Marc Behl

Multifunctional shape-memory polymers.

The thermally-induced shape-memory effect (SME) is the capability of a material to change its shape in a predefined way in response to heat. In shape-memory polymers (SMP) this shape change is the entropy-driven recovery of a mechanical deformation, which was obtained before by application of external stress and was temporarily fixed by formation of physical crosslinks. The high technological significance of SMP becomes apparent in many established products (e.g., packaging materials, assembling devices, textiles, and membranes) and the broad SMP development activities in the field of biomedical as well as aerospace applications (e.g., medical devices or morphing structures for aerospace vehicles). Inspired by the complex and diverse requirements of these applications fundamental research is aiming at multifunctional SMP, in which SME is combined with additional functions and is proceeding rapidly. In this review different concepts for the creation of multifunctionality are derived from the various polymer network architectures of thermally-induced SMP. Multimaterial systems, such as nanocomposites, are described as well as one-component polymer systems, in which independent functions are integrated. Future challenges will be to transfer the concept of multifunctionality to other emerging shape-memory technologies like light-sensitive SMP, reversible shape changing effects or triple-shape polymers.

Shape-memory polymers as a technology platform for biomedical applications

Polymeric materials are clinically required for medical devices, as well as controlled drug delivery systems. Depending on the application, the polymer has to provide suitable functionalities, for example, mechanical functions or the capability to actively move, so that an implant can be inserted in a compact shape through key-hole incisions and unfold to its functional shape in the body. Shape-memory polymers, as described herein regarding their general principle, compositions and architectures, have developed to a technology platform that allows the tailored design of such multifunctionality. In this way, defined movements of implants triggered either directly or indirectly, tailored mechanical properties, capability for sterilization, biodegradability, biocompatibility and controlled drug release can be realized. This comprehensive review of the scientific and patent literature illustrates that this technology enables the development of novel medical devices that will be clinically evaluated in the near future.

Actively moving polymers

The ability of polymers to move actively in response to an external stimulus such as heat or light is of high scientific and technological significance. In any instance stimuli-responsive effects on the molecular level are converted into macroscopic movement, whereby generally two different moving behaviors have to be differentiated for polymer-based materials: the shape-memory effect and the shape-changing capability. Basic concepts for the molecular design of suitable polymer architectures for shape-memory polymers as well as tailored programming processes are presented. The thermally-induced shape-memory effect of polymers is described as well as the extension of this concept to other stimuli than heat. Indirect actuation of the thermally-induced effect by IR-irradiation, electric current, humidity or alternating magnetic fields are outlined as well as recent work on light-induced shape-memory polymers. For shape-changing polymers, two basic concepts are presented: shape changes occurring during phase orientation of liquid crystal elastomers (LCE) and the photomechanical effect based on photoisomerization of moieties, such as azo-groups incorporated in suitable polymer systems.

Shape-Memory Polymers and Shape-Changing Polymers

The ability of polymers to respond to external stimuli is of high scientific and technological significance. In the last few years, research activities have been intensified substantially, exploring whether stimuli-sensitive polymers can be designed that move actively. In this review actively-moving materials were classified according to the underlying mechanisms enabling the shape changes: shape-memory polymers and shape-changing polymers/shape-changing gels were identified. The application spectra of these materials as well as the current developments were elucidated and general molecular design principles presented. When applicable, a further distinction according to the applied stimulus was made.

Reversible Bidirectional Shape‐Memory Polymers

Free-standing copolymer network samples with two types of crystallizable domains are capable of a fully reversible bidirectional shape-memory effect. One set of crystallizable domains determines the shape-shifting geometry while the other provides the thermally controlled actuation capability.

Triple-shape polymers

Shape-memory polymers (SMPs) are an emerging class of active materials, which are able to change their shape in a predefined way upon appropriate stimulation. As SMPs can switch from a temporary to their permanent shape they are dual-shape materials. Recently, multiphase polymer networks were explored, which are able to switch from a first shape (A) to a second shape (B) and from there to a third shape (C). Here we highlight this triple-shape effect (TSE) as a thermally triggered effect. The generality of the concept will be explained by describing suitable polymer network architectures and appropriate triple-shape creation processes (TSCPs). TSCP is a thermomechanical treatment typically consisting of two consecutive deformation steps resulting in shapes B and A. The molecular architecture of triple-shape polymers (TSPs) also contains the essential elements for the dual-shape effect (DSE), which therefore was systematically investigated. The understanding of the underlying mechanisms recently led to the discovery of a system, where a thermomechanical treatment with only one single deformation step resulted in a TSE. TSPs enable complex, active deformations on demand, having a high potential as enabling technology for application fields including intelligent medical devices, textile and assembling systems.

Temperature-memory polymer actuators

Reading out the temperature-memory of polymers, which is their ability to remember the temperature where they were deformed recently, is thus far unavoidably linked to erasing this memory effect. Here temperature-memory polymer actuators (TMPAs) based on cross-linked copolymer networks exhibiting a broad melting temperature range (ΔTm) are presented, which are capable of a long-term temperature-memory enabling more than 250 cyclic thermally controlled actuations with almost constant performance. The characteristic actuation temperatures Tacts of TMPAs can be adjusted by a purely physical process, guiding a directed crystallization in a temperature range of up to 40 °C by variation of the parameter Tsep in a nearly linear correlation. The temperature Tsep divides ΔTm into an upper Tm range (T > Tsep) forming a reshapeable actuation geometry that determines the skeleton and a lower Tm range (T < Tsep) that enables the temperature-controlled bidirectional actuation by crystallization-induced elongation and melting-induced contraction. The macroscopic bidirectional shape changes in TMPAs could be correlated with changes in the nanostructure of the crystallizable domains as a result of in situ X-ray investigations. Potential applications of TMPAs include heat engines with adjustable rotation rate and active building facades with self-regulating sun protectors.

Biodegradable multiblock copolymers based on oligodepsipeptides with shape-memory properties.

Thermoplastic phase-segregated multiblock copolymers with polydepsipeptides and PCL segments were prepared via coupling of diol and PCL-diol using an aliphatic diisocyanate. The obtained multiblock copolymers showed good elastic properties and a shape memory. Almost complete fixation of the mechanical deformation, resulting in quantitative recovery of the permanent shape with a switching temperature around body temperature, was observed. In hydrolytic degradation experiments, a quick decrease of the molecular weight without induction period was observed, and the material changed from elastic to brittle in 21 d. These materials promise a high potential for biomedical applications such as smart implants or medical devices.

Shape-memory polymers with multiple transitions: complex actively moving polymers

Shape-memory polymers (SMPs) are able to perform shape transitions in a pre-defined pathway in response to suitable external stimuli such as heat, magnetism, electricity, moisture, or light. Most of the SMPs are dual-shape materials, which enable a single shape transition from a temporary to a permanent shape. Recently, triple-shape polymers (TSPs), which are capable of accomplishing two shape transitions, as well as multi-shape polymers with shape changes have been introduced including temperature-memory polymers (TMPs) with tunable multiple shape transitions. Different concepts for obtaining multi-shape polymers are introduced and the sophisticated structural design concepts in combination with tailored shape-memory creation processes (SMCPs) are explained. Future opportunities emerge in alternative actuation methods and exploration of potential applications.

Non-contact actuation of triple-shape effect in multiphase polymer network nanocomposites in alternating magnetic field

Triple-shape polymers (TSP) can memorize two independent shapes, which are recovered when the temperature is subsequently increased. Certain applications do not allow triggering of the triple-shape effect (TSE) by environmental heating (e.g. potential damaging of surrounding tissue) and therefore require a non-contact activation. Here we explored whether polymer nanocomposites can be designed, which enable non-contact activation of TSE in an alternating magnetic field. A series of nanocomposites were synthesized by incorporation of silica coated iron(III)oxide nanoparticles into a polymer network matrix containing poly(e-caprolactone) (PCL) and poly(cyclohexyl methacrylate) (PCHMA) segments. Triple-shape functionalization of the materials was realized by application of different thermomechanical procedures (single or two dual-step), in which samples were deformed by bending to minimize changes in S/V ratio during shape recovery. For quantification of triple-shape properties inductive heating experiments were conducted in an alternating magnetic field at frequency of f = 258 kHz. By increasing the magnetic field strength H the triple-shape effect was triggered, while the maximum achievable temperature Tmax and the shape change was monitored using an infrared video camera. Excellent triple-shape properties were achieved for nanocomposites containing 40 wt-% of PCL exhibiting a two-step recovery of shapes B and C, when stimulated by step-wise increasing the magnetic field strength. In this way the TSE could be characterized by two distinct switching magnetic strengths Hsw,1(A → B) and Hsw,2(B → C) corresponding to the switching temperatures determined in cyclic, thermomechanical tensile tests for thermally-induced TSP.