Biomaterials
A stent can be inserted through a catheter at room temperature, and it will deploy when it reaches body temperature.
More recently, researchers and device developers have turned to shape-memory polymers, which can offer greater recoverable strain and tunability than alloys in some applications.
These smart materials include polylactide, polyurethane and polycaprolactone-based polymers. Their performance depends on the material and device design, including factors such as mechanical strength, response time and degradation. Researchers are investigating shape-memory polymers for applications ranging from stents and sutures to tissue-engineering scaffolds. Another medically relevant stimulus, particularly for implanted or ingested devices, is pH. Differences in pH between some tissues, body compartments or disease microenvironments may be used to activate pH-responsive materials, although these differences can be small, variable and heterogeneous. Depending on their design, pH-responsive materials can change properties such as swelling, conformation, solubility or drug-release behaviour. Many systems have limited selectivity when the relevant pH differences are small, while diseased tissues may exhibit only modest and heterogeneous changes. “There’s a lot of interest in making nanoparticles that are responsive to pH,” says Messersmith. For example, he adds, “you can administer a nanoparticle intravenously that finds its way to a tumour”. In practice, nanoparticle accumulation in a tumour depends on factors including the particle’s design, the tumour type and the delivery conditions. Alternatively, a pH-responsive device could track pH changes at a site. Experimental nanoparticle-loaded sutures, for instance, have been investigated for sensing pH at wound sites.
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Biomaterials can also be functionalised to respond to particular enzymes. Hydrogels, for instance, can be designed with enzyme-sensitive components and can undergo chemical or mechanical changes in response to enzyme activity. This approach has been investigated in hydrogel-based adhesives and drug- delivery systems designed to release therapeutic agents in response to disease-associated enzymes. Some externally worn medical devices detect stimuli such as movement, vibration or force and provide haptic feedback or assistance in response. Biomaterials are also engineered to respond to external stimuli such as light, radiofrequency energy or magnetic fields. This may allow physicians to control an ingested or implanted device from outside the body. However, the limited penetration depth of many wavelengths of light in biological tissue presents a challenge for light-responsive implants. In experimental systems, external magnetic fields can guide or actuate certain devices inside the body. Researchers have also investigated radio-frequency energy and magnetic fields to actuate devices or control drug release. “You can use an alternating magnetic field to change the shape or the phase of that material,” says Carmel Majidi, a professor of mechanical engineering at Carnegie Mellon University.
Stimuli-responsive medical devices Some responsive-material functions could have value across a range of medical applications. Biofilm control is one example. Biofilms can interfere with medical devices, particularly implants, and increase the risk of infection. Light, electrical and other stimuli-responsive antibacterial materials and coatings have shown antimicrobial effects in research settings. Whether these approaches translate into reliable infection prevention in clinical use remains to be established. Researchers are also investigating responsive biomaterials for diagnostic and imaging applications. Magnetic particle imaging, for instance, uses superparamagnetic iron oxide nanoparticles as tracers and images their spatial distribution in the body. Tumour imaging is among the applications being investigated.
The flexibility of soft materials is also opening up potential applications in minimally invasive surgery. Experimental soft robotic systems can use external stimuli to control the movement of devices delivered via a catheter or, in some cases, as ingestible devices. Light-responsive soft robots made from hydrogels or other polymers can bend or actuate in response to light. In contrast, soft grippers can be designed to grasp objects or tissues in laboratory and preclinical systems. Variable-stiffness catheter systems offer another approach. By changing stiffness through thermal, electrical or other mechanisms, a device can remain flexible while navigating through confined anatomy and become stiffer when greater force or positional
www.medicaldevice-developments.com
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