Evolution of the Microrobots: Stimuli-Responsive Materials and Additive Manufacturing Technologies Turn Small Structures into Microscale Robots.

The development of functional microsystems and microrobots that have characterized the last decade is the result of a synergistic and effective interaction between the progress of fabrication techniques and the increased availability of smart and responsive materials to be employed in the latter. Functional structures on the microscale have been relevant for a vast plethora of technologies that find application in different sectors including automotive, sensing devices, and consumer electronics, but are now also entering medical clinics. Working on or inside the human body requires increasing complexity and functionality on an ever-smaller scale, which is becoming possible as a result of emerging technology and smart materials over the past decades. In recent years, additive manufacturing has risen to the forefront of this evolution as the most prominent method to fabricate complex 3D structures. In this review, we discuss the rapid 3D manufacturing techniques that have emerged and how they have enabled a great leap in microrobotic applications. The arrival of smart materials with inherent functionalities has propelled microrobots to great complexity and complex applications. We focus on which materials are important for actuation and what the possibilities are for supplying the required energy. Furthermore, we provide an updated view of a new generation of microrobots in terms of both materials and fabrication technology. While two-photon lithography may be the state-of-the-art technology at the moment, in terms of resolution and design freedom, new methods such as two-step are on the horizon. In the more distant future, innovations like molecular motors could make microscale robots redundant and bring about nanofabrication.

Preparation of different conjugated polymers characterized by complementary electronic properties from an identical precursor.

The possibility of generating regions with different electronic properties within the same organic semiconductor thin film could offer novel opportunities for designing and fabricating organic electronic devices and circuits. This study introduces a new approach based on a novel type of highly processable polymer precursor that can yield two different conjugated polymers characterized by complementary electronic properties, i.e. promoting electron or hole transport, from the same starting material. In particular, these multipotent precursors comprise functionalized dihydroanthracene units that can offer several functionalization opportunities to improve the solubility or insert specific functionalities. This strategy also allows for the preparation of high-molecular-weight conjugated polymers comprising diethynylanthracene and anthraquinone units without the need for solubilizing side chains. Thin films of the polymer precursor can be used, after solid-state transformations, to prepare single organic layers comprising regions characterized by different chemical nature and electronic properties. Here, we present a detailed characterization of the chemical and electronic properties of the precursor and the obtained conjugated polymers, showing how it is possible to harvest their characteristics for potential applications such as electrochromic surfaces and organic field-effect transistors.

Invisible Thermoplasmonic Indium Tin Oxide Nanoparticle Ink for Anti-counterfeiting Applications.

In this study, we present a thermoplasmonic transparent ink based on a colloidal dispersion of indium tin oxide (ITO) nanoparticles, which can offer several advantages as anti-counterfeiting technology. The custom ink could be directly printed on several substrates, and it is transparent under visible light but is able to generate heat by absorption of NIR radiation. Dynamic temperature mapping of the printed motifs was performed by using a thermal camera while irradiating the samples with an IR lamp. The printed samples presented fine features (in the order of 75 µm) and high thermal resolution (of about 250 µm). The findings are supported by thermal finite-element simulations, which also allow us to explore the effect of different substrate characteristics on the thermal readout. Finally, we built a demonstrator comprising a QR Code invisible to the naked eye, which became visible in thermal images under NIR radiation. The high transparency of the printed ink and the high speed of the thermal reading (figures appear/disappear in less than 1 s) offer an extremely promising strategy toward low-cost, scalable production of photothermally active invisible labels.

Empirical Parameter to Compare Molecule-Electrode Interfaces in Large-Area Molecular Junctions.

This paper describes a simple model for comparing the degree of electronic coupling between molecules and electrodes across different large-area molecular junctions. The resulting coupling parameter can be obtained directly from current-voltage data or extracted from published data without fitting. We demonstrate the generalizability of this model by comparing over 40 different junctions comprising different molecules and measured by different laboratories. The results agree with existing models, reflect differences in mechanisms of charge transport and rectification, and are predictive in cases where experimental limitations preclude more sophisticated modeling. We also synthesized a series of conjugated molecular wires, in which embedded dipoles are varied systematically and at both molecule-electrode interfaces. The resulting current-voltage characteristics vary in nonintuitive ways that are not captured by existing models, but which produce trends using our simple model, providing insights that are otherwise difficult or impossible to explain. The utility of our model is its demonstrative generalizability, which is why simple observables like tunneling decay coefficients remain so widely used in molecular electronics despite the existence of much more sophisticated models. Our model is complementary, giving insights into molecule-electrode coupling across series of molecules that can guide synthetic chemists in the design of new molecular motifs, particularly in the context of devices comprising large-area molecular junctions.

A Simple Approach for Flexible and Stretchable Anti-icing Lubricant-Infused Tape.

Unwanted icing has major safety and economic repercussions on human activities, affecting means of transportation, infrastructures, and consumer goods. Compared to the common deicing methods in use today, intrinsically icephobic surfaces can decrease ice accumulation and formation without any active intervention from humans or machines. However, such systems often require complex fabrication methods and can be costly, which limits their applicability. In this study, we report the preparation and characterization of several slippery lubricant-infused porous surfaces (SLIPSs) realized by impregnating with silicone oil a candle soot layer deposited on double-sided adhesive tape. Despite the use of common household items, these SLIPSs showed anti-icing performance comparable to other systems described in the literature (ice adhesion < 20 kPa) and a good resistance to mechanical and environmental damages in laboratory conditions. The use of a flexible and functional substrate as tape allowed these devices to be stretchable without suffering significant degradation and highlights how these systems can be easily prepared and applied anywhere needed. In addition, the possibility of deforming the substrate can "allow" the application of SLIPS technology in mechanical ice removal methodologies, drastically incrementing their performance.

Toward Mechanochromic Soft Material-Based Visual Feedback for Electronics-Free Surgical Effectors.

A chromogenically reversible, mechanochromic pressure sensor is integrated into a mininvasive surgical grasper compatible with the da Vinci robotic surgical system. The sensorized effector, also featuring two soft-material jaws, encompasses a mechanochromic polymeric inset doped with functionalized spiropyran (SP) molecule, designed to activate mechanochromism at a chosen pressure and providing a reversible color change. Considering such tools are systematically in the visual field of the operator during surgery, color change of the mechanochromic effector can help avoid tissue damage. No electronics is required to control the devised visual feedback. SP-doping of polydimethylsiloxane (2.5:1 prepolymer/curing agent weight ratio) permits to modulate the mechanochromic activation pressure, with lower values around 1.17 MPa for a 2% wt. SP concentration, leading to a shorter chromogenic recovery time of 150 s at room temperature (25 °C) under green light illumination. Nearly three-times shorter recovery time is observed at body temperature (37 °C). To the best of knowledge, this study provides the first demonstration of mechanochromic materials in surgery, in particular to sensorize unpowered surgical effectors, by avoiding dramatic increases in tool complexity due to additional electronics, thus fostering their application. The proposed sensing strategy can be extended to further tools and scopes.

Engineering Microneedle Patches for Improved Penetration: Analysis, Skin Models and Factors Affecting Needle Insertion.

Transdermal microneedle (MN) patches are a promising tool used to transport a wide variety of active compounds into the skin. To serve as a substitute for common hypodermic needles, MNs must pierce the human stratum corneum (~ 10 to 20 µm), without rupturing or bending during penetration. This ensures that the cargo is released at the predetermined place and time. Therefore, the ability of MN patches to sufficiently pierce the skin is a crucial requirement. In the current review, the pain signal and its management during application of MNs and typical hypodermic needles are presented and compared. This is followed by a discussion on mechanical analysis and skin models used for insertion tests before application to clinical practice. Factors that affect insertion (e.g., geometry, material composition and cross-linking of MNs), along with recent advancements in developed strategies (e.g., insertion responsive patches and 3D printed biomimetic MNs using two-photon lithography) to improve the skin penetration are highlighted to provide a backdrop for future research.

Direct laser writing of liquid crystal elastomers oriented by a horizontal electric field.

Background: The ability to fabricate components capable of performing actuation in a reliable and controlled manner is one of the main research topics in the field of microelectromechanical systems (MEMS). However, the development of these technologies can be limited in many cases by 2D lithographic techniques employed in the fabrication process. Direct Laser Writing (DLW), a 3D microprinting technique based on two-photon polymerization, can offer novel solutions to prepare, both rapidly and reliably, 3D nano- and microstructures of arbitrary complexity. In addition, the use of functional materials in the printing process can result in the fabrication of smart and responsive devices. Methods: In this study, we present a novel methodology for the printing of 3D actuating microelements comprising Liquid Crystal Elastomers (LCEs) obtained by DLW. The alignment of the mesogens was performed using a static electric field (1.7 V/µm) generated by indium-tin oxide (ITO) electrodes patterned directly on the printing substrates. Results: When exposed to a temperature higher than 50°C, the printed microstructures actuated rapidly and reversibly of about 8% in the direction perpendicular to the director. Conclusions: A novel methodology was developed that allows the printing of directional actuators comprising LCEs via DLW. To impart the necessary alignment of the mesogens, a static electric field was applied before the printing process by making use of flat ITO electrodes present on the printing substrates. The resulting microelements showed a reversible change in shape when heated higher than 50 °C.

Mechanochromic LLDPE Films Doped with NIR Reflective Paliogen Black.

The perylene bisimide derivative Paliogen Black (P-black) is proposed as a new chromogenic probe that shows visible (vis) and near-infrared (NIR) responses after mechanical solicitations of host linear low-density polyethylene (LLDPE) films. P-black is reported to display strong absorption in the vis spectrum and unusual reflective and cooling features in the NIR region. Uniaxial deformation of the 2.5, 5, and 10 wt% P-black/LLDPE films yields a dichroic absorption under polarized light with color variations attributed by the computational analysis to the distinct anisotropic behavior of the transition dipole moments of P-black chromophores. When LLDPE films are deformed, P-black aggregates reduce their size from ≈30-40 µm to ≈5-10 µm that, in turn, causes reflectivity losses of about 30-40% at the maximum elongation. This gives rise to warming of 5-6 °C of the locally oriented film placed in contact with a black substrate under the illumination with an IR lamp for 5 s. These features combined with the high sensitivity of the vis-NIR response toward mechanical solicitations render P-black as a new solution to detect uniaxial deformations of plastic films through both optical and thermal outputs.

High-Performance Luminescent Solar Concentrators Based on Poly(Cyclohexylmethacrylate) (PCHMA) Films.

In this study, we report on the use of poly(cyclohexylmethacrylate) (PCHMA) as an alternative to the commonly used poly(methylmethacrylate) (PMMA) for the design of efficient luminescent solar concentrators (LSCs). PCHMA was selected due to its less polar nature with respect to PMMA, a characteristic that was reported to be beneficial in promoting the fluorophore dispersibility in the matrix, thus maximizing the efficiency of LSCs also at high doping. In this sense, LSC thin films based on PCHMA and containing different contents of Lumogen F Red 305 (LR, 0.2-1.8 wt%) demonstrated optical efficiencies (ηopt) comprising between 9.5% and 10.0%, i.e., about 0.5-1% higher than those collected from the LR/PMMA systems. The higher LR/polymer interactions occurred using the PCHMA matrix maximized the solar harvesting characteristics of the fluorophore and limited the influence of the adverse dissipative phenomena on the fluorophore quantum efficiency. These effects were also reflected by varying the LSC film thickness and reaching maximum ηopt of about 11.5% in the case of PCHMA films of about 30 µm.

Intermolecular Effects on Tunneling through Acenes in Large-Area and Single-Molecule Junctions.

This paper describes the conductance of single-molecules and self-assembled monolayers comprising an oligophenyleneethynylene core, functionalized with acenes of increasing length that extend conjugation perpendicular to the path of tunneling electrons. In the Mechanically Controlled Break Junction (MCBJ) experiment, multiple conductance plateaus were identified. The high conductance plateau, which we attribute to the single molecule conformation, shows an increase of conductance as a function of acene length, in good agreement with theoretical predictions. The lower plateau is attributed to multiple molecules bridging the junctions with intermolecular interactions playing a role. In junctions comprising a self-assembled monolayer with eutectic Ga-In top-contacts (EGaIn), the pentacene derivative exhibits unusually low conductance, which we ascribe to the inability of these molecules to pack in a monolayer without introducing significant intermolecular contacts. This hypothesis is supported by the MCBJ data and theoretical calculations showing suppressed conductance through the PC films. These results highlight the role of intermolecular effects and junction geometries in the observed fluctuations of conductance values between single-molecule and ensemble junctions, and the importance of studying molecules in both platforms.

Electrically-Conductive Polyketone Nanocomposites Based on Reduced Graphene Oxide.

In this work, we investigated the functionalization of polyketone 30 (PK30) with glycyl-glycine (Gly-Gly) via the Paal-Knorr reaction with the aim of homogenously dispersing two types of reduced graphene oxide (rGO, i.e., lrGO and hrGO, the former characterized by a lower degree of reduction in comparison to the latter) by non-covalent interactions. The functional PK30-Gly-Gly polymer was effective in preparing composites with homogeneously distributed rGO characterized by an effective percolation threshold at 5 wt. %. All the composites showed a typical semiconductive behavior and stable electrical response after several heating/cooling cycles from 30 to 115 °C. Composites made by hrGO displayed the same resistive behaviour even if flanked by a considerable improvement on conductivity, in agreement with the more reduced rGO content. Interestingly, no permanent percolative network was shown by the composite with 4 wt. % of lrGO at temperatures higher than 45 °C. This material can be used as an ON-OFF temperature sensor and could find interesting applications as sensing material in soft robotics applications.

Functional Materials for Two-Photon Polymerization in Microfabrication.

Direct laser writing methods based on two-photon polymerization (2PP) are powerful tools for the on-demand printing of precise and complex 3D architectures at the micro and nanometer scale. While much progress was made to increase the resolution and the feature size throughout the years, by carefully designing a material, one can confer specific functional properties to the printed structures thus making them appealing for peculiar and novel applications. This Review summarizes the state-of-the-art of functional resins and photoresists used in 2PP, discussing both the range of material functions available and the methods used to prepare them, highlighting advantages and disadvantages of different classes of materials in achieving certain properties.

Systematic experimental study of quantum interference effects in anthraquinoid molecular wires.

In order to translate molecular properties in molecular-electronic devices, it is necessary to create design principles that can be used to achieve better structure-function control oriented toward device fabrication. In molecular tunneling junctions, cross-conjugation tends to give rise to destructive quantum interference effects that can be tuned by changing the electronic properties of the molecules. We performed a systematic study of the tunneling charge-transport properties of a series of compounds characterized by an identical cross-conjugated anthraquinoid molecular skeleton but bearing different substituents at the 9 and 10 positions that affect the energies and localization of their frontier orbitals. We compared the experimental results across three different experimental platforms in both single-molecule and large-area junctions and found a general agreement. Combined with theoretical models, these results separate the intrinsic properties of the molecules from platform-specific effects. This work is a step towards explicit synthetic control over tunneling charge transport targeted at specific functionality in (proto-)devices.

Correction: Systematic experimental study of quantum interference effects in anthraquinoid molecular wires.

[This corrects the article DOI: 10.1039/C8NA00223A.].

Quantum interference mediated vertical molecular tunneling transistors.

Molecular transistors operating in the quantum tunneling regime represent potential electronic building blocks for future integrated circuits. However, due to their complex fabrication processes and poor stability, traditional molecular transistors can only operate stably at cryogenic temperatures. Here, through a combined experimental and theoretical investigation, we demonstrate a new design of vertical molecular tunneling transistors, with stable switching operations up to room temperature, formed from cross-plane graphene/self-assembled monolayer (SAM)/gold heterostructures. We show that vertical molecular junctions formed from pseudo-p-bis((4-(acetylthio)phenyl)ethynyl)-p-[2,2]cyclophane (PCP) SAMs exhibit destructive quantum interference (QI) effects, which are absent in 1,4-bis(((4-acetylthio)phenyl)ethynyl)benzene (OPE3) SAMs. Consequently, the zero-bias differential conductance of the former is only about 2% of the latter, resulting in an enhanced on-off current ratio for (PCP) SAMs. Field-effect control is achieved using an ionic liquid gate, whose strong vertical electric field penetrates through the graphene layer and tunes the energy levels of the SAMs. The resulting on-off current ratio achieved in PCP SAMs can reach up to ~330, about one order of magnitude higher than that of OPE3 SAMs. The demonstration of molecular junctions with combined QI effect and gate tunability represents a critical step toward functional devices in future molecular-scale electronics.

Two-Terminal Molecular Memory through Reversible Switching of Quantum Interference Features in Tunneling Junctions.

Large-area molecular tunneling junctions comprising self-assembled monolayers of redox-active molecules are described that exhibit two-terminal bias switching. The as-prepared monolayers undergo partial charge transfer to the underlying metal substrate (Au, Pt, or Ag), which converts their cores from a quinoid to a hydroquinoid form. The resulting rearomatization converts the bond topology from a cross-conjugated to a linearly conjugated π system. The cross-conjugated form correlates to the appearance of an interference feature in the transmission spectrum that vanishes for the linearly conjugated form. Owing to the presence of electron-withdrawing nitrile groups, the reduction potential and the interference feature lie close to the work function and Fermi level of the metallic substrate. We exploited the relationship between conjugation patterns and quantum interference to create nonvolatile memory in proto-devices using eutectic Ga-In as the top contact.

Controlling destructive quantum interference in tunneling junctions comprising self-assembled monolayers via bond topology and functional groups.

Quantum interference effects (QI) are of interest in nano-scale devices based on molecular tunneling junctions because they can affect conductance exponentially through minor structural changes. However, their utilization requires the prediction and deterministic control over the position and magnitude of QI features, which remains a significant challenge. In this context, we designed and synthesized three benzodithiophenes based molecular wires; one linearly-conjugated, one cross-conjugated and one cross-conjugated quinone. Using eutectic Ga-In (EGaIn) and CP-AFM, we compared them to a well-known anthraquinone in molecular junctions comprising self-assembled monolayers (SAMs). By combining density functional theory and transition voltage spectroscopy, we show that the presence of an interference feature and its position can be controlled independently by manipulating bond topology and electronegativity. This is the first study to separate these two parameters experimentally, demonstrating that the conductance of a tunneling junction depends on the position and depth of a QI feature, both of which can be controlled synthetically.

Conformation-driven quantum interference effects mediated by through-space conjugation in self-assembled monolayers.

Tunnelling currents through tunnelling junctions comprising molecules with cross-conjugation are markedly lower than for their linearly conjugated analogues. This effect has been shown experimentally and theoretically to arise from destructive quantum interference, which is understood to be an intrinsic, electronic property of molecules. Here we show experimental evidence of conformation-driven interference effects by examining through-space conjugation in which π-conjugated fragments are arranged face-on or edge-on in sufficiently close proximity to interact through space. Observing these effects in the latter requires trapping molecules in a non-equilibrium conformation closely resembling the X-ray crystal structure, which we accomplish using self-assembled monolayers to construct bottom-up, large-area tunnelling junctions. In contrast, interference effects are completely absent in zero-bias simulations on the equilibrium, gas-phase conformation, establishing through-space conjugation as both of fundamental interest and as a potential tool for tuning tunnelling charge-transport in large-area, solid-state molecular-electronic devices.

Pronounced Environmental Effects on Injection Currents in EGaIn Tunneling Junctions Comprising Self-Assembled Monolayers.

Large-area tunneling junctions using eutectic Ga-In (EGaIn) as a top contact have proven to be a robust, reproducible, and technologically relevant platform for molecular electronics. Thus far, the majority of studies have focused on saturated molecules with backbones consisting mainly of alkanes in which the frontier orbitals are either highly localized or energetically inaccessible. We show that self-assembled monolayers of wire-like oligophenyleneethynylenes (OPEs), which are fully conjugated, only exhibit length-dependent tunneling behavior in a low-O2 environment. We attribute this unexpected behavior to the sensitivity of injection current on environment. We conclude that, contrary to previous reports, the self-limiting layer of Ga2O3 strongly influences transport properties and that the effect is related to the wetting behavior of the electrode. This result sheds light on the nature of the electrode-molecule interface and suggests that adhesive forces play a significant role in tunneling charge-transport in large-area molecular junctions.