Multi-material additive manufacturing has become a powerful technique to fabricate complex microfluidic devices by integrating multiple functional materials, enabling the seamless integration of structural, mechanical, sensing, and fluidic functionalities. This paper reviews the state-of-the-art in this research field. Specifically, different additive manufacturing technologies are presented. Extrusion-based techniques (e.g. fused filament fabrication, polyjet printing, direct ink writing) allow sequential deposition of various polymers through multiple nozzles, enabling the fabrication of microfluidic devices with embedded electrodes. However, extrusion methods generally offer limited resolution and require surface post-processing to achieve smooth fluidic channels. Conversely, light-based techniques (e.g. stereolithography, digital light processing and two-photon polymerization) yield micrometer-scale resolution but they are constrained to single-material fabrication. Several approaches to overcome this limitation are reviewed, including multi-vat printers, single-vat resin swapping systems, (micro)fluidic resin exchange, and wavelength-controlled dual-curing resins, to enable the fabrication of integrated microfluidic devices, which exhibit a variety of material properties (rigid, flexible, conductive, etc.) and functional elements. The remaining challenges include material compatibility and biocompatibility, preventing cross-contamination, and balancing high-resolution with throughput. The ongoing developments in materials and process optimization have resulted in improvements in the reliability of multi-material printing, driving the adoption of this technology for the fabrication of microfluidic devices, with the potential to be used in a variety of applications, including lab-on-a-chip technologies, clinical diagnostics, and point-of-care devices.

Handling ultra-thin semiconductor dies during assembly remains challenging due to their mechanical fragility and sensitivity to placement forces. Contactless die transfer methods reduce mechanical loading and have potential to enable higher placement speeds and improved throughput but introduce challenges such as uncontrolled impact dynamics, die rebound, and misalignment during landing. This work investigates a liquid-assisted contactless adhesive bonding approach in which droplets deposited on stencil-printed conductive adhesive act as transient damping and alignment media during die landing. Dummy silicon dies with a gold (Au) surface finish were released from a short distance onto the adhesive. Four liquids with different physical and chemical properties—deionized (DI) water, isopropyl alcohol (IPA), acetone, and liquid flux—were investigated to evaluate the influence of key fluid parameters, including surface tension, viscosity, and density, on die impact dynamics, stabilization, and subsequent bonding quality. The results demonstrate that droplet material properties significantly influence die landing dynamics in liquid-assisted contactless assembly. Among the investigated liquids, deionized water enabled reliable die entrapment and damping. In contrast, for the lower-surface-tension solvents isopropyl alcohol (IPA) and acetone, the droplets were not sufficiently confined under the experimental conditions, and therefore no direct die entrapment was observed, although impact damping was still achieved. Die entrapment and stabilization behavior were analyzed using high-speed camera recordings and complementary two-dimensional simulations performed in COMSOL Multiphysics. Furthermore, the impact of the droplets on the resulting bond interface and joint quality was evaluated through post-bond characterization. The study provides insight into droplet-mediated die landing and offers guidelines for selecting liquids for reliable contactless micro-assembly.

Dielectric elastomers (DEs) have received significant attention for their good performance among different smart material transducers. This study demonstrates the feasibility of fabricating dielectric elastomer actuators (DEAs) using exclusively inkjet printing technique. The manufactured unimorph bending cantilevers are composed of a polydimethylsiloxane (PDMS) active layer, sandwiched between two compliant electrodes, and printed onto a thin polyimide (PI) substrate. This study addresses the key fabrication challenges associated with inkjet printing such a layered actuator structure. This entails the consistent printing of the Ag electrodes on the smooth PI substrate, a PDMS layer on the Ag electrodes, the Ag electrodes on the smooth PDMS surface, and the respective steps of processing and curing. The fully inkjet-printed DEAs exhibited a maximum tip displacement of 36 mu m in quasi-static operation (1 kV(pp)) and 12.8 mu m in resonant operation (50 Hz, 800 V-pp). This is the first time that inkjet-printing has been employed to print an entire dielectric elastomer actuator, broadening the outlooks to develop innovative devices that base on smart material transducers.

Organ-on-a-chip (OoC) devices require the precise control of various media. This is mostly done using several fluid control components, which are much larger than the typical OoC device and connected through fluidic tubing, i.e., the fluidic system is not integrated, which inhibits the system’s portability. Here, we explore the limits of fluidic system integration using off-the-shelf fluidic control components. A flow control configuration is proposed that uses a vacuum to generate a fluctuation-free flow and minimizes the number of components used in the system. 3D printing is used to fabricate a custom-designed platform box for mounting the chosen smallest footprint components. It provides flexibility in arranging the various components to create experiment-specific systems. A demonstrator system is realized for lung-on-a-chip experiments. The 3D-printed platform box is 290 mm long, 240 mm wide and 37 mm tall. After integrating all the components, it weighs 4.8 kg. The system comprises of a switch valve, flow and pressure controllers, and a vacuum pump to control the diverse media flows. The system generates liquid flow rates ranging from 1.5 Lmin ^-1 to 68 Lmin ^-1 in the cell chambers, and a cyclic vacuum of 280 mbar below atmospheric pressure with 0.5 Hz frequency in the side channels to induce mechanical strain on the cells-substrate. The components are modular for easy exchange. The battery operated platform box can be mounted on either upright or inverted microscopes and fits in a standard incubator. Overall, it is shown that a compact integrated and portable fluidic system for OoC experiments can be constructed using off-the-shelf components. For further down-scaling, the fluidic control components, like the pump, switch valves, and flow controllers, require significant miniaturization while having a wide flow rate range with high resolution.

Flexible assembly automation is one of the solutions that have been put forward to meet market demands on increased product diversity and smaller lot-sizes, improved product quality, reduced delivery times and shortening product life-cycles. However, despite considerable research effort in the field of flexible assembly automation, the penetration in industry of the concepts and techniques of flexible assembly automation is still relatively low.The goal of this paper is to initiate discussions in the assembly research community on increasing the application potential of flexible assembly automation. A research structure is proposed, showing three highly interrelated elements that need further research in order to achieve increased industrialisation of flexible assembly automation: the assembly process, rules for product design for automated assembly and equipment suitable for automated assembly.

This paper describes a concept for development of flexible assembly systems (FAS's). Analysis of the state-of-the-art in FAS's shows three obstructions to industrial use of FAS's: (1) they do not suit the manufacturer's needs, (2) solutions lack flexibility and (3) strategic investment aspects are not considered.The described concept enables development of manufacturer specific solutions. The concept is strongly assembly process based. It is divided into two parts: development of an assembly process based toolkit and configuration of a product assortment based FAS out of the toolkit.

Reciprocating piezoelectric micropumps enable miniaturization in microfluidics for lab-on-a-chip applications such as organs-on-chips (OoC). However, achieving a steady flow when using these micropumps is a significant challenge because of flow ripples in the displaced liquid, especially at low frequencies or low flow rates (<50 µL/min). Although dampers are widely used for reducing ripples in a flow, their efficiency depends on the driving frequency of the pump. Here, we investigated multi-phase rectification as an approach to minimize ripples at low flow rates by connecting piezoelectric micropumps in parallel. The efficiency in ripple reduction was evaluated with an increasing number (n) of pumps connected in parallel, each actuated by an alternating voltage waveform with a phase difference of 2π/n (called multi-phase rectification) at a chosen frequency. We introduce a fluidic ripple factor (RFfl.), which is the ratio of the root mean square (RMS) value of the fluctuations present in the rectified output to the average fluctuation-free value of the discharge flow, as a metric to express the quality of the flow. The fluidic ripple factor was reduced by more than 90% by using three-phase rectification when compared to one-phase rectification in the 2–60 μL/min flow rate range. Analytical equations to estimate the fluidic ripple factor for a chosen number of pumps connected in parallel are presented, and we experimentally confirmed up to four pumps. The analysis shown can be used to design a frequency-independent multi-phase fluid rectifier to the desired ripple level in a flow for reciprocating pumps.

One of the essential requirements to create nanoparticle (NP)-based applications and functions is the ability to control their deposition in specific locations. Many methods have been proposed, with wet direct writing (DW) techniques such as inkjet printing being the most employed. These methods generally depend on off-line and solvent-based NP synthesis leading to contamination and impurity in the final NP film as well as inhomogeneity in the deposition caused by solution-substrate interactions. This paper introduces a dry aerosol direct writing (dADW) method, which combines spark ablation-based and solvent-free NP synthesis with spatially selective deposition using aerodynamic focusing in a vacuum chamber. The challenge is to print high-resolution lines and spots of nanoparticles with a diameter < 100 nm. We study two aerodynamic nozzle concepts, a converging nozzle (CN) and a sheath gas nozzle (SGN), and investigate numerically how their design, as well as operating parameters, relate to the deposition process performance. This is quantified by three criteria: contraction factor, focusing ratio, and collection efficiency. We also compared our numerical results to experimental assays by manufacturing two SGNs and three CNs and evaluating the performance of each nozzle in terms of resolution, sharpness and thickness of the line. Using one of the SGN designs with an outlet diameter of 248 mu m and an aerosol to total flow rate ratio of 0.17, we achieved a high-resolution line with a width of 67 mu m, i.e., equal to 27% of the nozzle diameter, when printing < 100 nm Au NPs. The presented additive manufacturing method enables, therefore, the creation of high-resolution and sharp patterns of metallic nanoparticles, which can be employed in a wide range of applications, ranging from interconnects to optical and gas sensors.

Microfluidic organs-on-chips (OoCs) technology has emerged as the trend for in vitro functional modeling of organs in recent years. Simplifying the complexities of the human organs under controlled perfusion of required fluids paves the way for accurate prediction of human organ functionalities and their response to interventions like exposure to drugs. However, in the state-of-the-art OoC, the existing methods to control fluids use external bulky peripheral components and systems much larger than the chips used in experiments. A new generation of compact microfluidic flow control systems is needed to overcome this challenge. This study first presents a structured classification of OoC devices according to their types and microfluidic complexities. Next, we suggest three fundamental fluid flow control mechanisms and define component configurations for different levels of OoC complexity for each respective mechanism. Finally, we propose an architecture integrating modular microfluidic flow control components and OoC devices on a single platform. We emphasize the need for miniaturization of flow control components to achieve portability, minimize sample usage, minimize dead volume, improve the flowing time of fluids to the OoC cell chamber, and enable long-duration experiments.

Metastructures composed of snapping beams are capable of deforming into a series of stable states, enabling them to realize shape reconfigurations. In this paper, we present the design of a metastructure-based morphing surface that is able to exhibit a series of stable configurations with different curvatures. Using theoretical, numerical, and experimental approaches, we study the snap-through transition between the initially flat and the curved stable configurations. Effects of geometric parameters on the snap-through and curvatures are systematically investigated. Results show that the beam thickness is important for tuning the snap-through response, while the curvature can be tuned by changing the beam height and the horizontal span of the structures. Furthermore, an analytical model is developed to investigate the structural nonlinear deformations. It is shown that the proposed model can predict the snap-through transition properly. The structural stability can be controlled by setting proper values for t/L and h/L (t, h, and L represents the beam thickness, height and span, respectively). Finally, it is demonstrated that based on two-dimensional arrangements of bi-stable elements, various stable configurations, like corrugations in different directions, can be imposed to the surface.

The ability to tune the localised surface plasmon resonance (LSPR) behaviour of metal nanostructures has great importance for many optical sensor applications such as metal (plasmon) enhanced fluorescence spectroscopy and surface-enhanced Raman scattering (SERS). In this paper, we used Aerosol Direct Writing (ADW) to selectively deposit fine gold nanoparticles (AuNPs) patterns. A low-temperature thermal post-treatment (below 200 °C) provides enough energy to merge and transform AuNPs into larger features significantly different from non-thermally treated samples. The optical behaviour of non-treated and thermally treated AuNP films was investigated by photoluminescence (PL) spectroscopy. The PL measurements showed a red-shift, compared to bulk gold, using 488 nm and 514 nm laser excitation, and a blue-shift using 633 nm laser excitation. The thermal post-treatment leads to a further blue-shift compared to non-treated samples in the presence of both 514 and 633 nm laser. Finally, the AuNPs patterns were employed as a SERS-active substrate to detect low-concentrated (10-8 M) rhodamine B. This method's ability to selectively deposit 3D gold nanostructures and tune their optical behaviour through a low-temperature thermal treatment allows optimisation of the optical response and enhancement of the Raman signal for specific bio-analytes.

Multi-stable metastructures composed of curved beams can switch to a series of stable configurations via elastic snap-through transitions. The elastic deformations allow metastructures to function as reusable energy absorbers. However, conventional metastructure designs based on solid beams often result in relatively low energy dissipation. In this work, it is found that by increasing the beam unit's bending stiffness while keeping the volume/mass constant, energy dissipation of the metastructure can be largely improved. Based on this observation, we propose two types of structural designs (lattice and hollow cross-section design) as building blocks for multi-stable metastructures. The lattice design is realized by incorporating lattice structures into pre-shaped beams while for the hollow cross-section design, a box-shaped cross section is adopted. The proposed structures are experimentally characterized under cyclic loading and are shown to exhibit sequential snap-through transitions with relatively large energy dissipation. Results show the snap-through behavior can be further tailored through tuning structural in-plane thickness. Effects of geometric parameters on snap-through, local buckling and bi-stability are investigated, and the feasible design domains for selecting proper lattice and cross-section geometries are identified. In addition, we demonstrate that the proposed design is not restricted to beams, and can be extended to shell structures.

Metastructures consisting of planar arrangements of bi-stable snap-through beams are able to exhibit multiple stable configurations. Apart from the expected translational state transition, when all beam elements snap through, rotational states may exist as well. In this paper we explore the rotational properties of multi-stable metastructures on the basis of both experimental and theoretical investigations, and define the conditions for achieving rotational stable states. Results show that the metastructure is able to realize both translational and rotational states, while the rotational transitions require less energy as compared to their translational counterparts. The influence of geometric parameters on rotational stability is investigated via parametric studies. Furthermore, to determine the design criteria for rotational stability, a theoretical investigation based on mode superposition principle is performed to predict the nonlinear-deformation of a unit cell. The theoretical analysis predicts well the rotational snap-through transitions that are observed in finite element simulations. It is found that the rotational stability is determined by setting proper values for h/L and t/L (h, t, L represent apex height, thickness and span of the bi-stable beam structure, respectively). Finally, we experimentally demonstrate that the proposed metastructure with multiple layers is able to achieve large rotations and translations.

Microvalves are important flow-control devices in many standalone and integrated microfluidic applications. Polydimethylsiloxane (PDMS)-based pneumatic microvalves are commonly used but they generally require large peripheral connections that decrease portability. There are many alternatives found in the literature that use Si-based microvalves, but variants that can throttle even moderate pressures (1 bar) tend to be bulky (cm-range) or consume high power. This paper details the development of a low-power, normally-open piezoelectric microvalve to control flows with a maximum driving pressure of 1 bar, but also retain a small effective form-factor of 5 mm × 5 mm × 1.8 mm. A novel combination of rapid prototyping methods like stereolithography and laser-cutting have been used to realize this device. The maximum displacement of the fabricated piezoelectric microactuator was measured to be 8.5 μm at 150 V. The fabricated microvalve has a flow range of 0–90 μL min−1 at 1 bar inlet pressure. When fully closed, a leakage of 0.8% open-flow was observed with a power-consumption of 37.5 μW. A flow resolution of 0.2 μL min−1—De-ionized (DI) water was measured at 0.5 bar pressure.

Surface-enhanced Raman scattering (SERS) substrates are of great interest for detecting low-concentrated analytes. However, issues such as multistep processing, cost, and possible presence of hazardous substances in the fabrication still represent a significant drawback. In this paper, an innovative direct writing method is introduced for solvent-free and spatially selective deposition of fine metal copper nanoparticles (CuNPs), with size distribution below 20 nm, generated in-line through a spark ablation method (SAM). The deposited CuNPs' morphology and composition were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), and energy-dispersive X-ray spectroscopy (EDS). The resulting CuNP patterns feature porous 3D microdomains with nanometric structures serving as hot spots for Raman signal enhancement. Low-temperature post-treatment (below 200 degrees C) of the deposited CuNPs significantly evolves its morphology and leads to sintering of NPs into a semicrystalline structure with sharp geometric features, which resulted in a more than 10-fold increase of the enhancement factor (up to 2.1 x 10 5) compared to non-heat-treated samples. The proposed method allows creating SERS substrates constituted by sharp 3D metallic nanopatterns selectively deposited onto specific regions, which paves the way for new printed, highly sensitive SERS-based sensors.

Multi-stable structures are able to achieve significant geometric change and retain specific deformed configurations after the loads have been removed. This reconfiguration property enables, for example, to design metamaterials with tunable features. In this work, a type of multi-stable metastructures exhibiting both level and tilted stable configurations is proposed based on 2D and 3D arrangements of bi-stable elements. The resulting level and tilted configurations are enabled by the rotational compliance, bi-stability and spatial arrangement of unit cells. The bi-stability of the unit cells and multi-stability of the metastructures are demonstrated and characterized by experiments and finite element analysis. Results show that transitions between level stable configurations are symmetric in terms of load–deflection response while switching to the tilted stable configurations leads to asymmetric mechanical responses. The tilted stable configurations are less stable than the level configurations. Moreover, we demonstrate that the level and tilted stable configurations of the metastructure depend on the parallel and serial arrangement of the unit cells.

Photonic packaging, which includes high-precision assembly of photonic sub-systems, is currently a bottleneck in the development of commercially-available integrated photonic products. In the pursuit of a fully-automated, high-precision, and cost-effective photonic alignment scheme for two multi-channel photonic chips, this paper explores different designs of the on-chip electrothermal actuators for positioning mechanically-flexible waveguide structures. The final alignment goal is ∼100 nm waveguide to waveguide. The on-chip actuators, particularly for out-of-plane actuation, are built in a 16 μm-thick SiO 2 photonic-material stack with 5 μm-thick poly-Si as an electrothermal element. A major challenge of out-of-plane positioning is a 6 μm height difference of the waveguides to be aligned, due to different built-up material stacks, together with a misalignment tolerance of 1 μm–2 μm from the pre-assembly (flip-chip) process. Therefore, the bimorph-actuator design needs to compensate this height difference, and provide sufficient motion to align the waveguides. We propose to exploit the post-release deformation of so-called short-loop bimorph actuator designs to meet these joint demands. We explore different design variants based on the heater location and the integration of actuator beams with waveguide beams. The actuator design (with 30 μm poly-Si and 900 μm SiO 2 in length) has ∼8 μm out-of-plane deflection and is able to generate ∼4 μm motion, which meets the design goal.

This paper describes the design, fabrication and characterization of an in-plane positioning system within a thick (16 micrometers) silicon dioxide photonic-material stack. This is part of a proposed novel photonic alignment scheme, targeting at highly-automated assembly and high-precision alignment of multi-port photonic chips. Creating such functionality in thick silicon dioxide is challenging because of its low coefficient of thermal expansion, as well as the stresses present in the material. A design is proposed which addresses both challenges, and which in fact makes positive use of the present stress. The in-plane positioning system combines an electrothermal chevron actuator and a lever mechanism, aiming to achieve several micrometer displacement. The lever structure is proposed to amplify the motion of the chevron actuator. The shuttle of the chevron actuator and the lever are provided with a set of hooks. The hooks engage during the fabrication of the structure, because of the stress-induced retraction of the chevron actuator. With this design, a robust fabrication yield was achieved. The characterization work includes analyzing the engagement between the hook and chevron actuator, and the in-plane displacement with the lever enhancement.