SMART MATERIALS FOR STRETCHABLE ELECTRONICS, SENSORS AND SOFT ACTUATION

Smart materials can be exploited to facilitate disruptive or transformative changes in several fields like stretchable electronics, soft robotics or to develop new class of sensors. They are innovative materials that interact with the environment and respond to external stimuli altering their physical properties in a controlled fashion. They are made integrating different materials at the nanoscale in a nanocomposite to obtain novel functionalities that are not showed from individual constituents. Polymers are the best candidates to be used in smart material fabrication because of their structural and functional properties that can be easily tuned. Moreover, they are low-cost, versatile and can be processed into any shape including thin films. In order to exploit smart materials for soft robotics or stretchable electronic applications, it is required that they should be electrically conductive, patternable, have good mechanical properties and need to be able to transduce an electrical signal in a mechanical response. In addition, their functionalities should remain unchanged over a long period of time. Thus polymers are combined with hard materials like metals, semiconductors or standard electronic components. It is challenging to fabricate technologically relevant smart materials combining hard and soft materials because of their intrinsic physical diversities. Standard manufacturing processes fail to achieve the needed requirements. Among different processes to fabricate smart materials based on polymers, Supersonic Cluster Beam Implantation (SCBI) and Supersonic Cluster Beam Deposition (SCBD) are effective techniques to realize smart materials based on metal/polymer nanocomposites. In my thesis work, I have demonstrated that it is possible to produce new robust smart materials, designing both their electrical and mechanical properties with sharp precision. Metal/polymer nanocomposites have been designed at the nanoscale level to obtain sensors, actuators and electronic devices. Their electrical and mechanical properties have been characterized and their performances have been tested under different stress conditions

High-throughput shadow mask printing of passive electrical components on paper by supersonic cluster beam deposition

We report the rapid prototyping of passive electrical components (resistors and capacitors) on plain paper by an additive and parallel technology consisting of supersonic cluster beam deposition (SCBD) coupled with shadow mask printing. Cluster-assembled films have a growth mechanism substantially different from that of atom-assembled ones providing the possibility of a fine tuning of their electrical conduction properties around the percolative conduction threshold. Exploiting the precise control on cluster beam intensity and shape typical of SCBD, we produced, in a one-step process, batches of resistors with resistance values spanning a range of two orders of magnitude. Parallel plate capacitors with paper as the dielectric medium were also produced with capacitance in the range of tens of picofarads. Compared to standard deposition technologies, SCBD allows for a very efficient use of raw materials and the rapid production of components with different shape and dimensions while controlling independently the electrical characteristics. Discrete electrical components produced by SCBD are very robust against deformation and bending, and they can be easily assembled to build circuits with desired characteristics. The availability of large batches of these components enables the rapid and cheap prototyping and integration of electrical components on paper as building blocks of more complex systems

Facile fabrication of complex networks of memristive devices

We describe the memristive properties of cluster-assembled gold films. We show that resistive switching is observed in pure metallic nanostructured films at room temperature and atmospheric pressure, in response to applied voltage inputs. In particular, we observe resistance changes up to 400% and archetypal switching events that have remarkable symmetry with the applied voltage. We associated this symmetry with 'potentiation' and 'anti-potentiation' processes involving the activation of synapses and of pathways comprising multiple synapses. The stability and reproducibility of the resistance switching, which lasted over many hours, make these devices ideal test-beds for exploration of the basic mechanisms of the switching processes, and allow convenient fabrication of devices that may have neuromorphic properties

Embedding electronics in 3D printed structures by combining fused filament fabrication and supersonic cluster beam deposition

We present an integrated additive manufacturing approach for the rapid prototyping of objects with embedded electric circuits. Our approach relies on the combined use of standard fused filament fabrication (FFF) for the production of thermoplastic 3D freeform components, and supersonic cluster beam deposition (SCBD) for the fabrication of embedded electrical conducting lines and resistors with tailored conductivity. SCBD is an additive fabrication technique based on the deposition of neutral metallic clusters carried in a highly collimated supersonic beam. SCBD produces strongly adherent conducting layers onto polymeric substrates with electrical resistance depending only on the thickness of the cluster-assembled film. A multi-step fabrication procedure alternating FFF and SCBD was developed and optimized allowing the fabrication of conductive 3D oblique paths, bridging vias, and sockets for standard electronic components fitting. This resulted in the simplification of the topology of planar electric circuits by enabling out-of-plane connections, minimizing the implementation of bulky passive electrical components and avoiding the use of soldering and conductive adhesives for the integration of active electronic components. A dark-activated light sensor was produced as a demonstrator

Facile fabrication of complex networks of memristive devices

We describe the memristive properties of cluster-assembled gold films. We show that resistive switching is observed in pure metallic nanostructured films at room temperature and atmospheric pressure, in response to applied voltage inputs. In particular, we observe resistance changes up to 400% and archetypal switching events that have remarkable symmetry with the applied voltage. We associated this symmetry with 'potentiation' and 'anti-potentiation' processes involving the activation of synapses and of pathways comprising multiple synapses. The stability and reproducibility of the resistance switching, which lasted over many hours, make these devices ideal test-beds for exploration of the basic mechanisms of the switching processes, and allow convenient fabrication of devices that may have neuromorphic properties

Supersonic cluster beam printing of carbon microsupercapacitors on paper

Paper is a cheap, lightweight and renewable material with increasing applicative interest as a substrate for disposable and flexible electronics. The integration of planar energy storage devices on paper is a necessary and challenging step for the development of smart and autonomous flexible electronic platforms. Here we report the one-step, room temperature, fabrication of planar microsupercapacitors where nanostructured current collectors and carbon electrodes are deposited by supersonic cluster beam deposition (SCBD) on plain paper sheets and 1-Ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl) imide ([Emim][NTf2]) is used as ionic liquid electrolyte. As-prepared microsupercapacitors showed a high capacitance density of about 7 F cm 123 and good capacity retention upon prolonged cycling. The encapsulation of SCBD-made microsupercapacitors by means of a polydimethylsiloxane layer and their usability in driving a low power temperature sensor are demonstrate

Integrated Simultaneous Detection of Tactile and Bending Cues for Soft Robotics

Soft robots should move in an unstructured environment and explore it and, to do so, they should be able to measure and distinguish proprioceptive and exteroceptive stimuli. This can be done by embedding mechanosensing systems in the body of the robot. Here, we present a polydimethylsiloxane block sensorized with an electro-optical system and a resistive strain gauge made with the supersonic cluster beam implantation (SCBI) technique. We show how to integrate these sensing elements during the whole fabrication process of the soft body and we demonstrate that their presence does not change the mechanical properties of the bulk material. Exploiting the position of both sensing systems and a proper combination of the output signals, we present a strategy to measure simultaneously external pressure and positive/negative bending of the body. In particular, the optical system can reveal any mechanical stimulation (external from the soft block or due to its own deformation), while the resistive strain gauge is insensitive to the external pressure, but sensitive to the bending of the body. This solution, here applied to a simple block of soft material, could be extended to the whole body of a soft robot. This approach provides detection and discrimination of the two stimuli (pressure and bending), with low computational effort and without significant mechanical constraint

A simple scanning spectrometer based on a stretchable elastomeric reflective grating

We report a scanning optical spectrometer based on the use of a stretchable elastomeric reflective grating. The grating is obtained by supersonic cluster beam implantation of silver nanoparticles on polydimethylsiloxane previously grooved by molding to create a replica of a commercial digital versatile disk grating. The use of a stretchable grating allows the spectrometer spanning the whole optical wavelength range by solely extending the diffraction element by more than 100% of its original dimensions. The stretchable reflective optical grating shows excellent performances and stability upon thousands of stretching cycles. The use of this elastomeric element makes the optical layout and the mechanics of the spectrometer extremely simple and advantageous for those applications where spectral resolution is not a major requirement. As a proof of principle, we present the absorption spectrum of Rhodamine B in solution obtained by our spectrometer and compared to commercial instruments

Low-voltage dielectric elastomer actuators with stretchable electrodes fabricated by supersonic cluster beam implantation

Supersonic cluster beam implantation of Ag nanoparticles is proposed for the fabrication of stretchable and compliant electrodes for dielectric elastomer actuators (DEAs) with reduced thickness. Thanks to the low-energy and finely tunable implantation process, a nanocomposite Ag/polydimethylsiloxane electrode layer is produced with a moderate stiffening effect for the DEA in contrast with a common deposition strategy for electrodes. Thin DEAs with an overall thickness of 17 μm were fabricated and tested under different preloading conditions, demonstrating a max uniaxial actuation strain of 2.5% at an actuation voltage of 765 V, lower than the typical voltage values of DEAs. The electrodes remained conductive up to 40% strain, and they fully recovered the original resistance after 70% stretching. Our results represent a significant step towards the development of DEAs operating at reduced actuation voltages, by stacking of micrometer-thick elastomer films, paving the way to novel applications in soft robotics

Conducting shrinkable nanocomposite based on au-nanoparticle implanted plastic sheet: Tunable thermally induced surface wrinkling

A thermally shrinkable and conductive nanocomposite material is prepared by supersonic cluster beam implantation (SCBI) of neutral Au nanoparticles (Au NPs) into a commercially available thermo-retractable polystyrene (PS) sheet. Micronanowrinkling is obtained during shrinking, which is studied by means of SEM, TEM and AFM imaging. Characteristic periodicity is determined and correlated with nanoparticle implantation dose, which permits us to tune the topographic pattern. Remarkable differences emerged with respect to the well-known case of wrinkling of bilayer metal-polymer. Wrinkled composite surfaces are characterized by a peculiar multiscale structuring that promises potential technological applications in the field of catalytic surfaces, sensors, biointerfaces, and optics, among others