Reprint of: Pendant allyl crosslinking as a tunable shape memory actuator for vascular applications

Tunable shape memory properties of highly stretchable poly(ester urea) random copolymers based on α-amino acids

The exploration of biodegradable polymers with shape memory effects (SMEs) holds great promise in biomedical fields. Revealing the relationship between the SMEs and polymer structures not only contributes to interpreting the SME mechanisms, but also prompts the customization of materials properties for specific requirements. Herein, we developed a series of poly(ester urea) (PEU) random copolymers composed of two different diamine monomers based on L-alanine and L-valine, respectively. It was shown that the shape memory performance of the PEU copolymers strongly depended on the composition of two different diamine monomers in the PEU copolymers and other physical properties. This tunability likely arose from the change of polymer chain mobility and crystallinity, which were impacted by the choice of α-amino acids. Intriguingly, thin films of the PEU copolymers exhibited a high strain at break of 347-743% around the physiological temperature (35 °C). Moreover, the random copolymerization of two different sorts of diamine monomers has been demonstrated as a facile approach to precisely tailor the physical properties of the PEUs according to custom needs.

4D Printing: A Cutting-edge Platform for Biomedical Applications

Nature's materials have evolved over time to be able to respond to environmental stimuli by generating complex structures that can change their functions in response to distance, time, and direction of stimuli. A number of technical efforts are currently being made to improve printing resolution, shape fidelity, and printing speed to mimic the structural design of natural materials with three-dimensional (3D) printing. Unfortunately, this technology is limited by the fact that printed objects are static and cannot be reshaped dynamically in response to stimuli. In recent years, several smart materials have been developed that can undergo dynamic morphing in response to a stimulus, thus resolving this issue. Four-dimensional (4D) printing refers to a manufacturing process involving additive manufacturing, smart materials, and specific geometries. It has become an essential technology for biomedical engineering and has the potential to create a wide range of useful biomedical products. This paper will discuss the concept of 4D bioprinting and the recent developments in smart matrials, which can be actuated by different stimuli and be exploited to develop biomimetic materials and structures, with significant implications for pharmaceutics and biomedical research, as well as prospects for the future.

Smart scaffolds: shape memory polymers (SMPs) in tissue engineering

Smart scaffolds based on shape memory polymer (SMPs) have been increasingly studied in tissue engineering. The unique shape actuating ability of SMP scaffolds has been utilized to improve delivery and/or tissue defect filling. In this regard, these scaffolds may be self-deploying, self-expanding, or self-fitting. Smart scaffolds are generally thermoresponsive or hydroresponsive wherein shape recovery is driven by an increase in temperature or by hydration, respectively. Most smart scaffolds have been directed towards regenerating bone, cartilage, and cardiovascular tissues. A vast variety of smart scaffolds can be prepared with properties targeted for a specific tissue application. This breadth of smart scaffolds stems from the variety of compositions employed as well as the numerous methods used to fabricated scaffolds with the desired morphology. Smart scaffold compositions span across several distinct classes of SMPs, affording further tunability of properties using numerous approaches. Specifically, these SMPs include those based on physically cross-linked and chemically cross-linked networks and include widely studied shape memory polyurethanes (SMPUs). Various additives, ranging from nanoparticles to biologicals, have also been included to impart unique functionality to smart scaffolds. Thus, given their unique functionality and breadth of tunable properties, smart scaffolds have tremendous potential in tissue engineering.

Design and in vitro evaluation of electrospun shape memory polyurethanes for self-fitting tissue engineering grafts and drug delivery systems

Integration of multiple features including shape memory, biodegradation, and sustained drug delivery in a single material offers the opportunity to significantly improve the abilities of implantable devices for cardiovascular system regeneration. Two types of shape memory polyurethanes (SMPUs): PU-PLGA and PU-PLLA/PEG differing in soft segments composition that comprising blends of various biodegradable polyols, i.e. D,l-lactide-co-glycolide diol (o-PLGA), poly(e-caprolactone) diols (o-PCL) with various molecular weights, poly-l-lactide diol (o-PLLA), polyethylene glycol (o-PEG) were synthesized and further utilized to electrospun nanofibrous - rapamycin (Rap) delivery system. Structure characterization by Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DCS) and hydrophilicity measurements were performed to gain more insights on the influence of the particular units of the softs segments on the transition temperature (T_trans), shape recovery, degradation profile, and drug release kinetics. In vitro study in PBS solution revealed that incorporation of o-PLGA segments to SMPUs is favorable over o-PEG as increased shape memory performance was observed. Moreover, presence of PLGA in PU-PLGA gave more predictable degradation profile in comparison to PU-PLLA/PEG system. Human Cardiac Fibroblasts (HCF) viability tests in vitro confirmed that the amount of Rap released from evaluated PU-PLLA/PEG/Rap and PU-PLGA/Rap drug delivery systems was sufficient to inhibit cells growth on the surface of the tested materials.

Nasolacrimal stent with shape memory as an advanced alternative to silicone products

Epiphora is the overflow of tears typically caused by obstruction or occlusion of the nasolacrimal duct. More attention is required to address this global health issue owing to the increase in air pollution. Implantation of a silicone stent is the preferred treatment for epiphora; however, introducing a silicone stent into a narrow duct with complex geometry is challenging as it requires guidance by a sharp metal needle. Additionally, silicone can cause adverse reactions such as biofilm formation and tear flow resistance due to its extreme hydrophobicity. To overcome these problems, in this study we developed a new type of biocompatible shape memory polymer (SMP) stent with elasticity capacity for self-expansion. First, SMPs in the form of x%poly(ε-caprolactone)-co-y%poly(glycidyl methacrylate) (x%PCL-y%PGMA) were synthesized via ring opening polymerization by varying the molar ratio of PCL (x%) and PGMA (y%). Second, the shape memory and mechanical properties were tuned by controlling the crosslinking degree and concentration of x%PCL-y%PGMA solution to produce a test type of SMP stent. Lastly, this 94%PCL-06%PGMA stent exhibited more standout critical functions in a series of in vitro and in vivo experiments such as a cell growth-supporting level of biocompatibility with nasal epithelial cells without significant inflammatory responses, better resistance to biofilm formation, and more efficient capacity to drain tear than the silicone control. Overall, 94%PCL-06%PGMA can be suggested as a superior alternative to the currently used materials for nasolacrimal stents. STATEMENT OF SIGNIFICANCE: Silicone intubation (stenting) has been widely used to treat nasolacrimal duct obstruction, however, it can cause adverse clinical effects such as bacterial infection; presents procedural challenges because of the curved nasolacrimal duct structure; and shows poor drainage efficiency stemming from the highly hydrophobic nature of silicone. In this work, we describe an innovative shape memory polymer (SMP) as a superior alternative to conventional silicone-based materials for nasolacrimal duct intubation. We demonstrate the clear advantages of the SMP over conventional silicone, including a much higher drainage capacity and superior resistance to bacterial infection.

Development of a Shape-Memory Tube to Prevent Vascular Stenosis

Inserting a graft into vessels with different diameters frequently causes severe damage to the host vessels. Poor flow patency is an unresolved issue in grafts, particularly those with diameters less than 6 mm, because of vessel occlusion caused by disturbed blood flow following fast clotting. Herein, successful patency in the deployment of an ≈2 mm diameter graft into a porcine vessel is reported. A new library of property-tunable shape-memory polymers that prevent vessel damage by expanding the graft diameter circumferentially upon implantation is presented. The polymers undergo seven consecutive cycles of strain energy-preserved shape programming. Moreover, the new graft tube, which features a diffuser shape, minimizes disturbed flow formation and prevents thrombosis because its surface is coated with nitric-oxide-releasing peptides. Improved patency in a porcine vessel for 18 d is demonstrated while occlusive vascular remodeling occurs. These insights will help advance vascular graft design.

Stretchable degradable and electroactive shape memory copolymers with tunable recovery temperature enhance myogenic differentiation

Development of flexible degradable electroactive shape memory polymers (ESMPs) with tunable switching temperature (around body temperature) for tissue engineering is still a challenge. Here we designed and synthesized a series of shape memory copolymers with electroactivity, super stretchability and tunable recovery temperature based on poly(ε-caprolactone) (PCL) with different molecular weight and conductive amino capped aniline trimer, and demonstrated their potential to enhance myogenic differentiation from C2C12 myoblast cells. We characterized the copolymers by Fourier transform infrared spectroscopy (FT-IR), proton nuclear magnetic resonance (¹H NMR), cyclic voltammetry (CV), ultraviolet-visible spectroscopy (UV-vis), differential scanning calorimetry (DSC), shape memory test, tensile test and in vitro enzymatic degradation study. The electroactive biodegradable shape memory copolymers showed great elasticity, tunable recovery temperature around 37°C, and good shape memory properties. Furthermore, proliferation and differentiation of C2C12 myoblasts were investigated on electroactive copolymers films, and they greatly enhanced the proliferation, myotube formation and related myogenic differentiation genes expression of C2C12 myoblasts compared to the pure PCL with molecular weight of 80,000. Our study suggests that these electroactive, highly stretchable, biodegradable shape memory polymers with tunable recovery temperature near the body temperature have great potential in skeletal muscle tissue engineering application.

STATEMENT OF SIGNIFICANCE

Conducting polymers can regulate cell behavior such cell adhesion, proliferation, and differentiation with or without electrical stimulation. Therefore, they have great potential for electrical signal sensitive tissue regeneration. Although conducting biomaterials with degradability have been developed, highly stretchable and electroactive degradable copolymers for soft tissue engineering have been rarely reported. On the other hand, shape memory polymers (SMPs) have been widely used in biomedical fields. However, SMPs based on polyesters usually are biologically inert. This work reported the design of super stretchable electroactive degradable SMPs based on polycaprolactone and aniline trimer with tunable recovery temperature around body temperature. These flexible electroactive SMPs facilitated the proliferation and differentiation of C2C12 myoblast cells compared with polycaprolactone, indicating that they are excellent scaffolding biomaterials in tissue engineering to repair skeletal muscle and possibly other tissues.