Design, Modelling and Sensing Possibilities of Magneto-Rheological Based Devices

This thesis has been put in place during the development of an innovative medical device which consists in an intelligent footwear for foot plantar pressure redistribution in diabetic patients. In fact, despite the several sophisticated techniques developed in the last twenty years, diabetes remains one of the first causes of non-traumatic lower limb amputation worldwide. This is mainly due to the combination of peripheral neuropathy, which determines the loss of pain sensation in the lower extremities, and high plantar pressures, both recurrent among diabetic patients. The target application imposes severe constraints for what concerns the system requirements because of the high plantar pressure magnitude and dynamics achieved by diabetic people during walking. Furthermore, the need to maintain the offloading system portable requires at the same time a high level of miniaturisation and a reduced power consumption. Within a so challenging scenario, a regulating principle relying on Magneto-Rheological (MR) fluids, may represent a good solution. In fact, MR-based systems offer as main and common advantages high sustainable loads, high dynamic ranges of operation, low complexity, high reliability and low power consumption. MR fluids are a particular group of smart materials whose rheological properties (mainly the fluid internal yield stress which in turn determines the apparent viscosity of the fluid itself) can be controlled by and external magnetic field. With increasing levels of exciting field higher values of viscosity can be obtained, with the consequent possibility to control the material transition from the liquid to the semi-solid state. The research work presented in this thesis focuses on MR valves, the core element of the offloading system conceived. Nevertheless, the analysis has been conducted in order to be as broad as possible and most of the concepts presented can be extended to all MR-based devices. The development of an enhanced magnetic equivalent circuit to take into account relevant fringing and leakage phenomena is firstly addressed. High accuracy, flexibility and computational efficiency characterise the proposed approach which can be generalised to any axisymmetric structure. Analytical models are developed to describe three MR valves configurations and the analysis steps followed can be used as guidelines to define a design methodology. A dimensioning routine is implemented to shape the valves structures in order to fulfil some imposed design requirements and/or compare the different valves performances. A qualitatively consistent attempt for the dynamic modelling of MR valves is presented through considerations on energy exchanges between the different physical domains involved. This analysis underlined that MR-based systems behave like transducers and their sensing possibilities are demonstrated experimentally. Finally, all the contents addressed contribute to the conception and realisation of a miniature MR soft shock absorber, the basic constitutive element of the variable stiffness sole conceived. The research activities and the related results presented in this thesis do not pretend to definitely clarify and fix all points still open to question. The aim of this work is rather to provide some further elements and concepts to improve the design and modelling of MR- based devices

Mortality and pulmonary complications in patients undergoing surgery with perioperative SARS-CoV-2 infection: an international cohort study

Background: The impact of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) on postoperative recovery needs to be understood to inform clinical decision making during and after the COVID-19 pandemic. This study reports 30-day mortality and pulmonary complication rates in patients with perioperative SARS-CoV-2 infection. Methods: This international, multicentre, cohort study at 235 hospitals in 24 countries included all patients undergoing surgery who had SARS-CoV-2 infection confirmed within 7 days before or 30 days after surgery. The primary outcome measure was 30-day postoperative mortality and was assessed in all enrolled patients. The main secondary outcome measure was pulmonary complications, defined as pneumonia, acute respiratory distress syndrome, or unexpected postoperative ventilation. Findings: This analysis includes 1128 patients who had surgery between Jan 1 and March 31, 2020, of whom 835 (74·0%) had emergency surgery and 280 (24·8%) had elective surgery. SARS-CoV-2 infection was confirmed preoperatively in 294 (26·1%) patients. 30-day mortality was 23·8% (268 of 1128). Pulmonary complications occurred in 577 (51·2%) of 1128 patients; 30-day mortality in these patients was 38·0% (219 of 577), accounting for 81·7% (219 of 268) of all deaths. In adjusted analyses, 30-day mortality was associated with male sex (odds ratio 1·75 [95% CI 1·28–2·40], p\textless0·0001), age 70 years or older versus younger than 70 years (2·30 [1·65–3·22], p\textless0·0001), American Society of Anesthesiologists grades 3–5 versus grades 1–2 (2·35 [1·57–3·53], p\textless0·0001), malignant versus benign or obstetric diagnosis (1·55 [1·01–2·39], p=0·046), emergency versus elective surgery (1·67 [1·06–2·63], p=0·026), and major versus minor surgery (1·52 [1·01–2·31], p=0·047). Interpretation: Postoperative pulmonary complications occur in half of patients with perioperative SARS-CoV-2 infection and are associated with high mortality. Thresholds for surgery during the COVID-19 pandemic should be higher than during normal practice, particularly in men aged 70 years and older. Consideration should be given for postponing non-urgent procedures and promoting non-operative treatment to delay or avoid the need for surgery. Funding: National Institute for Health Research (NIHR), Association of Coloproctology of Great Britain and Ireland, Bowel and Cancer Research, Bowel Disease Research Foundation, Association of Upper Gastrointestinal Surgeons, British Association of Surgical Oncology, British Gynaecological Cancer Society, European Society of Coloproctology, NIHR Academy, Sarcoma UK, Vascular Society for Great Britain and Ireland, and Yorkshire Cancer Research

Experimental Study of Non-Ideal Phenomena Affecting Magneto-Rheological Elastomers Piezoresistivity

Magneto-Rheological Elastomers (MREs) represent emerging composite materials consisting of small magnetic particles dispersed in a highly elastic polymeric matrix. Particles interactions with external fields (magnetic and electric) and external stresses result in a variation of rheological and physical properties of the material. In particular, MRE samples exhibit piezoresistivity, i.e. a change in the intrinsic material resistivity if subjected to an external stress. While literature reports experimental sessions aimed to establish the MREs piezoresistive characteristics and sensing capabilities, tests assessing the presences of hysteresis and cyclic drifts for multiple loading/unloading cycles of the MREs are not diffused. Nevertheless, these information are crucial to establish the quality and reliability of a sensing system. The presented work addresses the investigation of such parasitic phenomena for different MRE samples in order to assess their existence and relevance, to provide a more detailed and comprehensive description of the MRE piezoresistive effect as well as to enlighten further important elements useful to determine the possibility of using such materials for the realization of force or pressure sensors

Bingham-Papanastasiou and Approximate Parallel Models Comparison for the Design of Magneto-Rheological Valves

Magneto-Rheological Fluids (MRFs) are smart materials whose physical properties can be controlled by an exciting magnetic field. MRFs are described as Bingham plastics with variable magnetic field dependent yield stress. Thanks to their particular features, MRFs have been largely employed to realize controllable power dissipating devices and, among them, regulable valves without moving parts. The most commonly configuration used for MRF based valves consists on fluid flow through an annular duct. The conception of such valves implies to deal with different physics. In particular, the magnetic circuit is usually designed and verified by mean of FE (Finite Element) analysis, while the duct geometry is usually dimensioned using an approximated formula based on fluid flow between parallel plates. In the presented work, a complete and detailed derivation of the analytical model is discussed in order to describe the flow of MRFs through an annulus using an approximate parallel plate geometry. Successively, the Bingham-Papanastasiou regularization is chosen as the mean to accurately describe the continuous non-linear yield stress and shear dependent viscosity of a commercially available MRF and it is then implemented into a FE software. This step allows to built a complete multiphysics problem for the design of MRFs based devices. Results obtained from the analytical model and FE analysis are then compared and the different steps in the proposed approaches are validated

Large angle flexure pivot development for future science payloads for space applications

An innovative design of a Large Angle Flexure Pivot (LAFP) is described. It combines the advantages of flexure mechanisms while surpassing one of their few flaws, small displacement strokes. The LAFP design exceeds these angular limitations to reach a deflection of 180° (±90°). The centre shifts laterally by less than ±35 μm throughout the full rotation range. The LAFP is meant to be mounted in pairs, coaxially and with the payload between them. The intended application of the LAFP is to angularly guide an optical component in a space environment for future science missions operating in a cryogenic environment. A dedicated performance test bench was developed and manufactured to test the pivot characteristics notably the lateral shift using Eddy current sensors. The test bench incorporates a representative dummy payload for mass and inertia. Extensive FEM analysis has been performed to validate the design at component level and further analysis with the pivots mounted with a representative payload on a test bench for random vibration, shock and thermal cycling environment. The second test bench for the vibration and shock tests has been manufactured incorporating a simplified launch locking device. The performance tests have confirmed a lateral shift of less than ±35 μm over an angular range of ±90°. The pivots have been successfully tested and survived vibration loads for high level sine at 24 g and random vibration at 12 grms in all three directions