Materials The material evolution
Self-healing polymers illustrate both the potential and the complexity of this new frontier. Designed to restore structural integrity after sustaining damage, they can extend the life of implants and wearable systems. Some achieve this through embedded reservoirs of reactive agents that polymerise when a crack forms; others rely on reversible chemical bonds that reform under specific conditions. In the lab, the effect is striking; microfractures vanish, restoring strength with minimal loss of performance. In the body, the implications are significant. Orthopaedic implants, cardiovascular components, or even soft robotic surgical tools could repair themselves, reducing the risk of sudden failure. But performance in a controlled environment is only half the equation. Before they reach a patient, all medical devices must be sterilised – often via high- pressure steam, radiation or chemical agents. These processes are harsh. Heat can break the very bonds that enable self-healing. Radiation can alter polymer microstructures, rendering them brittle. Chemical sterilant may leave residues that interfere with healing chemistry. While a handful of polymer systems have been engineered to survive such treatment, they remain exceptions. Until these materials can endure sterilisation without degradation, their use in long-term or load-bearing devices will remain limited. If self-healing polymers aim to make devices more resilient, shape-memory alloys focus on making them more adaptable. Nitinol (NiTi), an alloy of nickel and titanium, is the most widely known example. Its ability to change shape in response to temperature – or recover from large deformations without permanent damage – has transformed the design of stents, filters and surgical instruments. “The transformation temperature of NiTi can be controlled by the addition of third elements such as niobium, copper and iron,” Roy explains. “NiTi fatigue resistance can be controlled by changing internal material properties, the manufacturing processes used and external conditions.” Delivered in a compact form, these devices expand, or shift shape once deployed in the body, often with remarkable precision. This property is the result of a reversible transformation between two crystal phases, tuned so that the change occurs within the narrow range of human body temperature. Recent advances in additive manufacturing have broadened the design possibilities for NiTi devices. Techniques such as laser powder bed fusion now allow engineers to create complex, patient-specific geometries directly from digital models, reducing material waste and post-processing. As well as this, “internal properties can be changed by processing methods like precipitation and grain refinement, which can significantly improve fatigue life. External factors could involve thermomechanical
www.medicaldevice-developments.com Nitinol’s shape-memory superpower
Nitinol, a near-equiatomic alloy of nickel and titanium (NiTi), is the workhorse of shape-memory and superelastic materials in medical device engineering. Its ability to return to a pre-set shape upon encountering body temperature makes it invaluable for life-changing implant applications. A recent clinical review highlighted how nitinol’s shape-memory effect empowers self-expandable stents, which are crimped in the martensite phase (below transformation temperature) and, once deployed in the body, naturally expand and conform to vessel walls as they re-enter the austenite phase. “Nitinol is an equiatomic alloy of nickel and titanium. Its unique properties like ‘superelasticity’ and ‘shape memory’ have made it one of the most commonly used materials for manufacturing hardware in endovascular neurosurgery. The solid state of nitinol has two interconvertible (austenite and martensite) phases. With increasing temperature, the martensite phase gets transformed into the austenite phase (thermal phase transformation), and thus remembers the shape.” Source: ‘NiTinol: A Review of Its Smart Properties That Make It a Smart Alloy and a Strong Ally in Endovascular Neurosurgery’
treatment and cyclic loading conditions, which can also improve fatigue life,” Roy shares. However, these methods also introduce new challenges. Microstructural variations, inclusions and surface roughness can compromise fatigue resistance, an essential consideration for implants subjected to millions of loading cycles. Electropolishing and other finishing processes can restore surface quality but add time and cost. Sterilisation is a similarly delicate issue. Nitinol’s corrosion resistance and biocompatibility rely on a stable oxide layer. Certain sterilisation methods can disrupt this layer, increase nickel ion release and potentially trigger adverse reactions. Managing these effects without compromising device function is a balancing act, particularly when scaling production for global markets.
Stimuli-responsive hydrogels offer yet another approach to “enhance mechanical properties”, Roy explains. “Smart hydrogels can incorporate sensors for continuous monitoring of various physiological indicators and biomarkers, as well as having adaptive drug release.” Temperature-sensitive hydrogels might release a drug when exposed to a mild fever; pH-sensitive variants could deliver chemotherapy directly to the acidic microenvironment of a tumour. Some formulations are injectable, flowing into irregular anatomical spaces before solidifying into a functional form. Others serve as scaffolds for tissue engineering, supporting cell growth and gradually degrading as new tissue forms. Such versatility is unmatched by conventional materials, but so are the manufacturing constraints. Hydrogels’ high water content makes them vulnerable to drying, swelling or structural collapse during sterilisation. Steam can alter their network structure; radiation may trigger unwanted chemical reactions; and chemical sterilant can be hard to remove without damaging the gel. Emerging techniques such as supercritical carbon dioxide processing may preserve function more effectively, but they are not yet standard
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