Degradable shape-memory polymer networks from oligo[(l-lactide)-ran-glycolide]dimethacrylates

Research Progress of Shape Memory Polymer and 4D Printing in Biomedical Application

As a kind of smart material, shape memory polymer (SMP) shows great application potential in the biomedical field. Compared with traditional metal-based medical devices, SMP-based devices have the following characteristics: 1) The adaptive ability allows the biomedical device to better match the surrounding tissue after being implanted into the body by minimally invasive implantation; 2) it has better biocompatibility and adjustable biodegradability; 3) mechanical properties can be regulated in a large range to better match with the surrounding tissue. 4D printing technology is a comprehensive technology based on smart materials and 3D printing, which has great application value in the biomedical field. 4D printing technology breaks through the technical bottleneck of personalized customization and provides a new opportunity for the further development of the biomedical field. This paper summarizes the application of SMP and 4D printing technology in the field of bone tissue scaffolds, tracheal scaffolds, and drug release, etc. Moreover, this paper analyzes the existing problems and prospects, hoping to provide a preliminary discussion and useful reference for the application of SMP in biomedical engineering.

The Thermal and Mechanical Properties of Poly(ethylene-co-vinyl acetate) Random Copolymers (PEVA) and its Covalently Crosslinked Analogues (cPEVA)

The thermal and mechanical properties of poly(ethylene-co-vinyl acetate) random copolymers (PEVA) and its covalently crosslinked analogues (cPEVA) were controlled by the overall crystallinity of the polymer networks. The cPEVAs with different VA-content were synthesized by thermally-induced crosslinking of linear PEVA with dicumyl peroxide (DCP). This work was mainly concerned with the effect of vinyl acetate (VA) content on the crosslinking density, thermal and mechanical properties of PEVAs and cPEVAs, respectively. The chemical composition was analyzed by thermogravimetric analysis and 1H-NMR. The thermal and mechanical properties of PEVAs and cPEVAs have been studied through a series of conventional analytical methods, including gel content determination, different scanning calorimetry, thermogravimetric analysis, dynamic mechanical thermal analysis and traditional mechanical measurements. The experimental results show that the thermal and mechanical properties of PEVAs and cPEVAs increase with decreasing the VA-content. A broad melting transition with a ΔTm in the range from 78 °C to 95 °C was observed for all polymer networks.

A rapidly recoverable shape memory polymer with a topologically well-controlled poly(ethyl methacrylate) structure

Many of the unique properties of a conventionally crosslinked shape memory network are not found at the same time, and this is a large challenge for the development of advanced shape memory functional materials. In this work, a topologically well-controlled network shape memory poly(ethyl methacrylate) (CN-SMPEMA) is designed and fabricated by introducing two tetra-armed functional structures simultaneously as well-defined structure units to promote switch segment and net-point uniform distribution via the combined technology of the unique controllable atom transfer radical polymerization (ATRP) and copper(i)-catalyzed azide-alkyne cycloaddition (CuAAC). Compared with conventionally crosslinked networks, the as-prepared CN-SMPEMA not only exhibits a combination of excellent mechanical properties, shape fixity, shape recovery ratios and outstanding cycling stability, but also displays rapid recoverability. Additionally, a feasible molecular mechanism for the shape memory effect of the CN-SMPEMA system is analyzed and proposed. We anticipate that such a topologically well-defined network shape memory material with multiple excellent properties will broaden the practical application range of acrylate-based shape memory polymer materials.

Recent advances in degradable lactide-based shape-memory polymers

Biodegradable polymers are versatile polymeric materials that have a high potential in biomedical applications avoiding subsequent surgeries to remove, for example, an implanted device. In the past decade, significant advances have been achieved with poly(lactide acid) (PLA)-based materials, as they can be equipped with an additional functionality, that is, a shape-memory effect (SME). Shape-memory polymers (SMPs) can switch their shape in a predefined manner upon application of a specific external stimulus. Accordingly, SMPs have a high potential for applications ranging from electronic engineering, textiles, aerospace, and energy to biomedical and drug delivery fields based on the perspectives of new capabilities arising with such materials in biomedicine. This study summarizes the progress in SMPs with a particular focus on PLA, illustrates the design of suitable homo- and copolymer structures as well as the link between the (co)polymer structure and switching functionality, and describes recent advantages in the implementation of novel switching phenomena into SMP technology.

Viability, morphology and function of primary endothelial cells on poly(n-butyl acrylate) networks having elastic moduli comparable to arteries

Soft hydrophobic poly(n-butyl acrylate) networks (cPnBA) were developed as entropy elastic substrates for passive mechanical stimulation of cells, where the elastic modulus of the cPnBAs could be systematically adjusted by variation of the cross-link density. The networks were synthesized by thermally-induced radical polymerization from n-butyl acrylate, with poly(propylene glycol) dimethacrylate (PPGDMA) acting as cross-linker, whereby the purity of the cPnBAs was confirmed by(1) H-NMR spectroscopy and gas chromatography. In this work two cPnBA polymer networks with an elastic modulus around 200 kPa and 1 MPa were investigated having an elastic modulus similar to that of arteries. Both cPnBAs exhibited an almost smooth surface with a surface roughness (R q) in the wet state ranging from 17 to 37 nm and a similar zetapotential, indicating an almost identical chemical composition within the topmost surface layer in terms of functional groups. In contrast, wettability of the samples was found to be different with an advancing angle ( advancing) of 123 ± 3.8° for cPnBA0250, while for cPnBA1100 significantly lower values for advancing (111 ± 3.8°) were obtained. First in vitro tests were performed with primary endothelial cells (HUVEC) to study its effects on vascular cell functions. Within the time period of cultivation (72 h), the cells on the cPnBA samples reached subconfluence and showed a viability rate of almost 100%. Although cell density differed after 72 h with more cells on cPnBA0250 than on cPnBA1100, both materials showed no significant effect on the cell morphology, the cellular LDH-release, which was used as marker for the integrity of the cell membrane, and the organisation of the VE-cadherin. However, lower cell density and less actin stress fibre formation on cPnBA1100 might indicate that cell-material interaction was weaker on cPnBA1100 than on cPnBA0250. The secretion of the vasoactive cytokines prostacyclin (PGI2) and thromboxane A2 (TXA2) was low compared to previously reported values. However, the anti-thrombogenic ratio of PGI2/TXA2 - which is balanced under physiological conditions - with much higher PGI2 compared to TXA2 (up to 17.6-fold after 72 h for cPnBA1100) suggests that this material might be effective to preventing thrombosis.

Combined, Independent Small Molecule Release and Shape Memory via Nanogel-Coated Thiourethane Polymer Networks

Drug releasing shape memory polymers (SMPs) were prepared from poly(thiourethane) networks that were coated with drug loaded nanogels through a UV initiated, surface mediated crosslinking reaction. Multifunctional thiol and isocyanate monomers were crosslinked through a step-growth mechanism to produce polymers with a homogeneous network structure that exhibited a sharp glass transition with 97% strain recovery and 96% shape fixity. Incorporating a small stoichiometric excess of thiol groups left pendant functionality for a surface coating reaction. Nanogels with diameter of approximately 10 nm bearing allyl and methacrylate groups were prepared separately via solution free radical polymerization. Coatings with thickness of 10-30 μm were formed via dip-coating and subsequent UV-initiated thiol-ene crosslinking between the SMP surface and the nanogel, and through inter-nanogel methacrylate homopolymerization. No significant change in mechanical properties or shape memory behavior was observed after the coating process, indicating that functional coatings can be integrated into an SMP without altering its original performance. Drug bioactivity was confirmed via in vitro culturing of human mesenchymal stem cells with SMPs coated with dexamethasone-loaded nanogels. This article offers a new strategy to independently tune multiple functions on a single polymeric device, and has broad application toward implantable, minimally invasive medical devices such as vascular stents and ocular shunts, where local drug release can greatly prolong device function.

Bio-based (co)polylactide-urethane networks with shape memory behavior at body temperature

The novel bio-based polymer networks with shape memory behavior were synthesized from polylactide-based tetraols and hexamethylene diisocyanate trimer.

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.

Low density biodegradable shape memory polyurethane foams for embolic biomedical applications

Low density shape memory polymer foams hold significant interest in the biomaterials community for their potential use in minimally invasive embolic biomedical applications. The unique shape memory behavior of these foams allows them to be compressed to a miniaturized form, which can be delivered to an anatomical site via a transcatheter process and thereafter actuated to embolize the desired area. Previous work in this field has described the use of a highly covalently crosslinked polymer structure for maintaining excellent mechanical and shape memory properties at the application-specific ultralow densities. This work is aimed at further expanding the utility of these biomaterials, as implantable low density shape memory polymer foams, by introducing controlled biodegradability. A highly covalently crosslinked network structure was maintained by use of low molecular weight, symmetrical and polyfunctional hydroxyl monomers such as polycaprolactone triol (PCL-t, Mn= 900 g), N,N,N0,N0-tetrakis(hydroxypropyl)ethylenediamine and tris(2-hydroxyethyl)amine. Control over the degradation rate of the materials was achieved by changing the concentration of the degradable PCL-t monomer and by varying the material hydrophobicity. These porous SMP materials exhibit a uniform cell morphology and excellent shape recovery, along with controllable actuation temperature and degradation rate. We believe that they form a new class of low density biodegradable SMP scaffolds that can potentially be used as "smart" non-permanent implants in multiple minimally invasive biomedical applications.

Influence of different heating regimes on the shape-recovery behavior of poly(L-lactide) in simulated thermomechanical tests

AIM

Multifunctional polymer-based biomaterials, which combine degradability with a shape-memory capability and in this way enable the design of actively moving implants such as self-anchoring implants or controlled release systems, have been recently introduced. Of particular interest are approved degradable polymers such as poly(L-lactide) (PLLA), which can be easily functionalized with a shape-memory effect. In the case of semicrystalline PLLA, the glass transition can be utilized as shape-memory switching domain.

METHODS

In this work we applied a fully atomistic molecular dynamics simulation to study the shape-memory behavior of PLLA. A heating-deformation-cooling programming procedure was applied to atomistic PLLA packing models followed by a recovery module under stress-free conditions allowing the shape recovery. The recovery was simulated by heating the samples from Tlow = 250 K to Thigh = 500 K with different heating rates β of 125, 40 and 4 K·ns(-1).

RESULTS

We could demonstrate that the obtained strain recovery rate (Rr) was strongly influenced by the applied simulation time and heating rate, whereby Rr values in the range from 46% to 63% were achieved. On its own the application of a heating rate of 4 K·ns(-1) enabled us to determine a characteristic switching temperature of Tsw = 473 K for the modeled samples.

CONCLUSIONS

We anticipate that the atomistic modeling approach presented should be capable of enabling further study of Tsw with respect to the molecular structure of the investigated SMP and therefore could be applied in the context of design and development of new shape-memory (bio)materials.

Poly(ε-caprolactone)-based copolymers bearing pendant cyclic ketals and reactive acrylates for the fabrication of photocrosslinked elastomers

Block copolymers of poly(ethylene glycol) and poly(ε-caprolactone) (PCL) with chemically addressable functional groups were synthesized and characterized. Ring-opening polymerization of ε-caprolactone (CL) and 1,4,8-trioxaspiro-[4,6]-9-undecanone (TSU) using α-methoxy, ω-hydroxyl poly(ethylene glycol) as the initiator afforded a copolymer with cyclic ketals being randomly distributed in the hydrophobic PCL block. At an initiator/catalyst molar ratio of 10/1 and a TSU/CL weight ratio of 1/4, a ketal-carrying copolymer (ECT2-CK) with Mn of 52 kDa and a ketal content of 15 mol.% was obtained. Quantitative side-chain deacetalization revealed the reactive ketones without noticeable polymer degradation. In our study, 10 mol.% of cyclic ketals were deprotected and the ketone-containing copolymer was designated as ECT2-CO. Reaction of ECT2-CO with 2-(2-(aminooxy)acetoxy)-ethyl acrylate gave rise to an acrylated product (ECT2-AC) containing an estimated 3-5 acrylate groups per chain. UV-initiated radical polymerization of ECT2-AC in dichloromethane resulted in a crosslinked network (xECT2-AC). Thermal and morphological analyses employing differential scanning calorimetry and atomic force microscopy operated in PeakForce Tapping mode revealed the semicrystalline nature of the network, which contained stiff crystalline lamellae dispersed in a softer amorphous interstitial. Macroscopic and nanoscale mechanical characterizations showed that ECT2-CK exhibited a significantly lower modulus than PCL of a similar molecular weight. Whereas ECT2-CK undergoes a plastic deformation with a distinct yield point and a cold-drawing region, xECT2-AC exhibits a compliant, elastomeric deformation with a Young's modulus of 0.5±0.1 MPa at 37°C. When properly processed, the crosslinked network exhibited shape-memory behaviors, with shape fixity and shape recovery values close to 1 and a shape recovery time of less than 4s at 37°C. In vitro studies showed that xECT2-AC films did not induce any cytotoxic effects on the cultured mesenchymal stem cells. The crosslinkable polyester copolymers can be potentially used as tissue engineering scaffolds and minimally invasive medical devices.

Shape memory performance of thermoplastic polyvinylidene fluoride/acrylic copolymer blends physically cross-linked by tiny crystals

Polyvinylidene fluoride (PVDF)/acrylic copolymer (ACP) blends are a typical miscible crystalline/amorphous system over the whole composition range. Our previous investigation indicated that blend samples with controlled component ratios and crystallization conditions exhibit good shape memory properties (J. Phys. Chem. B 2012, 116, 1256-1264). In this work, we systematically investigated the cold crystallization temperature effects on the crystal morphologies and the shape memory properties for the 50/50 blend. It was found that the quenched blend is an amorphous material with a low glass transition temperature. Annealing at temperatures above T(g) of the blend induces crystallization of PVDF from the miscible amorphous PVDF/ACP phase, leading to an increased glass transition temperature of the blend. High annealing temperature results in large PVDF spherulites, while low annealing temperature produces tiny crystals in the blend. Furthermore, tiny crystals serve as the physical cross-link points and the amorphous regions among them act as the reversible phase for the blend materials during the mechanical deformations. Therefore, the PVDF/ACP blends with tiny crystals show not only high shape fixity but also excellent recovery ratios.

A NiTi alloy-based cuff for external banding valvuloplasty: a six-week follow-up study in pigs

OBJECTIVE

The study aimed to test a Nitinol(®)-based vein cuff model for external banding valvuloplasty.

METHOD

In 12 adult minipigs, the vena jugularis externa was covered for 42 days by a cuff with an inner diameter adapted to the outer vein diameter in supine position. By changing from supine into prone position hypostatically vein dilation was induced to simulate varicose vein dilation. Cuff position and the inner diameter of the vein lumen under the cuff were examined by computer tomography scanning. Also, histological analysis of the vein wall within the cuff was performed.

RESULTS

The preset tubular shape of the cuff and the cuff position did not change in both prone and supine position, but due to fibrosis the luminal vein diameter within the cuff was decreased (P < 0.01) already after 21 days.

CONCLUSION

A foreign body response resulted in a fibrous capsule covering the cuff which might limit cuff functionality.

Shape-memory properties and degradation behavior of multifunctional electro-spun scaffolds

Multifunctional polymer-based biomaterials, which combine degradability and shape-memory capability, are promising candidate materials for the realization of active self-anchoring implants. In this work we explored the shape-memory capability as well as the hydrolytic and enzymatic in vitro degradation behavior of electro-spun scaffolds prepared from a multiblock copolymer, containing hydrolytically degradable poly(p-dioxanone) (PPDO) and poly(e-caprolactone) (PCL) segments, which we have named PDC. Electro-spun PDC scaffolds with an average deposit thickness of 80 ± 20 µm and a porosity in the range from 70% to 80% were prepared, where the single fiber diameter was around 3 µm. Excellent shape-memory properties were achieved with high recovery rate (Rr) values in the range of Rr = 92% to 98% and a recovery stress of smax = 4.6 MPa to 5.0 MPa. The switching temperature (Tsw) and the characteristic temperature obtained under constant strain recovery conditions (Ts,max) were found in the range from 32 °C to 35 °C, which was close to the melting temperature (Tm,PCL) associated to the poly(e-caprolactone) domains. A linear mass loss was observed in both hydrolytic and enzymatic degradation experiments. The mass loss was substantially accelerated, in enzymatic degradation when Pseudomonas cepacia lipase was added, which was reported to accelerate the degradation of PCL. During hydrolytic degradation a continuous decrease in elongation at break (eB) from eB = 800% to 15% was observed in a time period of 92 days, while in enzymatic degradation experiments a complete mechanical failure was obtained after 4 days.

Hyaluronic acid-based hydrogels crosslinked by copper-catalyzed azide-alkyne cycloaddition with tailorable mechanical properties

Biopolymers of the extracellular matrix are attractive starting materials for providing degradable and biocompatible biomaterials. In this study, hyaluronic acid-based hydrogels with tunable mechanical properties were prepared by the use of copper- catalyzed azide-alkyne cycloaddition (known as "click chemistry"). Alkyne-functionalized hyaluronic acid was crosslinked with linkers having two terminal azide functionalities, varying crosslinker density as well as the lengths and rigidity of the linker molecules. By variation of the crosslinker density and crosslinker type, hydrogels with elastic moduli in the range of 0.5-4 kPa were prepared. The washed materials contained a maximum of 6.8 mg copper per kg dry weight and the eluate of the gel crosslinked with diazidostilbene did not show toxic effects on L929 cells. The hyaluronic acid-based hydrogels have potential as biomaterials for cell culture or soft tissue regeneration applications.

Design of cross-linked semicrystalline poly(ε-caprolactone)-based networks with one-way and two-way shape-memory properties through Diels-Alder reactions

Cross-linked poly(ε-caprolactone) (PCL)-based polyesterurethane (PUR) systems have been synthesized through Diels-Alder reactions by reactive extrusion. The Diels-Alder and retro-Diels-Alder reactions proved to be useful for enhancing the molecular motion of PCL-based systems, and therefore their crystallization ability, in the design of cross-linked semicrystalline polymers with one-way and two-way shape-memory properties. Successive reactions between α,ω-diol PCL (PCL(2) ), furfuryl alcohol, and methylene diphenyl 4,4'-diisocyanate straightforwardly afforded the α,ω-furfuryl PCL-based PUR systems, and subsequent Diels-Alder reactions with N,N-phenylenedimaleimide afforded the thermoreversible cycloadducts. The cross-linking density could be modulated by partially replacing PCL-diol with PCL-tetraol. Interestingly, the resulting PUR systems proved to be semicrystalline cross-linked polymers, the melting temperature of which (close to 45 °C) represented the switching temperature for their shape-memory properties. Qualitative and quantitative measurements demonstrated that these PUR systems exhibited one-way and two-way shape-memory properties depending on their cross-linking density.

Demonstrating the Influence of Water on Shape-Memory Polymer Networks Based on Poly[(Rac-Lactide)-Co-Glycolide] Segments in Vitro

Thermally-responsive shape-memory polymers (SMP) are highly promising implant materials for applications in minimally-invasive surgery since the shape-memory effect (SME) enables the implantation of a bulky device in a compressed temporary state through a small incision. When heated to a temperature exceeding the material switching temperature (Tsw), the device recovers its original bulky shape. Therefore, SMP implants with Tsw ~ 37°C are required for such applications because the body cannot withstand excessive applications of heat. Here, Tsw of networks based on poly[(rac-lactide)-co-glycolide] star-shaped macrotriol or macrotetrols with 19–22 wt% glycolide content, varying oligomer molecular weight (Mn=3000–10000 g·mol−1), and netpoint functionality (f=3 or 4) were lowered from 55–66°C to below body temperature via the uptake of water, which also induced SME at body temperature. Programmed samples kept their temporary shape at room temperature in water as well as at 37°C under dry conditions but recovered in 37°C water. Water uptake/swelling studies and FTIR analysis indicated that the mechanism of solvent-induced SME involved the plasticization of water in switching domains as opposed to changes in swelling or hydrogen bonding. This indirect actuation of SME by using a combination of solvent and heat could be exploited in easy-to-handle shape-memory implant with slower degradation kinetics.

Progress in depsipeptide-based biomaterials

Polydepsipeptides - alternating copolymers of an alpha-amino acid and an alpha-hydroxy acid - are a group of biodegradable polymers. Versatile polydepsipeptides with or without pendant functional groups, as well as various polymer architectures, for example, providing alternative, random, diblock, triblock, multiblock or graft sequence structures, can be synthesized via ring-opening copolymerization of various morpholine-2,5-dione derivatives. They are potential candidates for a wide range of biomedical applications. Polydepsipeptides are described in this review from the aspect of materials science, especially as biomaterials. We mainly focus on various techniques developed to synthesize polydepsipeptides and their copolymers, biodegradation behaviors, shape-memory properties and possible biomedical applications.

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.

Biomedical applications of thermally activated shape memory polymers

Shape memory polymers (SMPs) are smart materials that can remember a primary shape and can return to this primary shape from a deformed secondary shape when given an appropriate stimulus. This property allows them to be delivered in a compact form via minimally invasive surgeries in humans, and deployed to achieve complex final shapes. Here we review the various biomedical applications of SMPs and the challenges they face with respect to actuation and biocompatibility. While shape memory behavior has been demonstrated with heat, light and chemical environment, here we focus our discussion on thermally stimulated SMPs.