. SOS imaging in SAM uses unstained light microscopic (LM) sections to prevent the bias of staining variation. Digital SOS values are comparable in different organ components and diseases. Mouse organs with the lowest mean SOS values included the adipose tissue, bone marrow, calcified cartilage, and nucleus pulposus; those with intermediate values included hyaline cartilages, osteoid, skeletal muscles, cortical and trabecular bones, and ligaments; and those with the highest values comprised fibrocartilages of the vertebral disc and meniscus. Water contents and delipidating procedures decreased SOS values. Collagenous density and arrangement affected higher SOS values. The trabecular bones of mice were thinner and showed significantly lower values of SOS than those of humans. Various orthopedic diseases and disorders displayed the characteristic SOS images. In osteoporosis, the trabecular bone becomes thin with lower SOS, indicating lesser stiffness to cause fractures. Comparison of woven and lamellar bones revealed that woven bones with lower SOS had lesser stiffness to fracture. Changes in SOS values indicated intramembranous bone formation. The trabecular bone develops from the connective tissues with an abrupt increase in SOS values. The regenerating process of bone fractures was monitored using SOS images, in which the granulation tissues transformed into calli in the osteoid to grow a new mineralized bone. The stiffness increased in phases, which appeared in SOS values. Although several methods have been used to visualize the stiffness of biological tissues, SAM only needs 10-µm unstained slides and can simultaneously compare mechanical stiffness and histology. SOS images provide informative mechanical alterations of the bone, cartilage, and connective tissues to assess the status and diagnose a disorder.

Design of a Felid-like Humanoid Foot for Stability Enhancement.

The foot is an important part of humanoid robot locomotion that can help with shock absorption while making contact with the ground. The mechanism of the foot directly affects walking stability. A novel foot mechanism inspired by the toes of felids is proposed. The foot has four bionic modules with soft pads and sharp claws installed at the four corners of a flat foot. This foot can reduce the impact experienced during foot landing and increase the time that the foot is in contact with the ground, which can improve the adaptability of the robot to different ground surface conditions with different levels of stiffness. The main structure of the bionic module is a four-bar linkage consisting of a slide way and a spring. Furthermore, the length of the four-bar linkage and the posture of the claw during insertion into soft ground are optimized to improve the stability and buffering performance. The validity of the proposed foot mechanism has been proved in simulations.

Experimental and numerical research of the steel frames with replaceable column foot

For steel moment-resisting frames, the plastic hinge often occurs in column foot under the action of earthquakes and it is difficult to repair the column foot developed plastic. To avoid the damage concentration effect of column foot, the innovative replaceable column foot is proposed in this paper, which can protect the column on the bottom floor and reduce the residual drifts of column. The stiffness and deformation of the replaceable column foot were investigated and compared with conventional rigid column foot. The pseudo-static test program of a one-third scaled steel frame with the replaceable column foot was carried out. The design approaches were recommended and verified based on the test results. The residual drifts, ductility and load-bearing capacity of the steel frame with replaceable column foot were investigated by finite element analysis and the results were compared to the steel frame with rigid column foot. It was found that the replaceable column foot can reduce the damage concentration effect of column on the bottom floor and the plastic deformations of replaceable column foot were mainly concentrated in energy dissipating components as expected. The residual drifts of the steel frame with replaceable column foot are less than that of the structure with rigid column foot. The parametric analysis of the replaceable column foot with different values of rational stiffness was performed. The results showed that the residual drifts, initial stiffness and load-bearing capacity of replaceable column foot can be accurately evaluated by the proposed design approaches. By the rational design of rational stiffness of replaceable column foot, different levels of stiffness and load-bearing capacity demand can be satisfied.

Accuracy and reproducibility of shear wave elastography according to the size and elasticity of lesions: A phantom study.

While the extrinsic factors affecting reproducibility of shear wave elastography (SWE) have been well documented, there are few resources assessing intrinsic characteristics of the lesion affecting the reproducibility and accuracy of SWE. In this regard, this study aimed to evaluate the accuracy of measured elasticity and the reproducibility of SWE according to the lesion size and stiffness. Two breast radiologists examined 20 targets of 4 different levels of stiffness and 5 different sizes (2.5, 4, 7, 11, and 18 mm) in a customized elasticity phantom. The B-mode image, color elastography image, and kPa measurement were obtained twice by each examiner with a 1-week interval. Inter- and intra-observer reproducibility and the accuracy of measured kPa were analyzed using intraclass correlation coefficient (ICC) and Bland-Altman analysis. Subgroup analysis was run to evaluate the effect of lesion size and stiffness on the reproducibility and accuracy of measured kPa. Inter- and intraobserver reproducibility for measuring kPa showed excellent agreement (ICC: 0.9742 and 0.9582; ICC: 0.9932 and 0.9294). The size and stiffness of the targets did not affect reproducibility. The overall accuracy of measured kPa was very high (ICC: 0.8049). In the subgroup analysis, targets that were ≤4 mm in size showed lower accuracy (ICC: 0.542), whereas targets that were 7 and 11 mm in size showed higher accuracy (ICC: 0.9832 and 0.9656, respectively). SWE shows excellent reproducibility regardless of lesion size or stiffness in phantom targets. The accuracy of measured kPa is high in lesions that are 7 and 11 mm in size but is low in lesions that are ≤4 mm in size.

SEMG Analysis of Upper Limb Muscles during Backhand Smash Using Badminton Rackets of Different Stiffness

The analysis of racket stiffness effect on muscle activity during movement associated with badminton is essential for a better understanding of which badminton racket is better for the athletes to use. The present study aimed to evaluate the effect of racket stiffness on muscle activity and activation time of the upper limb muscles—biceps (Bicp), lateral head (TLat) and long head (TLong) of the triceps—when performing the backhand smash (BH) movement using two rackets with different levels of stiffness. A group of 6 volunteers, with an average age of 21.8 (±4.5) years, and an average badminton practice time of 10.7 (±5.3) years, performed 10 valid repetitions of the movement with each racket, and the muscle activity was collected using surface electromyography. Observing the results shows that the most excited muscle in the BH smash, in terms of (%MVC), is the TLat, regardless of the stiffness of the racket, which shows 56.9% for the Duora 33 and about 68.9% for the Duora 88, comparing with the Bicp and the TLong, respectively. Also, it was observed that the more rigid racket (Duora 88) showed a higher muscular activity for the Bicp and TLat muscles.

Antagonistic Control of a Cable-Driven Prosthetic Hand with Neuromorphic Model of Muscle Reflex.

In this paper, a novel prototype of a cable-driven prosthetic hand with biorealisitic muscle property was developed. A pair of antagonistic muscles controlled the flexion and extension of the prosthetic index finger. Biorealistic properties of muscle were emulated using a neuromorphic model of muscle reflex in real time. The model output was coupled to a servo motor that tracked the computed muscle force. The servo motor was able to track model output within a frequency range from 0 to 8.29 (Hz) with a phase shift from 2 to 205 (deg). Surface electromyography signals collected from the amputee's forearm were used as α commands to drive the muscle model. With this prototype system, we evaluated its characteristics for force and stiffness control. Results of the force variability test showed that the standard deviation of fingertip force was linear to the mean fingertip force, indicating that force variability was proportional to the background force. At different levels of antagonistic co-contraction, the index finger and muscles displayed different levels of stiffness corresponding to the degree of co-activation. This prototype system showed the similar compliant behaviors of human limbs actuated with biological muscles. In further studies, this prototype system would be thoroughly evaluated for its biorealistic properties, and integrated with sensors to investigate feedback strategies of various sensory information for individuals with amputation. Clinical Relevance- This article established an antagonistic control of a cable-driven prosthetic hand with biorealistic properties of muscle reflex for application to individuals with amputation.

Design, modeling, and control of a Series Elastic Actuator with Discretely Adjustable Stiffness (SEADAS)

The recent application of the principle of discrete adjustment of stiffness has substantially enhanced the performance of Clutched Elastic Actuators (CEA), especially in terms of their capability for rapid stiffness switching. This key improvement plays a vital role in safety considerations, which are of particular relevance to physical Human–Robot Interaction (pHRI) applications. However, currently the only proposed CEA for this purpose suffers from bulkiness, which limits its potential application in multi-degree-of-freedom manipulators. In this work, we introduce a Series Elastic Actuator with Discretely Adjustable Stiffness (SEADAS) as a novel CEA, in which the stiffness can be modulated by adding/subtracting the appropriate number of elastic elements via clutches. The novelty lies in the unique design topology, where three concentric shafts are connected in series through clutches. Each shaft is further attached to a pair of elastic elements. The number of involved spring pairs, which depends on the engagement/disengagement of the clutches, allows for the attainment of three different levels of stiffness. The realized design is significantly compact relative to the previously proposed CEA that was based on this principle. This paper details the working principle, design and development of the actuator, stiffness and dynamic modeling, as well as a gain-scheduled PID controller used for accurate, stiffness-adaptive joint position control.

Comparison of 2 shear wave elastography systems in reproducibility and accuracy using an elasticity phantom

This study aimed to compare the accuracy and inter- and intra-observer reproducibility of the measured elasticity between 2 shear wave elastography systems. Three breast radiologists examined 8 targets of 4 different levels of stiffness (size: 11 mm, 4 mm) in an elasticity phantom (Customized 049A Elasticity QA Phantom, CIRS, Norfolk, VA, USA) using 2 different shear wave ultrasound elastography systems: SuperSonic Imagine (SSI) (SSI, Aix en Provence, France) and ShearScan (RS-80A, Samsung Medison, Seoul, Korea). Three radiologists performed ultrasound (US) elastography examinations for the phantom lesions using 2 equipment over a 1-week interval. Intra- and inter-observer reproducibility and the accuracy of the measured elasticity were analyzed and compared between the 2 systems. The accuracy of shape was also analyzed by shape-matching between B-mode and elastography color image. Intra-class correlation coefficients (ICC) were used in statistical analysis. For measured elasticity, the intra-observer and inter-observer reproducibility were excellent in both SSI and ShearScan (0.994 and 0.998). The overall accuracy was excellent in both systems, but the accuracy in small lesions (4 mm target) was lower in SSI than ShearScan (0.780 vs 0.967). The accuracy of shape-matching on the elastography image was 59.0% and 81.4% in the SSI and ShearScan, respectively. In conclusion, the SSI and ShearScan showed excellent intra- and inter-observer reproducibility. The accuracy of the Young's modulus was high in both the SSI and ShearScan, but the SSI showed decreased accuracy in measurement of elasticity in small targets and poor shape-matching between the B-mode image and color-coded elastography image.

Linearly implicit GARK schemes

Systems driven by multiple physical processes are central to many areas of science and engineering. Time discretization of multiphysics systems is challenging, since different processes have different levels of stiffness and characteristic time scales. The multimethod approach discretizes each physical process with an appropriate numerical method; the methods are coupled appropriately such that the overall solution has the desired accuracy and stability properties. The authors developed the general-structure additive Runge–Kutta (GARK) framework, which constructs multimethods based on Runge–Kutta schemes.This paper constructs the new GARK-ROS/GARK-ROW families of multimethods based on linearly implicit Rosenbrock/Rosenbrock-W schemes. For ordinary differential equation models, we develop a general order condition theory for linearly implicit methods with any number of partitions, using exact or approximate Jacobians. We generalize the order condition theory to two-way partitioned index-1 differential-algebraic equations. Applications of the framework include decoupled linearly implicit, linearly implicit/explicit, and linearly implicit/implicit methods. Practical GARK-ROS and GARK-ROW schemes of order up to four are constructed.

Photoelastic observation of toughness-dominant hydraulic fracture propagation across an orthogonal discontinuity in soft, viscoelastic layered formations

Hydraulic fracture (HF) unavoidably encounters natural pre-existing discontinuities in geologic formations, which complicates the propagation and containment behavior across a discontinuity. This study explored the HF propagation across an orthogonal discontinuity in layered formations by exploiting the photoelastic, transparent, soft (or deformable), and viscoelastic characteristics of gelatin. First, photoelastic HF experiments in homogeneous gelatin media were carried out while monitoring the fluid pressure and stress intensity factor (SIF). The SIF was observed to stay constant during steady-state HF propagation. Second, photoelastic HF experiments in layered gelatin media were conducted, in which biwing-type fractures encountered the bounding layers with different levels of stiffness. Two contrasting containment behaviors — crossing or dilation/arrested — were observed. The fracture crossed the discontinuity when it encountered the bounding layer with a lower toughness. By contrast, the fracture was arrested by the bounding layer and/or dilated the discontinuity, propagating along the discontinuity when the bounding layer had a greater toughness. In addition, competition between debonding of the layer interface and creating a fresh fracture in the bounding layer was found to play a significant role in fracture containment behavior. This study provides unique experimental data with photoelastic images that are comparable to analytical or numerical models. Furthermore, the results contribute to a better understanding of HF propagation and containment behaviors across a discontinuity in viscoelastic media.

CoboSkin: Soft Robot Skin With Variable Stiffness for Safer Human–Robot Collaboration

Conventional industrial robots are unable to guarantee the inherent safety when working together with humans due to the use of rigid components and the lack of force sensation. To enhance the safety of human-robot collaboration (HRC), the new collaborative robot skin (CoboSkin) with the features of softness, variable stiffness, and sensitivity is designed and studied in this article. The CoboSkin is composed of an array of inflatable units and sensing units. The sensing units made of soft porous materials are capable of measuring distributed contact force in a real-time manner. By leveraging the foaming process, the sensing units are interconnected with inflatable units fabricated by the elastomer of which the deformation is limited by the textile wrapped around it. Variation of stiffness is enabled by adjusting the internal air pressure supplied to inflatable units, thereby changing the sensitivity of the sensing units and reducing the peak impact force. Soft porous materials endowed the CoboSkin with increased sensitivity, minimal hysteresis, excellent cycling stability, and response time in the millisecond range, which enabled sensing feedback for controlling a robot arm at different levels of stiffness. Finally, the validation of the CoboSkin for safer HRC was conducted with a robot arm to detect an unintended collision, illustrating its potential application in robotics.

Design, Modeling, Additive Manufacturing, and Polishing of Stiffness-Modulated Porous Nitinol Bone Fixation Plates Followed by Thermomechanical and Composition Analysis

The use of titanium bone fixation plates is considered the standard of care for skeletal reconstructive surgery. Highly stiff titanium bone fixation plates provide immobilization immediately after the surgery. However, after the bone healing stage, they may cause stress shielding and lead to bone resorption and failure of the surgery. Stiffness-modulated or stiffness-matched Nitinol bone fixation plates that are fabricated via additive manufacturing (AM) have been recently introduced by our group as a long-lasting solution for minimizing the stress shielding and the follow-on bone resorption. Up to this point, we have modeled the performance of Nitinol bone fixation plates in mandibular reconstruction surgery and investigated the possibility of fabricating these implants. In this study, for the first time the realistic design of stiffness-modulated Nitinol bone fixation plates is presented. Plates with different levels of stiffness were fabricated, mechanically tested, and used for verifying the design approach. Followed by the design verification, to achieve superelastic bone fixation plates we proposed the use of Ni-rich Nitinol powder for the AM process and updated the models based on that. Superelastic Nitinol bone fixation plates with the extreme level of porosity were fabricated, and a chemical polishing procedure used to remove the un-melted powder was developed using SEM analysis. Thermomechanical evaluation of the polished bone fixation plates verified the desired superelasticity based on finite element (FE) simulations, and the chemical analysis showed good agreement with the ASTM standard.