Shape-changing polymers for biomedical applications

Smart polymers that are capable of controlled shape transformations under external stimuli have attracted significant attention in the recent years due to the resemblance of this behavior to the biological intelligence observed in nature. In this review, we focus on the recent progress in the field of shape-morphing polymers, highlighting their most promising applications in the biomedical field.

Biodegradable Shape Memory Polymers in Medicine

Shape memory materials have emerged as an important class of materials in medicine due to their ability to change shape in response to a specific stimulus, enabling the simplification of medical procedures, use of minimally invasive techniques, and access to new treatment modalities. Shape memory polymers, in particular, are well suited for such applications given their excellent shape memory performance, tunable materials properties, minimal toxicity, and potential for biodegradation and resorption. This review provides an overview of biodegradable shape memory polymers that have been used in medical applications. The majority of biodegradable shape memory polymers are based on thermally responsive polyesters or polymers that contain hydrolyzable ester linkages. These materials have been targeted for use in applications pertaining to embolization, drug delivery, stents, tissue engineering, and wound closure. The development of biodegradable shape memory polymers with unique properties or responsiveness to novel stimuli has the potential to facilitate the optimization and development of new medical applications.

Enhancing Stent Effectiveness with Nanofeatures

Drug-eluting stents are an effective therapy for symptomatic arterial obstructions, substantially reducing the incidence of restenosis by suppressing the migration and proliferation of vascular smooth muscle cells into the intima. However, current drug-eluting stents also inhibit the growth of endothelial cells, which are required to cover the vascular stent to reduce an excessive inflammatory response. As a result, the endothelial lining of the lumen is not regenerated. Since the loss of this homeostatic monolayer increases the risk of thrombosis, patients with drug-eluting stents require long-term antithrombotic therapy. Thus, there is a need for improved devices with enhanced effectiveness and physiological compatibility towards endothelial cells. Current developments in nanomaterials may enhance the function of commercially available vascular devices. In particular, modified design schemes might incorporate nanopatterns or nanoparticle-eluting features that reduce restenosis and enhance re-endothelialization. The intent of this review is to discuss emerging nanotechnologies that will improve the performance of vascular stents.

Change in surface roughness by dynamic shape-memory acrylate networks enhances osteoblast differentiation

Microscale surface roughness has been shown to enhance osseointegration of titanium implants through increased osteoblast differentiation while osteoblast proliferation remains greater on smooth titanium. Taking advantage of these phenomena, we developed a shape memory (meth)acrylate copolymer with thermomechanical properties that created a time-dependent dynamic surface change from smooth to rough under in vitro cell culture conditions and evaluated the effect of the shape recovery on osteoblast response. Rough topographies were created using soft lithography techniques to mimic those found on clinically-used Ti surfaces (machined smooth; acid-etched; grit-blasted). The surface roughness was then reduced to smooth via compression and shown to fully recover within 24 h in culture conditions. When grown under static conditions, osteoblast number, alkaline phosphatase specific activity (ALP), and osteoprotegerin (OPG) and vascular endothelial growth factor (VEGF) production were unaffected by polymer surface roughness, but osteocalcin (OCN) was increased on the grit-blasted polymer mimic. Under dynamic conditions, DNA was reduced but OCN and OPG were increased on the compressed grit-blasted polymer at 3 days compared to static surfaces. The present study indicates that responses to polymer surface are sensitive to time-dependent changes in topography. The use of a shape memory polymer with dynamic surface roughness may improve osseointegration.

Novel design approaches for multifunctional information carriers

Information carriers with an unprecedented combination of thermochromic and shape memory properties are presented. The studied material systems may be used in anti-counterfeiting applications.

Chitosan scaffolds with a shape memory effect induced by hydration

Chitosan-based porous scaffolds exhibit a shape memory effect triggered by hydration, and they are candidates for applications in minimally invasive surgery.

Drug-releasing implants: current progress, challenges and perspectives

This review presents the different types and concepts of drug-releasing implants using new nanomaterials and nanotechnology-based devices.

PCL-based Shape Memory Polymers with Variable PDMS Soft Segment Lengths

Thermoresponsive shape memory polymers (SMPs) are stimuli-responsive materials that return to their permanent shape from a temporary shape in response to heating. The design of new SMPs which obtain a broader range of properties including mechanical behavior is critical to realize their potential in biomedical as well as industrial and aerospace applications. To tailor the properties of SMPs, "AB networks" comprised of two distinct polymer components have been investigated but are overwhelmingly limited to those in which both components are organic. In this present work, we prepared inorganic-organic SMPs comprised of inorganic polydimethyl-siloxane (PDMS) segments of varying lengths and organic poly(ε-caprolactone) (PCL) segments. PDMS has a particularly low T(g) (-125 °C) which makes it a particularly effective soft segment to tailor the mechanical properties of PCL-based SMPs. The SMPs were prepared via the rapid photocure of solutions of diacrylated PCL(40)-block-PDMS(m)-block-PCL(40) macromers (m = 20, 37, 66 and 130). The resulting inorganic-organic SMP networks exhibited excellent shape fixity and recovery. By changing the PDMS segment length, the thermal, mechanical, and surface properties were systematically altered.

Shape memory polymer foams for cerebral aneurysm reparation: effects of plasma sterilization on physical properties and cytocompatibility

Shape memory polyurethanes (SMPUs) represent promising candidate materials for aneurysm embolization, since they could enable clinical problems still associated with these clinical procedures to be overcome. In this work, we report on the characterization of physicochemical, thermomechanical and in vitro interface properties of two SMPU foams (Cold Hibernated Elastic Memory, CHEM), proposed as a material for embolization devices in minimally invasive procedures. Moreover, because device sterilization is mandatory for in vivo applications, effects on the properties of the foams after plasma sterilization were also evaluated. Both foams (CHEM 3520 and CHEM 5520) showed excellent shape recovery ability (recovery rate, R(r), up to 99%) in conventional shape recovery tests, performed at constant heating rate. Transition temperatures (T(trans)), determined by tandelta peaks in dynamic mechanical analysis (DMA), were 32.2 and 45.1 degrees C, for CHEM 3520 and 5520, respectively. The value of T(trans) affects shape memory ability in the recovery test at 37 degrees C, which simulates the behavior after implantation of the device: in fact, R(r) was significantly higher for lower T(trans) foam (R(r) approximately 82% and R(r) approximately 46%, respectively, for CHEM 3520 and CHEM 5520). After plasma sterilization performed by a Sterrad sterilization system, an increase in open porosity was observed: this is probably due to the sterilization cycle; however, no effects on shape recovery behavior were observed. Furthermore, plasma treatment had no significant effect on L929 cells in in vitro cytotoxicity tests, performed on cell culture medium extracts in contact with foams for up to 7 days. Moreover, direct cytocompatibility tests showed a good colonization and growth from L929 cells on CHEM foams, suggesting the effectiveness of an in vivo healing process. All these results seem to suggest that CHEM foams could be advantageously used for manufacturing devices for mini-invasive embolization procedures of aneurysms.