Stefano Pandini

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.

Network architecture and shape memory behavior of cold-worked epoxies

The shape memory behavior of polymers derives from a combination of their molecular architecture and thermomechanical history. In this study, several epoxies with various network architectures were prepared using mixtures of a diepoxide resin, a monoepoxide resin, and an aliphatic diamine hardener. A nonconventional cold-working programming, carried out below Tg, was employed to set the materials in a temporary configuration and allowed to fix considerable amounts of the applied strain. The shape memory behavior was evaluated through transient heating and isothermal recovery tests. All the resins are capable of complete recovery, which occurs as a sequence of an early process taking place below Tg and a major one close to Tg, which acted as the switching temperature (Tswitch). The proximity of the deformation temperature to Tg influenced the amount of strain recovered within each process. It was shown that resins with different structures, although presenting similar Tswitch, may have different recovery kinetics, and the roles of the network density and the chain stiffness on the recovery rate were evidenced.

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.

Analysis of tongue pressure sensor for biomedical applications

The tongue is an important muscle and the contact with the hard palate during the articulation of a syllable or during swallowing is fundamental. Patients who have had cerebrovascular or other neurological disorders may have impaired speech and swallowing problems due to decreased ability to control the tongue. In this work, a device with the aim of providing a non-invasive aid for the rehabilitation is described. The proposed device has been designed with the purpose of measuring the tongue pressure on the palate directly in the oral cavity and transmitting the data wirelessly. The device is minimally invasive, because no cable is used to connect the pressure sensor placed in the oral cavity with the reading unit placed outside. A first prototype was developed and preliminary testing data for the analysis of the sensor behavior have been performed. A specific experimental setup has been designed and realized and a testing protocol has been defined and adopted. The obtained preliminary experimental results show a wide measurement range, up to 100 kPa. Application fields of this device are the treatment of people with swallowing or phonetic disorders.

Antiplasticization-driven stiffening in epoxy-amine networks: Effects of the resin formulation

In this work several epoxies with various and structurally related network architectures were prepared using mixtures of a diepoxide resin, a monoepoxide resin, and an aromatic diamine hardener. The effects of the systems formulation on their mechanical and thermal properties were investigated through dynamic-mechanical tests and room temperature tensile testing. The change in glass transition temperature and the stiffening effects measured at room temperature were interpreted at the light of the macromolecular architecture and the chain mobility hindrance connected to the antiplasticization phenomenology.

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.