Shape Memory Materials for Biomedical Applications

Shape memory materials represent a promising class of dual-shape materials that can move from one shape to another in response to a stimulus such as light, heat, electricity or magnetism. In this regard, the biomedical field is showing large interest in this class of materials, especially in shape memory polymers (SMPs), whose mechanical properties make them extremely attractive for many biomedical applications. However, diverse characteristics including also the mechanical behaviour are still part of research. In this contribution the shape memory properties of polymers will be quantified by cyclic thermomechanical investigations. One cycle includes the "programming" of the sample and the recovery of its permanent shape. To describe this phenomenon, a three-dimensional thermomechanical coupled model is proposed. This macromechanical constitutive model is based on the physical understanding of the material behaviour and a mechanical interpretation of the stress-strain-temperature changes observed during thermomechanical loading. The main focus of this work is the influence of both, the material constants and heat transfer boundary conditions on the response of shape memory polymers. Therefore we illustrate different general simulations as well as examples of application.

Shape memory polymers (SMP) are a class of stimuli-responsive materials that are able to respond to external stimulus such as heat by altering their shape. Bio-compatible SMPs have a number of advantages over static materials and are being studied extensively for biomedical and clinical applications (such as tissue stents and scaffolds). A previous study has demonstrated that the bio-compatible polymer blend of polylactic acid (PLA)/ thermoplastic polyurethane (TPU) (50/50 and 70/30) exhibit good shape memory properties. In this study, the mechanical and thermo-mechanical (shape memory) properties of TPU/PLA SMP blends were characterized; the compositions studied were 80/20, 65/35, and 50/50 TPU/PLA. In addition, porous TPU/PLA SMP blends were fabricated with a gas-foaming technique; and the morphology of the porous structure of these SMPs foams were characterized with scanning electron microscopy (SEM). The TPU/PLA bio-compatible SMP blend was fabricated with melt-blending and compression molding. The glass transition temperature (Tg) of the SMP blends was determined with a differential scanning calorimeter (DSC). The mechanical properties studied were the stress-strain behavior, tensile strength, and elastic modulus; and the thermomechanical (or shape memory) properties studied were the shape fixity rate (Rf), shape recovery rate (Rr), response time, and the effect of recovery temperature on Rr. The porous 80/20 PLA/TPU SMP blend was found to have the highest tensile strength, toughness and percentage extension, as well as the lowest density and uniform pore structure in the micron and submicron scale. The porous 80/20 TPU/PLA SMP blend may be further developed for specific biomedical and clinical applications where a combination of tensile strength, toughness, and low density are required.

Shape memory polymer (SMP) systems exhibiting semicrystalline- elastomer blends, such as thermoplastic polyurethane and polylactic acid have been well studied, but their use in biomedical shape memory applications has been limited by their high activation temperature. SMPs are capable of deformation and recovery through the activation of an external stimuli, such as temperature. Critical criteria for SMPs used in biomedical applications is achieving a stimulus temperature close to 37°C while still experiencing sufficient shape recovery. A polymer’s glass transition temperature has been well defined as the SMP system’s activation temperature and therefore should be decreased to achieve a decreased activation temperature. In this work, a well-known, biocompatible plasticizer, polyethylene oxide (PEO), was added to thermoplastic polyurethane (TPU) – polylactic acid (PLA) SMP blends to observe the plasticizing effect on the structural, thermal, mechanical, and shape memory properties of the polymer blends. Additionally, the geometry of the fabricated SMP samples was tailored to further enhance the shape memory effect through a bowtie honeycomb structure. Our results suggest that the addition of PEO into the SMP system may be an effective method for decreasing the polymer’s glass transition temperature through the alteration of the polymer chain structure. With the addition of 30% PEO, the glass transition temperature of the TPU/PLA blend was successfully decreased from 62.4°C to 34.6°C while achieving 86.5% shape recovery when activated at 37°C, which is only a 5% decrease in shape recovery when activated at 50°C. These results suggest that the addition of a biocompatible plasticizer may overcome the limitation of employing temperature activated SMP systems in biomedical applications, and enhances the potential of these materials for reconfigurable structures, energy dissipation systems, and SHM in civil engineering applications.

Shape memory polymers (SMPs) possess the remarkable ability to recover their original shape after being severely deformed, triggered by the right stimulus. This chapter reviews recent progress in shape memory materials, focusing on traditional types like shape memory alloys (SMAs) and SMPs, as well as emerging shape memory hybrids, which enable customizable properties for specific applications in a “do-it-yourself” manner. SMPs, in particular, show great promise for morphing aircraft applications due to their temperature- and time-dependent mechanical properties. Tests on SMPs under monotonic loading in high shear demonstrate their ability to recover original shapes while resisting large loads. Advances in SMP composites (SMPCs), which incorporate functional materials and nanostructures, further enhance their deformation recovery, mechanical properties, and controllable actuation. SMPs have broad applications across aerospace, biomedical devices, flexible electronics, and more. This chapter also provides an in-depth analysis of the shape recovery mechanisms, multifunctionality, and potential future directions for SMP development.

Chapter 14 reviews shape memory polymers (SMPs). Shape memory polymers belong to the family of shape memory materials (SMMs), which can be deformed into a predetermined shape under some imposed specific conditions such as temperature, electric or magnetic field, as well as strain and stress. These shapes can be relaxed back to their original field-free shapes under thermal, electrical, magnetic, strain, stress, temperature, laser, or environmental stimuli. These transformations are essentially due to the elastic energy stored in SMMs during initial deformation. As a member of SMMs, SMPs are stimuli-sensitive polymers. Shape memory polymers normally use either heat or laser light energy as a stimulant to change shape. The thermally-induced shape memory effect can be observed by irradiation with infrared light, exposure to alternating magnetic fields, application of an electric field or immersion in water.

Shape-memory materials (SMMs) are characterized by their unique ability to remember and recover their shape in response to external stimuli. Over recent decades, the use of SMMs in biomedical areas such as tissue engineering, drug delivery, endovascular surgery, orthodontics, orthopedics, etc. has attracted significant attention from both academia and industry. Recently, additive manufacturing (AM) has also attracted growing interest for biomedical applications because of its ability to produce on-demand, patient-tailored devices for medical treatments. This article provides a review of current state-of-the-art AM techniques for producing SMMs for biomedical applications. First, both shape-memory alloys and shape-memory polymers are discussed regarding their fundamental characteristics and compositions, and the general principles governing their shape-memory effects. Next, current and potential biomedical applications of SMMs are presented, then available AM techniques that have been used for the fabrication of SMM-based medical devices are discussed and explored. Finally, an outlook on AM of SMMs for biomedical applications is provided.

In this study, we developed a variable-stiffness and deformable link using shape-memory material and the jamming transition phenomenon. Above its glass transition temperature (Tg), a shape-memory polymer (SMP) can be deformed by applying a small load. SMPs maintain the deformed shape after they have been cooled below Tg, and they return to their original shape when heated above Tg. The reversible change in the elastic modulus between the glassy and rubbery states of SMPs can be on the order of 100–1000 times. We exploited the characteristics of SMPs to develop robot components with variable stiffness and sensitivity. The jamming transition phenomenon for granular material has been widely used as a method to change the stiffness of robots. This phenomenon is the change from fluid-like to solid-like conditions by removing air from a space containing particles. In this study, we developed a variable-stiffness link by combining the SMP and the jamming transition phenomenon. Moreover, by replacing the SMP with shape-memory alloys (SMAs), whose recovery force and elastic modulus are larger than those of SMPs, we prepared a second prototype with variable stiffness. We evaluated the performance of both prototypes, using the SMP or the SMA, with experiments and confirmed the motion principle of the proposed link (e.g., shape recovery and shape fixity). Moreover, it was confirmed that the stiffness of these links can be changed among four states.

This chapter investigates the tribological behavior of shape memory material (SMM) thin films, a critical aspect of their successful implementation in diverse applications such as microelectromechanical systems (MEMS), biomedical devices, and smart coatings. SMMs, encompassing shape memory alloys (SMAs) like NiTi and shape memory polymers (SMPs), exhibit unique shape recovery capabilities, enabling innovative functionalities. The chapter begins by reviewing common fabrication techniques for SMM thin films, including PVD, CVD, sol-gel methods, electrospinning, and spin coating. Subsequently, it discusses key tribological testing and characterization methods, encompassing friction and wear tests (e.g., pin-on-disk, ball-on-disk, and scratch tests) and characterization techniques. The core of the chapter delves into the tribological properties of SMM films, analyzing factors influencing friction (e.g., surface roughness, and phase transformations) and wear (e.g., hardness, and elastic recovery). Strategies for mitigating wear and friction, such as solid lubrication, self-lubricating coatings, and surface texturing, are explored. This chapter includes the critical role of wear resistance in MEMS devices, the importance of biocompatibility and minimal wear debris in biomedical implants, and the potential of SMM-based smart coatings for adaptive friction control in various industrial settings.

ABSTRACTIn the field of smart materials, an increasing demand is being observed for shape memory materials that exhibit advanced stimuli‐sensing and responsive capabilities. Two‐way shape memory polymers (2 W‐SMPs), as a novel type of smart shape memory material, offer numerous advantages over traditional one‐way shape memory polymers (1 W‐SMPs), particularly the fully reversible shape memory effect (SME), where the two‐way process (low‐temperature shape fixation and high‐temperature recovery to the original shape) occurs without the need for external energy input. These polymers are lightweight and exhibit remarkable deformation abilities, reversible changes in modulus, and multiple actuation modes. Additionally, they feature a high elastic modulus and increased response stress, making them particularly suitable and indispensable in fields ranging from medical devices and robotics to aerospace engineering and smart textiles. This review focuses on 2 W‐SMPs, detailing the underlying mechanisms of both 1 W‐SMPs and 2 W‐SMPs, as well as the various driving modes. We further discuss quasi‐2 W‐SMPs (with applied load force σ ≠ 0) and 2 W‐SMPs (with applied load force σ = 0) across different cross‐linking strategies. It also covers the broad applications of SMP materials and outlines the current challenges and potential avenues for future research.

Four-dimensional (4D) printing is regarded as a methodology that links 3D printing to time, which is characterized by the evolution of predetermined structures or functions for the printed object after applying stimulation. This dynamic feature endows 4D printing the potential to be intelligent, attracting wide attention from academia and industry. The transformation of shape and function is both obtained from the programming of the object endowed by the intrinsic characteristics of the material or by the manufacturing technology. Therefore, it is necessary to understand 4D printing from the perspective of both mechanism and manufacturing. Here, the state-of-the-art 4D printing polymer was summarized, beginning with the classifications, and leading to the mechanisms, stimulations, and technologies. The links and differences between 4D printing polymer and shape memory polymer, between 4D printing and 3D printing were highlighted. Finally, the biomedical applications were outlined and the perspectives were discussed.

[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI--COLUMBIA AT REQUEST OF AUTHOR.] How can research on mechanical engineering and materials science contribute to human health? The fabrication of biomedical scaffolds could be a good entry point. Scaffolds are broadly applied in biomedical fields with multiple functions, such as repair, replacement, and stimulation and monitoring when they are integrated with electronic/optoelectronic devices. Besides biocompatible, the scaffolds should be soft and in form of three-dimensional (3D) structures in order to mechanically and geometrically match the natural tissues and organs. Polymers are the most promising candidate materials for the scaffold fabrication. Compared to metals and ceramics, substantial polymers have biocompatibility and all of them have low Young's modulus and high processability. Benefiting from the high processability, a variety of approaches can be used to shape polymeric scaffolds with 3D architectures. The major three approaches are flexibility, stress induced assembly, and printing. However, none of them is flawless: (1) For flexibility, the scaffolds that integrated with electronic devices have large thickness which exponentially lower the flexibility. (2) For stress-induced assembly, the assembly operation requires complicated actuation equipment and the assembled scaffolds are usually tethered on cumbersome elastomeric substrates. (3) For printing, few of scaffolds fabricated by emerging 4D printing technologies are responsive to biocompatible stimuli. This dissertation aims at addressing these three problems. First, a new device structure, i.e., lateral electrode, is proposed to reduce the thickness and then improve the flexibility of the scaffolds with electronics, which is validated by fabricating flexible photodetectors on polyimide substrates. The photodetectors have excellent flexibility and can be bent to 3D structures. Second, a new stress-induced assembly strategy, i.e., responsive buckling, is developed in which the elastomeric substrates are replaced with deft responsive polymeric substrates. Various 3D polymeric scaffolds either with or without electronic devices are assembled when the substrates are exposed to external stimuli without manual intervention. This strategy is first verified by an acetone responsive organogel and then developed toward biomedical applications by using a body temperature responsive hydrogel. Third, a new shape memory polymer, i.e., poly (glycerol dodecanoate) acrylate (PGDA), whose transition temperature is in the range of 20-37 [degrees]C, is exploited for 4D printing of scaffolds. Because of the propriate transition temperature, the shape memory process of the scaffolds can be completed by using room temperature and body temperature as stimuli, which are harmless for human body. Moreover, a variety of delicate 3D structures including an artery-like tube are printed.

Shape memory polymers (SMPs) have emerged as a revolutionary class of smart materials with enormous potential for applications in engineering and science. This manuscript provides a comprehensive review, examining their processing techniques, compositions, characteristics, and applications. It also looks at different shape memory polymer materials, blends, and composites in a critical way by looking at their features and improved mechanical, thermal, electrical, and other properties. It discusses all reinforcements, such as carbon nanotubes, graphene, and metal nanoparticles, used in earlier studies and their impact on various shape memory composites and their properties. Recent trends in research related to shape memory polymer composites (SMPCs) have focused on providing solutions to these problems by adding fillers or reinforcements and/or combining them with other polymers. Further, it provides major applications of SMPS in biomedical, aerospace, smart textiles, sensors, and self-healing materials. A detailed review of all these applications, along with their challenges also discussed thoroughly. This review helps in providing a bridge between existing knowledge and future directions of SMPs and composites involve developing multi-stimuli responsive materials, improving material properties using hybrid reinforcements, developing complex shape recovery mechanisms, including self-healing and functional futures in emerging areas like biomedical, soft robotics, and other applications.

The temperature memory effect (TME) has been reported as another interesting feature of many shape memory materials (SMMs). In this paper, we briefly review the recent progress in monitoring minor over-heating/cooling for random thermal fluctuation based on the TME via differential scanning calorimeter test. Two typical SMMs, namely shape memory alloy and shape memory polymer, are investigated. Two methods, namely the turning point method and enthalpy method are discussed together with their underlying mechanisms based on the experimental results. While the enthalpy method has the advantage of far more less number of pre-experiment, a systematical investigation is still required to confirm its potential.

Thermochemically responsive shape memory polymers (SMPs) can recover from a customized temporary shape to their permanent form in response to both thermal activation and solvent diffusion. This dual-driving characteristic effectively mitigates the potential risks associated with high driving temperatures in biomedical applications and enables SMPs to achieve the multi-shape memory effect (multi-SME). Heating induces complex solvent-polymer interactions that create structural disorder, posing significant challenges for controlling shape memory performance. In this study, we develop a nonequilibrium thermodynamic model that incorporates the concept of phase transition into the Avramov-Milchev framework to address this issue. Furthermore, we validate the efficacy of our proposed model by predicting the thermomechanical behavior and shape memory performance of tert-butyl acrylate (tBA) SMP and Nafion SMP, respectively. This study aims to develop a modeling framework for thermochemically driven SMPs to elucidate the mechanism of disorder-assisted glass transition and enable precise control of the multi-SME, thereby advancing their potential in biomedical applications.

Personalized 4D printing of bioinspired tracheal scaffold concept based on magnetic stimulated shape memory composites