Effect of Topological Defects on the Elasticity of Near-Ideal Polymer Networks

In recent years, new types of polymer gels have emerged, which have a well-controlled network structure and few topological defects. These so-called near-ideal polymer networks constitute a good model system to revisit the long-standing problem of structure–property relationships in polymer networks, as well as a promising platform for the development of polymer gels with outstanding mechanical properties. In this study, we investigate the relative contributions of network defects (dangling chains and second-order loops) on the stress–stretch response of near-ideal polymer networks using a computational discrete network model. We identify the average chain prestretch as a key parameter to capture the effect of network topology on the elastic modulus and maximum extensibility. Proper account of the chain prestretch further leads to scaling relations for the elastic properties in terms of topology parameters that differ from classical estimates of rubber elasticity theory. Stress–stretch curves calculated using the discrete network model are also compared to semi-analytical estimates.

Time course of changes in the long-latency feedback response parallels the fast process of short-term motor adaptation

We investigated whether changes in the feedback stretch response were related to the proposed fast and slow processes of motor adaptation. We found that the long-latency component of the feedback stretch response was upregulated in the early stages of learning and the time course was correlated with the fast process. While some propose that the fast process reflects an explicit strategy, we argue instead that it may be a proxy for the feedback controller.

Time course of changes in the long latency feedback response parallels the fast process of short term motor adaptation

Adapting to novel dynamics involves modifying both feedforward and feedback control. We investigated whether the motor system alters feedback responses during adaptation to a novel force field in a manner similar to adjustments in feedforward control. We simultaneously tracked the time course of both feedforward and feedback systems via independent probes during a force field adaptation task. Participants (n=35) grasped the handle of a robotic manipulandum and performed reaches to a visual target while the hand and arm were occluded. We introduced an abrupt counter-clockwise velocity-dependent force field during a block of reaching trials. We measured movement kinematics and shoulder and elbow muscle activity with surface EMG electrodes. We tracked the feedback stretch response throughout the task. Using force channel trials we measured overall learning, which was later decomposed into a fast and slow process. We found that the long-latency feedback response (LLFR) was upregulated in the early stages of learning and was correlated with the fast component of feedforward adaptation. The change in feedback response was specific to the long-latency epoch (50-100 ms after muscle stretch) and was observed only in the triceps muscle, which was the muscle required to counter the force field during adaptation. The similarity in time course for the LLFR and the estimated time course of the fast process suggests both are supported by common neural circuits. While some propose that the fast process reflects an explicit strategy, we argue instead that it may be a proxy for the feedback controller.New & NoteworthyWe investigated whether changes in the feedback stretch response were related to the proposed fast and slow processes of motor adaptation. We found that the long latency component of the feedback stretch response was upregulated in the early stages of learning, and the time course was correlated with the fast process. While some propose that the fast process reflects an explicit strategy, we argue instead that it may be a proxy for the feedback controller.

Dimethicone Crosspolymer Effect on Bio Pad Material

Bio pad wound dressing is one of the current material in wound healing technology. This aim of this paper is to study the effects of dimethicone cross polymer on the biomaterial and to investigate the mechanical properties of the bio pad by the integration of experimental and numerical approach. In vitro uniaxial tensile test was performed to compute the stress-stretch response of the materials using ASTM D412 standard. The determination of material constants for the materials via numerical approach can be done by comparing with two hyper elastic constitutive models (Ogden and Neo-Hookean). The results show that Ogden’s exponent and coefficient for the subject estimated to be (μ = 0.434 MPa, α = 1.299) for Sample 1, (μ = 0.428 MPa, α = 1.424) for Sample 2, (μ = 0.463 MPa, α = 1.256) for Sample 3 and (μ = 0.633 MPa, α = 1.001) for Sample 4 respectively. Meanwhile, value of material constants for Neo-Hookeen were estimated to be (C1 = 0.00814 MPa), (C2 = 0.0121 MPa), (C3 = 0.00597 MPa) and (C4 = 0.00739 MPa) for Sample 1, Sample 2, Sample 3 and Sample 4 respectively. Therefore, this study could be useful in future studies in analysis of healing especially in dermatology area.

Spring-Dashpot Simulations of Polyester Ropes: Validation of the Syrope Model

The way polyester ropes change their length in response to tension is often represented using a static-dynamic model with parameters for dual linear stiffness, based on testing according to defined procedures. This may not be sufficient for the mooring analysis since the stretch response to tension of fiber ropes is nonlinear. In the Syrope Joint Industry Project, a model to be used in mooring analyses was developed together with a test procedure to obtain the parameters for this model. Based on these parameters and experience from the testing, an equivalent non-linear spring-dashpot model was developed. This model can predict the response in the rope also for other test procedures than those used for establishing the parameters in the analysis model. This demonstrates the validity of this Syrope analysis model.

The influence of kinesthetic motor imagery and effector specificity on the long-latency stretch response

The long-latency “reflexive” response (LLR) following an upper-limb mechanical disturbance is generated by neural circuitry shared with voluntary control. This feedback response supports many task-dependent behaviours and permits the expression of goal-directed corrections at latencies shorter than voluntary reaction time. An extensive body of literature has demonstrated that the LLR shows flexibility akin to voluntary control, but it has never been tested whether instruction-dependent LLR changes can also occur in the absence of an overt voluntary response. The present study used kinesthetic motor imagery (Experiment 1) and instructed participants to execute a voluntary response in a non-stretched contralateral muscle (Experiment 2) to explore the relationship between the overt production of a voluntary response and LLR facilitation. Activity in stretched right wrist flexors were compared to standard “not-intervene” and “compensate” conditions. Our findings revealed that on ~40% of imagery and ~50% of contralateral trials, a partial voluntary response “leaked-out” into the stretched right wrist flexor muscle. On these “leaked” trials, the early portion of the LLR (R2) was facilitated and displayed a similar increase to compensate trials. The latter half of the LLR (R3) showed further modulation, mirroring the patterns of voluntary response activity. By contrast, the LLR on “non-leaked” imagery and contralateral trials did not modulate. We suggest that even though a hastened voluntary response cannot account for all instruction-dependent LLR modulation, the overt execution of a voluntary response in the same muscle(s) as the LLR is a pre-requisite for facilitation of this rapid feedback response.New and NoteworthyWe examined volitional modulation of the long-latency stretch response (LLR) using two novel approaches: motor imagery and the execution of contralateral movements. The LLR was only facilitated on imagery or contralateral trials when a voluntary response “leaked-out” into stretched muscle suggesting that a voluntary response in the same muscle as the LLR is a prerequisite for facilitation. Our findings also demonstrate an important distinction between the early (R2) and late (R3) portions of the LLR.

The Syrope Method for Stiffness Testing of Polyester Ropes

The load-elongation properties of polyester ropes are often modelled through a static-dynamic model with parameters based on different proposed test procedures, which may not be sufficient for the mooring analysis since the stretch response to tension of fiber ropes is nonlinear. In the Syrope Joint Industry Project, a new procedure for tension vs. stretch testing of polyester ropes for mooring applications was developed. The test allows for determining parameters in a rope model, which was developed in the JIP, where the length and dynamic stiffness of the rope are given by the actual mean tension and the previous highest mean tension in each mooring line. Test results from this new procedure have been compared with results from the test procedures provided in API, ABS and ISO documents. All tests were performed on samples from the same sub-rope. Differences and similarities between the different test procedures and analysis methods have been investigated.

Coordinating long-latency stretch responses across the shoulder, elbow, and wrist during goal-directed reaching

The long-latency stretch response (muscle activity 50–100 ms after a mechanical perturbation) can be coordinated across multiple joints to support goal-directed actions. Here we assessed the flexibility of such coordination and whether it serves to counteract intersegmental dynamics and exploit kinematic redundancy. In three experiments, participants made planar reaches to visual targets after elbow perturbations and we assessed the coordination of long-latency stretch responses across shoulder, elbow, and wrist muscles. Importantly, targets were placed such that elbow and wrist (but not shoulder) rotations could help transport the hand to the target—a simple form of kinematic redundancy. In experiment 1 we applied perturbations of different magnitudes to the elbow and found that long-latency stretch responses in shoulder, elbow, and wrist muscles scaled with perturbation magnitude. In experiment 2 we examined the trial-by-trial relationship between long-latency stretch responses at adjacent joints and found that the magnitudes of the responses in shoulder and elbow muscles, as well as elbow and wrist muscles, were positively correlated. In experiment 3 we explicitly instructed participants how to use their wrist to move their hand to the target after the perturbation. We found that long-latency stretch responses in wrist muscles were not sensitive to our instructions, despite the fact that participants incorporated these instructions into their voluntary behavior. Taken together, our results indicate that, during reaching, the coordination of long-latency stretch responses across multiple joints counteracts intersegmental dynamics but may not be able to exploit kinematic redundancy.