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The opportunity to create magneto‐responsive soft materials (MSMs) with in‐process tailorable and locally controllable magnetic properties is highly desirable across many technological and biomedical applications. In this paper, this capability is demonstrated for the first time using computer‐controlled dual‐material aerosol jet printing (DMAJP) technology. This approach allows controlled variation of composition between the aerosols of a magnetic nanoparticles (MNPs) ink and a photocurable polymer during the printing process. The mixing ratio of the two aerosols determines the MNPs loading in the nanocomposite, which can be used to locally control the magnetic properties of the printed structures. The printing process is structured in a layer‐by‐layer fashion in combination with a sacrificial layer approach for building fully freestanding MSM structures that combine magnetoactive and non‐magnetoactive elements in a single process multi‐material printing method with no further assembly requirements. Using this method, the direct manufacturing of small‐scale multi‐material soft objects with complex shapes and programmable functions whose movements can be controlled by the application of an external magnetic field is demonstrated. [ABSTRACT FROM AUTHOR]

Endothelial cells (ECs) line the inner walls of blood vessels, respond to shear stress by elongating in the direction of flow. Engineering aligned ECs in vitro is essential for modeling human vascular diseases and for drug testing. Current microfluidic approaches mainly rely on unidirectional laminar flow, uniform coating of surfaces to improve cellular adhesion or alteration of the surface topography. Challenges persist due to shear stress‐induced changes in cellular behavior, especially in complex multicellular environments and the time needed for the cells to align and polarize inside the microfluidic conduits. Generally, protein coating processes and physical treatments are also not compatible with the steps required for the assembly of microfluidic devices. This approach employs aerosol jet printing (AJP) to precisely pattern poly(3,4‐ethylenedioxythiophene) polystyrene sulphonate (PEDOT:PSS) within microfluidic chambers in a single step. It is shown that the PEDOT:PSS is biocompatible and facilitates EC adhesion, patterning, elongation, and alignment. Under capillary flow, the cells retain their pattern‐induced morphology over 7 d, confirming the efficacy of the approach in promoting cellular organization, eliminating the need for external pumps. Furthermore, it is demonstrated that the PEDOT:PSS pattern retains structural integrity and electrical stability following oxygen plasma treatment, required for assembling of fully enclosed microfluidic devices. [ABSTRACT FROM AUTHOR]

Additive manufacturing (AM) is rapidly revolutionizing modern manufacturing with recent progress in advanced printing methods and improved properties of printed materials. However, traditional AM methods are limited by their input‐oriented nature, which demands tedious trial‐and‐error tuning of printing parameters to achieve desired output properties. Here, an output‐oriented artificial intelligence‐integrated AM (AIAM) method is reported that enables an user to specify desired output properties while the printer autonomously discovers the optimal input printing parameters by integrating hybrid machine learning models and in situ measurements. Based on a predictive mapping between the input printing parameters and the output properties of interests established with <20 experiments designed by active learning, inverse design tasks are performed to intelligently generate the printing parameter settings that lead to desired outcomes using reinforcement learning. This method is demonstrated by autonomous aerosol jet printing (AJP) of conductive polymer films and achieving user‐defined electrical resistances with an ultralow error of 3.7%. The AIAM method, with its output‐oriented nature, holds the potential to significantly improve the autonomy, predictability, efficiency, and accessibility of the AM processes, which will unlock new possibilities in the autonomous and intelligent printing of a broad range of functional materials and devices. [ABSTRACT FROM AUTHOR]

Aerosol jet printing (AJP) is an emerging direct write tool enabling rapid prototyping and fabrication of electronics components. AJP provides faster and less expensive production of devices with feature sizes of >10 microns compared to traditional MEMS processes. Herein, the fabrication of a resistance temperature detector‐micro hotplate (RTD‐µHP) is reported using AJP printed silver. AJP eliminates sophisticated MEMS processes such as masking, alignment, and etching. The compatibility of the AJP process with a broad range of materials is demonstrated by printing highly resolved lines on rigid and flexible substrates. Optimal thermal sintering conditions of AJP printed silver (Ag) lines are found by in situ resistance measurements. To stabilize the temperature coefficient of resistance (TCR) of the RTD‐µHP at high operating current levels, electrical sintering is performed on the RTD‐µHPs. Electrical sintering improves the conductivity of fully thermally sintered Ag RTD‐µHPs by 32% and provides a burn‐in mechanism for stabilizing the TCR. The TCR of the RTD‐µHPs after electrical sintering is 3.80 × 10−3 at 22 °C, close to the value for bulk Ag. The performance of the RTD‐µHPs is tested using a reference thermocouple. The RTD‐µHPs show reliable and repeatable heating and temperature sensing up to 70 °C in air. [ABSTRACT FROM AUTHOR]

Multi‐dimensional tactile force sensors with high sensitivity, large measuring range, and direction recognition are essential for human‐machine interaction, medical rehabilitation, and humanoid robotics. Current tactile force sensors mainly make use of bump structure or 3D patterned composite layers to achieve multi‐dimensional force sensing, which not only results in thick sensors with unadjustable measuring performance, but brings about significant difficulties in fabrication. Here, an organic electrochemical transistor (OECT) based thin multi‐dimensional force sensor with tunable range/sensitivity has been designed with unilateral conductive micro‐riblets array, and aerosol jet printing was developed to build the unilateral conductive structure and multi‐material layers. Such unilateral conductive micro‐riblets array's directional dependent deformation endows the sensor with the capacities of lateral force response and force direction recognition. With the profound contact area sensitivity from super capacitance effect and signal amplifying of OECT, the aerosol jet printing OECT force sensor shows an adjustable sensing performance with high sensitivity of ≈1.45 kPa−1 at low‐pressure range of 0–500 Pa and a large measuring range even up to as high as 50 kPa. Finally, several tactile sensing applications have been demonstrated for the sensor's multifunctional performance. [ABSTRACT FROM AUTHOR]

Nanoparticle 3D printing and sintering is a promising method to achieve freeform interconnects on compliant substrates for applications such as soft robotics and wearable healthcare devices. However, previous strategies to sinter metallic nanoparticles while preserving the soft polymer substrate are rife with problems such as cracking and low conductivity of the metallic features. In this paper, the mechanisms of cracking in nanoparticle‐based 3D printed and sintered stretchable interconnects are identified and architecture and processing strategies are demonstrated to achieve crack‐free interconnects fully embedded in thin (<100 μm in thickness) stretchable polydimethylsiloxane (PDMS) with external connectivity. Capillary forces between nanoparticles developed through rapid solvent evaporation in the colloidal ink is hypothesized to initiate cracking during drying. Additionally, the presence of oxygen promotes the removal of organic surfactants and binders in the nanoparticle ink which increases nanoparticle agglomeration, grain growth, and subsequently conductivity. An experimental step‐wise variation of the thermal/atmospheric process conditions supports this hypothesis and shows that the presence of air during a low temperature drying step reduces the capillary stress to produce crack‐free interconnects with high conductivities (up to 56% of bulk metal) while having an excellent compatibility with the underlying polymer materials. Finally, stretchable interconnects fully‐encapsulated in PDMS polymer, with 3D pillar architectures for external connectivity are demonstrated, thus also solving an important "last‐mile" problem in the packaging of stretchable electronics. [ABSTRACT FROM AUTHOR]

This work demonstrates a novel fabrication approach based on the combination of screen and aerosol jet printing to manufacture fully printed organic electrochemical transistors (OECTs) and OECT‐based logic circuits on PET substrates with superior performances. The use of aerosol jet printing allows for a reduction of the channel width to ≈15 µm and the estimated volume by a factor of ≈40, compared to the fully screen printed OECTs. Hence, the OECT devices and OECT‐based logic circuits fabricated with the proposed approach emerge with a high ON/OFF ratio (103–104) and remarkably fast switching response, reaching an ON/OFF ratio of >103 in 4–8 ms, which is further demonstrated by a propagation delay time of just above 1 ms in OECT‐based logic inverter circuits operated at a frequency of 100 Hz. All‐printed monolithically integrated OECT‐based five‐stage ring oscillator circuits further validated the concept with a resulting self‐oscillation frequency of 60 Hz. [ABSTRACT FROM AUTHOR]

Aerosol jet printing is a popular digital additive manufacturing method for flexible and hybrid electronics, but it lacks sophisticated real‐time process control schemes that would enable more widespread adoption in manufacturing environments. Here, an optical measurement system is introduced to track the aerosol density upstream of the printhead. The measured optical extinction, combined with the aerosol flow rate, is directly related to deposition rate and accurately predicts functional materials properties such as the electrical resistance of printed graphene films. This real‐time system offers a compelling solution for process drift and batch‐to‐batch variability, rendering it a valuable tool for both real‐time control of aerosol jet printing and fundamental studies of the underlying process science. [ABSTRACT FROM AUTHOR]

Thermoelectric generators are an environmentally friendly and reliable solid‐state energy conversion technology. Flexible and low‐cost thermoelectric generators are especially suited to power flexible electronics and sensors using body heat or other ambient heat sources. Bismuth telluride (Bi2Te3) based thermoelectric materials exhibit their best performance near room temperature making them an ideal candidate to power wearable electronics and sensors using body heat. In this report, Bi2Te3 thin films are deposited on a flexible polyimide substrate using low‐cost and scalable manufacturing methods. The synthesized Bi2Te3 nanocrystals have a thickness of 35 ± 15 nm and a lateral dimension of 692 ± 186 nm. Thin films fabricated from these nanocrystals exhibit a peak power factor of 0.35 mW m−1·K−2 at 433 K, which is among the highest reported values for flexible thermoelectric films. In order to evaluate the flexibility of the thin films, static and dynamic bending tests are performed while monitoring the change in electrical resistivity. After 1000 bending cycles over a 50 mm radius of curvature, the change in electrical resistance of the film is 23%. Using Bi2Te3 solutions, the ability to print thermoelectric thin films with an aerosol jet printer is demonstrated, highlighting the potential of additive manufacturing techniques for fabricating flexible thermoelectric generators. [ABSTRACT FROM AUTHOR]

Modern electronic devices, particularly those intended for wearable or human health monitoring applications, require high levels of flexibility and stretchability. Hence devices, as well as interconnects, need to be capable of retaining functionality even when being mechanically deformed. Most approaches towards achieving this rely on printing or transferring structures onto elastomeric substrates that can withstand stretching. However, the processing involved can often be cumbersome, and the structures themselves tend to suffer from poor fatigue and/or are limited by the mechanical properties of the underlying substrate. Here, we have developed a novel aerosol jet printing technique capable of building fully freestanding functional structures layer by layer, which are robust and reliable upon thousands of stretching cycles. The process involves printing a combination of layers of different materials with the desired functionality, onto a substrate coated with a sacrificial film that is subsequently dissolved to release the printed structure. Using this method, we demonstrate freestanding conductive wires that can be used as stretchable interconnects/electrodes, and that also function as strain‐sensors. Additionally, we show that a freestanding capacitive structure functions as a robust, stretchable humidity sensor, paving the way for the development of other multilayer, multifunctional stretchable devices and sensors. [ABSTRACT FROM AUTHOR]

In this paper the use of aerosol jet printing as a fabrication and rapid prototyping method for terahertz metamaterials is presented. Since this technique combines the very precise deposition of metallic inks with line widths down to 10 µm and the use of digitally generated tool paths, it is highly suitable for fabricating arbitrary 2D microstructures for terahertz applications. The fabrication of a metamolecule composed of four closed ring resonators with two different radii on conventional polyester foil is shown. Furthermore, the metamaterials are characterized thoroughly using microscopic images, confocal microscopy, and terahertz time‐domain spectroscopy, which demonstrate impressively the suitability of this fabrication technology for terahertz applications. For an additional validation the experimentally gathered data with numerical simulations are compared. Digital aerosol jet printing is demonstrated as a very promising fabrication and prototyping method for novel 2D metastructures. In this paper, asymmetric closed ring metamolecule resonators are fabricated with line widths of merely 10 µm. The printed structures are evaluated by THz time‐domain spectroscopy and validated by numerical simulations. [ABSTRACT FROM AUTHOR]