Katia Paderni

Shape-memory polymer networks from sol–gel cross-linked alkoxysilane-terminated poly(ε-caprolactone)

A novel type of covalently cross-linked semi-crystalline polymer with shape-memory and biocompatibility properties was prepared from alkoxysilane-terminated poly(ε-caprolactone) (PCL) by sol–gel process that allowed the generation of silica-like cross-linking points. A fine tuning of the cross-linking density and thermal properties (melting temperature) of the materials was obtained by controlling the molecular weight of the PCL precursor (and thus the molecular structure of the resulting network) and the curing conditions. The shape-memory behaviour was investigated with bending tests. Recovery times of less than one second were observed in water depending on the temperature, and a linear correlation of the recovery time with cross-linking density and molecular weight of PCL network precursor was observed.

Poly(vinyl alcohol)-Based Film Potentially Suitable for Antimicrobial Packaging Applications

This work aimed at developing a thin and water-resistant food-grade poly(vinyl alcohol) (PVOH)-based matrix able to swell when in contact with high moisture content food products without rupturing to release antimicrobial agents onto the food surface. This film was prepared by blending PVOH and 7.20% (wt/wt of PVOH) of poly(ethylene glycol) (PEG) with citric acid as crosslinking agent. The film-forming solution was then casted onto a flat surface and the obtained film was 60 μm in thickness and showed a good transparency (close to T = 100%) in the visible region (400 to 700 nm). After immersion in water for 72 h at room temperature, the crosslinked matrix loses only 19.2% of its original weight (the percentage includes the amount of unreacted crosslinking agent, antimicrobial in itself). Water content, degree of swelling, and crosslinking density of the film prove that the presence of PEG diminishes the hydrophilic behavior of the material. Also the mechanical properties of the wet and dry film were assessed. Alongside this, 2.5% (wt/wt of dry film) of grapefruit seed extract (GSE), an antimicrobial agent, was added to the film-forming solution just before casting and the ability of the plastic matrix to release the additive was then evaluated in vitro against 2 GSE-susceptible microorganisms, Salmonella enteritidis and Listeria innocua. The results indicate that the developed matrix may be a promising food-grade material for the incorporation of active substances.

The two-way shape memory behaviour of crosslinked poly(ε-caprolactone) systems with largely varied network density

The two-way shape memory behaviour of semicrystalline networks was investigated on systems based on poly(ε-caprolactone) featuring significantly different network architecture. Crosslinked poly(ε-caprolactone)s were prepared by thermal curing from methacrylic end-capped linear chains having various methacrylation degrees. By conveniently reducing the methacrylation degree, the crosslink density of cured materials was varied over a range of one order of magnitude, leading to comparable changes in the material compliance in the rubbery region, but only to moderate variations in melting and crystallization temperatures (Tm and Tc) and in the crystallinity content. When subjected to constant non-zero stress and to cooling–heating cycles from above Tm to below Tc, the materials undergo a reversible two-way elongation–contraction effect, whose extent depends on material structure and applied stress. The structural changes in the crystalline phase accompanying the cooling-induced elongation were studied through differential scanning calorimetry and X-ray diffraction analyses. The elongation process involves different contributions of entropy- and crystallization-driven processes, whose amounts were investigated as a function of the loading conditions and the molecular architecture. The role of the network density towards a controlled two-way response is evidenced, showing that a proper value of the crosslink density has to be identified to maximize the two-way elongation capabilities.

Tailored One-Way and Two-Way Shape Memory Capabilities of Poly(ε-Caprolactone)-Based Systems for Biomedical Applications

This paper investigates the shape memory capabilities of semicrystalline networks, focusing the attention on poly(ε-caprolactone) (PCL) systems, a class of materials that allows to satisfy important requirements for their applications as biomedical devices, such as the good biocompatibility, the fast recovery of large “temporary” shape configurations, and the easy tailoring of the transformation temperatures. The materials were prepared with various crosslink densities and crosslinking methodologies; in particular, beside a thermal crosslinking based on reactive methacrylic end groups, a novel type of covalently crosslinked semicrystalline systems was prepared by a sol-gel approach from alkoxysilane-terminated PCL precursors, so as to avoid potentially toxic additives typically used for free-radical thermal curing. The materials were subjected to biological tests, to study their ability in sustaining cell adhesion and proliferation, and to thermal characterizations, to evaluate the possibility to tailor their melting and crystallization temperatures. The one-way shape memory (i.e., the possibility to set the material in a given configuration and to recover its pristine shape) and the two-way shape memory response (i.e., the triggered change between two distinguished shapes on the application of an on-off stimulus) were studied by applying optimized thermo-mechanical cyclic histories. The ability to fix the applied shape and to recover the original one on the application of heating (i.e., the one-way effect) was evaluated on tensile bars; further, to investigate a potential application as self-expandable stents, isothermal shape memory experiments were carried out also on tubular specimens, previously folded in a temporary compact configuration. The two-way response was studied through the application of a constant load and of a heating/cooling cycle from above melting to below the crystallization temperature, leading to a reversible elongation/contraction effect, involving maximum strain changes up to about 80%, whose extent may be controlled through the crosslink density.

Facile fabrication of shape memory poly(ε-caprolactone) non-woven mat by combining electrospinning and sol–gel reaction

Poly(e-caprolactone)-based non-woven fibrous mats showing excellent one-way shape memory properties were obtained through a straightforward approach by combining electrospinning process and sol–gel reaction. A solution of partially crosslinked α,ω-triethoxysilane-terminated poly(e-caprolactone) was used to obtain bead-free fibers through electrospinning. Non-woven mats with different crosslinking degrees have been prepared and the effect of the different crosslinking extent and of the microfibrous structure were correlated to the mechanical and shape memory properties of the material. The evolution of fiber architecture within the non-woven mat following deformation and shape memory cycles was also investigated.

Evaluation of the shape memory performances of poly(ε-caprolactone)-based tubular devices for potential biomedical applications

The shape memory behavior of tubular specimens based on crosslinked poly(e-caprolactone) was investigated in order to evaluate their ability i) to restore their shape after being folded in a more compact one, and ii) to exert stress under external confinement (recovery stress). The specimens were prepared following different crosslinking methodologies and with different network densities, in order to tailor the material response in terms of transformation temperatures and recovery stress capabilities. The devices are able to fully recover their shape once heated close to the melting temperature and to exert moderate stresses, that may be controlled through thickness and crosslink density, and whose values were employed to develop a new testing apparatus for the measurement of radial dilation capabilities.

Tailored one-way and two-way shape memory response of poly(ε-caprolactone)-based systems for biomedical applications

A series of crosslinked poly(ε-caprolactone) (PCL) materials were obtained starting from linear, three- and four-arm star PCL functionalized with methacrylate end-groups, allowing to tune the melting temperature (Tm) on a range between 36 and 55°C. After deforming the specimens at 50% above Tm, the materials are seen to fully restore their original shape by heating them on a narrow region close to Tm; further, when the shape memory effect is triggered under fixed strain conditions, the materials are able to exert stress on a range between 0.2 and 7 MPa. The materials also display two-way shape memory features, reversibly moving between two shapes when cooled and heated under a fixed load. Finally, to investigate the application of the PCL materials as self-expandable stents, one-way shape memory experiments are currently carried out on tubular specimens.

Two-Way Shape Memory Behavior of Electrospun Non-Woven Mats Prepared from Sol-Gel Crosslinked Poly(ε-Caprolactone)

Non-woven fibrous mats based on semicrystalline networks were prepared starting from poly(ε-caprolactone) and by combining electrospinning process and sol-gel crosslinking reaction. The mats were subjected to proper thermo-mechanical cycles to investigate their two-way shape memory capabilities (i.e. the possibility to change between two distinguished shapes upon heating and cooling), and an improvement of the two-way behaviour was researched through the application of a training cycle. An ex-situ SEM analysis described the microstructural evolution accompanying the two-way shape memory cyclic response.

Shape Memory Properties of PBS-Silica Hybrids

A series of novel Si–O–Si crosslinked organic/inorganic hybrid semi-crystalline polymers with shape memory properties was prepared from alkoxysilane-terminated poly(butylene succinate) (PBS) by water-induced silane crosslinking under organic solvent-free and catalyst-free conditions. The hydrolyzation and condensation of alkoxysilane end groups allowed for the generation of silica-like crosslinking points between the polymeric chains, acting not only as chemical net-points, but also as inorganic filler for a reinforcement effect. The resulting networks were characterized using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), dynamic-mechanical analysis (DMA) and tensile and shape memory tests to gain insight into the relationship between the polymeric structure, the morphology and the properties. By controlling the molecular weight of the PBS precursor, a fine tuning of the crosslinking density and the inorganic content of the resulting network was possible, leading to different thermal, mechanical and shape memory properties. Thanks to their suitable morphology consisting of crystalline domains, which represent the molecular switches between the temporary and permanent shapes, and chemical net-points, which permit the shape recovery, the synthesized materials showed good shape memory characteristics, being able to fix a significant portion of the applied strain in a temporary shape and to restore their original shape above their melting temperature.

Experimental kinematics of a special shape actuator

This paper investigates the kinematical behavior of a polymer based star-shaped actuator, able to produce mechanical work through the shape memory effect, that allows a significant shape variations on the application of an external stimulus. The adopted material is a semicrystalline network based on poly(e-caprolactone) crosslinked by thermal curing; the material was adopted due to its fast recovery process when heated close to the melting temperature and the high recovery degree, and, due to its good biocompatibility, it may suitable for biomedical application. The original, or “permanent”, material shape is that of a cylindrical annulus, which is set in a “temporary” configuration as a six spikes star. The temporary shape is fixed through a thermo-mechanical program, involving deformation above melting temperature and cooling under fixed strain and carried out by means of an ad-hoc designed fixture. By heating the deformed specimen above the melting temperature, the system is able to recover the original cylindrical shape realizing a motion and a mechanical power. This peculiar response, consisting in a progressive radial expansion activated by temperature, may be considered for application as self-expanding stenting device triggered by the human body temperature.The shape of the system, that changes during the transformation, can be described as a two dimensional temporal function that represents the mean line of the section of the cylindrical annulus (perpendicular to the height of the annulus). This temporal function is a combination of a circular function and of a modified rhodoneal function and, after a proper calibration through experimental tests, is used to evaluate the kinematics of the system. The function is able to describe adequately the shape evolution experimentally displayed by the samples, with a very good agreement at the starting and final instants of the transformation, while the accuracy during the transformation is acceptable for the proposed application.Copyright