Search

Shape memory polymers (SMPs) are an exciting class of stimuli-responsive smart materials that demonstrate reactive and reversible changes in mechanical property, usually by switching between different states due to external stimuli. We report on the development of a polyurethane-based SMP foam for effective pressure redistribution that demonstrates controllable changes in dynamic pressure redistribution capability at a low transition temperature (∼24 °C)-ideally suited to matching modulations in body contact pressure for dynamic pressure relief (e.g., for alleviation or pressure ulcer effects). The resultant SMP material has been extensively characterized by a series of tests including stress-strain testing, compression testing, dynamic mechanical analysis, optical microscopy, UV-visible absorbance spectroscopy, variable-temperature areal pressure distribution, Fourier transform infrared spectroscopy, Raman spectroscopy, X-ray diffraction, differential scanning calorimetry, dynamic thermogravimetric analysis, and 1H nuclear magnetic resonance spectroscopy. The foam system exhibits high responsivity when tested for plantar pressure modulation with significant potential in pressure ulcers treatment. Efficient pressure redistribution (∼80% reduction in interface pressure), high stress response (∼30% applied stress is stored in fixity and released on recovery), and excellent deformation recovery (∼100%) are demonstrated in addition to significant cycling ability without performance loss. By providing highly effective pressure redistribution and modulation when in contact with the body's surface, this SMP foam offers novel mechanisms for alleviating the risk of pressure ulcers. ispartof: ACS OMEGA vol:4 issue:13 pages:15348-15358 ispartof: location:United States status: published

Shape memory polymers are stimulus responsive smart materials that can be applied in several forms such as films, fibers, and foams for a wide range of applications. Novel stress-memory behavior at a fiber level is yet to be uncovered, which would be favorable to control stress in the broad horizon of smart materials for numerous functions. In this work, a semi-crystalline segmented polyurethane was synthesized to prepare filaments/fibres and films. A rational experimental design was established and the stress-memory behavior of both the films and filaments was systematically studied for comparison. Tensile stress-memory programming was performed at three strain levels (20%, 40%, and 60%) to record the memory stress response as a function of temperature with time. The characterization of the thermal and mechanical properties of the stress-memory programmed specimens has objectively proven the reason behind the higher stress response in the filaments than in the films. Melt spinning has induced perfect crystallization with ordered polymer packing and enabled maximum memory stress to be retrieved in the filaments. The evolution of memory stress follows a linear trend with an increase in strain and temperature (r2 = 0.91–1). In addition, pressure related studies were also carried out for smart filament integrative fabrics to realize stress-memory behavior. This unprecedented and novel approach of unveiling the memory behavior specifically at the filament level will enable material scientists to comprehend the fundamental aspects for precise optimization and control of memory stress in smart structures for applications such as compression stockings that require stimuli responsive force.

In current practice of compression therapy, there has been a challenge of compression controlling and need for multiple devices to achieve different functions. Herein, the potential of a thermal-sensitive memory polymer, namely, polyurethane (MPU), is explored in designing a prototype that acts as a flexible bandage system to provide dual benefits (heat and pressure) simultaneously for a desired compression. After preconditioning, the thermomechanical behavior of the MPU becomes repeatable and consistent, which can be programmed to remember (store) and retrieve reversibly a prescribed internal stress, responsive to an external stimulus around its transition temperature (Ttrans). The smart bandage system is then built using the MPU actuator which integrates electric resistive wires as both a heat source for thermal therapy and stimulus for trigger control. The compression can also be easily switched between static and dynamic (massage) modes using a pro-set thermal stimulus. This paper also addresses several challenging issues and design considerations for the prototype. Theoretical analyses have been applied to predict the device performance, showing excellent agreement with experimental results. This work sheds insights for broadening the applications of stress memory and for engineering smart products needing stimulus responsive forces such as dynamic cushions, pressure garment and electronic actuators. Keywords: Shape memory polymer, Stress memory, Polyurethane, Compression therapy, Massaging, Venous disorders, Pressure, Bandage

Similar to shape memory, the stress in a stimulus responsive polymer can also be programmed, stored, and retrieved reversibly upon an external stimulus, and known as stress memory. Herein, the stress analysis in a semicrystalline polyurethane is investigated to unveil the total stress–strain components of the memory polymer. The evolution of stress under different temperature and strain levels is determined experimentally. A constitutive model based on phase transition was further used to predict and characterize the individual stress components during the thermomechanical process. In contrast to earlier models, a new approach of using relaxed modulus (RM) has been proposed to predict the stress components in tensile programming condition. The predicted results are having significant agreement with the experimental data. The quantitative stress analysis can help in engineering the products more precisely, where the controllable stimulus responsive stress is needed in multidisciplinary arenas such as pressure garments, massage devices, and artificial muscles etc. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 941–947

Like shape memory polymers, a novel phenomenon of stress memory was shown in which the stress of a material can respond to an external stimulus. This concept was further enlightened by a switch-spring-frame model that would eliminate the limitation of existing models which overlooked the stimulus responsive nature of such polymers. The discovery being reported in this article was stemmed from a real case study into shape memory polymer fibers in compression stocking for varicose veins. The breakthrough of stress memory enabled researchers to develop applications needing stimuli-responsive forces, which can broaden the horizon of such smart polymers in emerging smart products in many multidisciplinary fields such as sensors, stress garments, and massage devices, electronic skins, and artificial muscles. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 893–898

Materials, structures and systems, responsive to an external stimulus are smart and adaptive to our human demands. Among smart materials, polymers with shape memory effect are at the forefront of research leading to comprehensive publications and wide applications. In this paper, we extend the concept of shape memory polymers to stress memory ones, which have been discovered recently. Like shape memory, stress memory represents a phenomenon where the stress in a polymer can be programmed, stored and retrieved reversibly with an external stimulus such as temperature and magnetic field. Stress memory may be mistaken as the recovery stress which was studied quite broadly. Our further investigation also reveals that stress memory is quite different from recovery stress containing multi-components including elastic and viscoelastic forces in addition to possible memory stress. Stress memory could be used into applications such as sensors, pressure garments, massage devices, electronic skins and artificial muscles. The current revelation of stress memory potentials is emanated from an authentic application of memory fibres, films, and foams in the smart compression devices for the management of chronic and therapeutic disorders.