During the resorbable-polymer-boom of the 1970s and 1980s, polycaprolactone (PCL) was used in the biomaterials field and a number of drug-delivery devices. Its popularity was soon superseded by faster resorbable polymers which had fewer perceived disadvantages associated with long term degradation (up to 3-4 years) and intracellular resorption pathways; consequently, PCL was almost forgotten for most of two decades. Recently, a resurgence of interest has propelled PCL back into the biomaterials-arena. The superior rheological and viscoelastic properties over many of its aliphatic polyester counterparts renders PCL easy to manufacture and manipulate into a large range of implants and devices. Coupled with relatively inexpensive production routes and FDA approval, this provides a promising platform for the production of longer-term degradable implants which may be manipulated physically, chemically and biologically to possess tailorable degradation kinetics to suit a specific anatomical site. This review will discuss the application of PCL as a biomaterial over the last two decades focusing on the advantages which have propagated its return into the spotlight with a particular focus on medical devices, drug delivery and tissue engineering.

The return of a forgotten polymer : Polycaprolactone in the 21st century

Thermally-induced two-way shape memory polymers: Mechanisms, structures, and applications

One of the most promising fields for shape-memory polymers is the biomedical field Shape-memory polymers that can be triggered by various (physiological) conditions such as temperature and moisture have been reported, as well as remotely triggerable shape-memory polymers that make use of electromagnetic fields, ultrasound, etc These polymers have shown great promise in in vitro studies as well as in early in vivo studies Their biocompatibility, and in some cases biodegradability, renders them excellent candidates for minimal invasive surgery and for the design of triggerable biomedical devices The current review provides a nonexhaustive overview of recent developments realized throughout the last decade in the field of shape-memory polymers serving specific biomedical applications while considering relevant triggers and biocompatible chemistries

Shape-memory polymers for biomedical applications

Photo-cross-linking: A powerful and versatile strategy to develop shape-memory polymers

This work reports the reversible shape memory polymer (RSMP) by semicrystalline poly(ethylene-co-vinyl acetate) (PEVA) with dynamic covalent polymer networks. The ester linkages in the cross-linked PEVA acted as dynamic covalent bonds in the presence of a transesterification catalyst. This dynamic covalent network imparted the material with good thermal plasticity in a non-flow state at high temperature. The resulting PEVA possessed both the reversible shape memory effect (RSME) and thermadapt property. The thermadapt performance gave little adverse effect on the RSME. This work provides an approach to improve the reprocessability/recyclability of thermoset RSMPs through simple modification of commercially available polymers.

Reversible Shape Memory Polymer from Semicrystalline Poly(ethylene-co-vinyl acetate) with Dynamic Covalent Polymer Networks

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 © 2014 by ASME

Experimental Kinematics of a Special Shape Actuator

On-board comfort measurement system development for buses

The increasing integration between electronics and mechanical engineering brings to the industrial market very hi-tech sensors, often non-linear, capable of more than a single input and single output. A problem more and more relevant for sensors like these is calibration. Classic l calibration procedures, when applied to this extremely engineered sensors, lead to poor accuracy and are generally not satisfactory. The case study is the calibration of a bi laser based position sensor, in particular a positive sensitive detector, that is an optical position transducer based on series of photodiodes commonly used as multidimensional sensor. To perform the calibration a micrometric positioning table was used to test the whole photodiode active area in both directions. The sensor studied showed a very linear behaviour in the central region of the working range, and a limited nonlinearity closer to the range limits and was to be used to verify robot movement capabilities; to reduce uncertainty associated with nonlinearities, standard, non-linear, calibrations were performed, pointing out residual values in order to compare different algorithms. In a previous work, authors have already tested a linear model against an algorithm based on radial basis functions (RBF) and Nelder-Mead simplex method. Object of this paper is the definition of a procedure based on RBF and genetic algorithms for multi-dimensional interpolation of data cloud and a comparison between this updated procedure results and the ones of the previou s studied algorithms. The reference model for calibration was a black box with two inputs, X and Y position of the laser spot, and two outputs, voltages Vx and Vy, while the calibration procedure was split in two separate layers, one for each output depend ing on both inputs. Given N data points in a M-dimensional environment and N values that represent the non linearity residual, purpose of the algorithm is to approximate a data cloud with a real function, that is represented as a sum of a polynomial (linea radial basis functions, each associated with a different center (node) and weighted by an appropriate coefficient, that the procedure also allow to assess. When no starting guess for nodes are given in input, nodes coordinates are the output of a non based on a genetic algorithm, whose goal is to locally minimize the objective function. The algorithm stops itself whenever it reaches a certain tolerance level, a user specified number of nodes or when the previous iteration has a better value of the objective function. This study has been performed for various RBF classes, and shows an increased accuracy, thus a better metrological behaviour, with respect to the standard linear (planar) calibration model traditionally used.

Mimo non -linear sensors calib ration based on genetic alg orithms

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.

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

Transport service suppliers try to provide faster trips to ensure users to spend as little time as possible travelling. On the other hand, previous studies have been conducted to investigate the value of travel time as an opportunity, and to consider interventions that can increase the perceived value of time so that reducing it becomes less important. Such studies seek to understand to what extent different activities are carried out, how personal objects are used, and how travellers’ levels of sociability could influence their perceptions of the journey. The present paper extends the scope of the literature by reporting on a comparative investigation of two different contests, Bristol (UK) and Brescia (IT), by analysing the quality of time spent on board urban buses. The aim is to identify relationships between objective factors and users’ perceptions, and to pinpoint similarities and differences between the two contexts. Furthermore, Travel Experience Indices have been developed to provide a quantitative evaluation of the travel experience.

Travel experience on board urban buses: a comparison between Bristol and Brescia

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