Multi-material 3D printed shape memory polymer with tunable melting and glass transition temperature activated by heat or light

Shape memory polymers are attractive smart materials that have many practical applications and academic interest. Three-dimensional (3D) printable shape memory polymers are of great importance for the fabrication of soft robotic devices due to their ability to build complex 3D structures with desired shapes. We present a 3D printable shape memory polymer, with controlled melting and transition temperature, composed of methacrylated polycaprolactone monomers and N-Vinylcaprolactam reactive diluent. Tuning the ratio between the monomers and the diluents resulted in changes in melting and transition temperatures by 20, and 6 °C, respectively. The effect of the diluent addition on the shape memory behavior and mechanical properties was studied, showing above 85% recovery ratio, and above 90% fixity, when the concentration of the diluent was up to 40 wt %. Finally, we demonstrated multi-material printing of a 3D structure that can be activated locally, at two different temperatures, by two different stimuli; direct heating and light irradiation. The remote light activation was enabled by utilizing a coating of Carbon Nano Tubes (CNTs) as an absorbing material, onto sections of the printed objects

Stress-Free Two-Way Shape Memory Effect of Poly(ethylene glycol)/ Poly(epsilon-caprolactone) Semicrystalline Networks

In this work, poly(ethylene glycol) (PEG)/poly(epsilon- caprolactone) (PCL) semicrystalline networks were prepared by photo-cross-linking of methacrylated macromonomers with different molecular weights and in different proportions to obtain amphiphilic materials capable of displaying properly designed shape memory effects. Networks based on PCL 10 kDa and PEG 3 kDa showed suitable thermal and mechanical properties with well-separated crystallization and melting regions to achieve a self-standing two-way shape memory effect. Particularly, after the application of a specific thermomechanical history, these materials are capable of cyclically changing their shape between two configurations upon cooling-heating cycles in the absence of any external load applied. The effect of the composition of the networks and of the employed thermomechanical parameters, such as the applied strain and the actuation temperature, was investigated to shed light on the shape memory mechanism for this class of materials, which are considered promising for applications in the biomedical field and as reversible actuators for soft robotics

Reversible Stress-Driven and Stress-Free Two-Way Shape Memory Effect in a Sol-Gel Crosslinked Polycaprolactone

The two-way shape memory effect is the ability of a material to change its shape between two configurations upon application and removal of a stimulus, and, among shape memory polymers, it is featured only by few systems, such as semicrystalline networks. When studied under tensile conditions, it consists of elongation-contraction cycles along cooling and heating across the crystallization and melting region, typically under the application of a constant load. However, recent studies on crosslinked semicrystalline co-polymers demonstrate that also a completely stress-free, or self-sustained, two-way effect may be achieved through specific thermomechanical cycles. This effect is currently regarded with interest for the development of intrinsically reversible sensors and actuators, and it may also be displayed by simpler materials, as homopolymer-based semicrystalline networks. Only seldom articles investigate this possibility, therefore in this work the two-way shape memory behavior is studied on a poly(e-caprolactone) system, crosslinked by means of a sol-gel approach. The effect is studied both under stress-driven and stress-free condition, by applying properly set-up thermo-mechanical histories. The results allow to describe the effect as a function of temperature, to reveal the dependence on specific testing parameters and to compare the extent of the reversible strain variation under these two conditions

Modeling, Topology Optimization and Experimental Validation of Glass-Transition-based 4D-printed Polymeric Structures

In recent developments in the field of multi-material additive manufacturing, differences in material properties are exploited to create printed shape memory structures, which are referred to as 4D-printed structures. New printing techniques allow for deliberate introduction of prestresses in the specimen during manufacturing. This prestress is combined with a heat-induced glass transition, which lowers the materials Young's modulus. Upon the decrease in stiffness, the prestress is released, which results in the realization of a pre-programmed deformation. Coupled with the right design, this enables new functionalities. As the design of such functional multi-material structures is crucial but far from trivial, a systematic methodology is developed, where a finite element model is combined with a density-based topology optimization method to describe the material layout. The coupling between the definition of the prestress and the material interpolation function used in the topology description is addressed. The efficacy of topology optimization to design 4D-printed structures is explored by applying the methodology to a variety of design problems. Tests are performed with printed samples to calibrate the prestress and to validate the modeling approach. This study demonstrates that by combining topology optimization and 4D-printing concepts, stimuli-responsive structures with specific properties can be designed and realized

Stakes sensitivity and credit rating: a new challenge for regulators

The ethical practices of credit rating agencies (CRAs), particularly following the 2008 financial crisis, have been subject to extensive analysis by economists, ethicists, and policymakers. We raise a novel issue facing CRAs that has to do with a problem concerning the transmission of epistemic status of ratings from CRAs to the beneficiaries of the ratings (investors, etc.), and use it to provide a new challenge for regulators. Building on recent work in philosophy, we argue that since CRAs have different stakes than the beneficiaries of the ratings in the ratings being accurate, what counts as knowledge (and as having ‘epistemic status’) concerning credit risk for a CRA may not count as knowledge (as having epistemic status) for the beneficiary. Further, as it stands, many institutional investors (pension funds, insurance companies, etc.) are bound by law to make some of their investment decisions dependent on the ratings of officially recognized CRAs. We argue that the observation that the epistemic status of ratings does not transmit from CRAs to beneficiaries makes salient a new challenge for those who think current regulation regarding the CRAs is prudentially justified, namely, to show that the harm caused by acting on a rating that does not have epistemic status for beneficiaries is compensated by the benefit from them acting on a CRA rating that does have epistemic status for the CRA. Unlike most other commentators, therefore, we offer a defeasible reason to drop references to CRAs in prudential regulation of the financial industry

Hierarchical motion of 4D-printed structures using the temperature memory effect

The temperature memory effect (TME) refers to the ability of a shape memory polymer to display recovery around the temperature at which its predeformation occurred so that the material expresses its shape memory response not only in terms of shape but also for what concerns the deformation temperature. This peculiar effect, displayed only by certain classes of polymers, allows to control of the triggering temperature for the shape memory effect as well as to provide multiple shape memory responses for specific, properly designed predeformation histories. Moreover, when combined with 3D printing, such an effect opens new powerful perspectives for designing autonomous structures with customized architectures and programmable/controllable shape changes. However, the design of such structures and of their active response is not trivial and requires careful attention at different levels, i.e., during printing, experimental characterization, modeling, and simulation. The topic of the present chapter concerns 4D-printed structures exhibiting the TME, and it aims at providing the reader with both an analysis and discussion, helpful in guiding toward the design of functional structures capable of controlled motions, also in a hierarchical manner. Particularly, a methodological approach is proposed and includes three main stages: evaluation of material properties, experimental characterization of 3D-printed structures, and modeling/simulation. A discussion about the steps of each stage is provided, together with an overview of the current state of the art, and a case study is presented. Potential application fields and future perspectives are also explored and discussed

MODELING, TOPOLOGY OPTIMIZATION AND EXPERIMENTAL VALIDATION OF GLASS-TRANSITION-BASED 4D-PRINTED POLYMERIC STRUCTURES

In recent developments in the field of multi-material additive manufacturing, differences in material properties are exploited to create printed shape memory structures, which are referred to as 4D-printed structures. New printing techniques allow for deliberate introduction of prestresses in the specimen during manufacturing. This prestress is combined with a heat-induced glass transition, which lowers the materials Young's modulus. Upon the decrease in stiffness, the prestress is released, which results in the realization of a pre-programmed deformation. Coupled with the right design, this enables new functionalities. As the design of such functional multi-material structures is crucial but far from trivial, a systematic methodology is developed, where a finite element model is combined with a density-based topology optimization method to describe the material layout. The coupling between the definition of the prestress and the material interpolation function used in the topology description is addressed. The efficacy of topology optimization to design 4D-printed structures is explored by applying the methodology to a variety of design problems. Tests are performed with printed samples to calibrate the prestress and to validate the modeling approach. This study demonstrates that by combining topology optimization and 4D-printing concepts, stimuli-responsive structures with specific properties can be designed and realized