In this paper, we propose a novel tactile sensor with a “fingerprint” design, named due to its spiral shape and dimensions of 3.80 mm × 3.80 mm. The sensor is duplicated in a four-by-four array containing 16 tactile sensors to form a “SkinCell” pad of approximately 45 mm by 29 mm. The SkinCell was fabricated using a custom-built microfabrication platform called the NeXus which contains additive deposition tools and several robotic systems. We used the NeXus’ six-degrees-of-freedom robotic platform with two different inkjet printers to deposit a conductive silver ink sensor electrode as well as the organic piezoresistive polymer PEDOT:PSS-Poly (3,4-ethylene dioxythiophene)-poly(styrene sulfonate) of our tactile sensor. Printing deposition profiles of 100-micron- and 250-micron-thick layers were measured using microscopy. The resulting structure was sintered in an oven and laminated. The lamination consisted of two different sensor sheets placed back-to-back to create a half-Wheatstone-bridge configuration, doubling the sensitivity and accomplishing temperature compensation. The resulting sensor array was then sandwiched between two layers of silicone elastomer that had protrusions and inner cavities to concentrate stresses and strains and increase the detection resolution. Furthermore, the tactile sensor was characterized under static and dynamic force loading. Over 180,000 cycles of indentation were conducted to establish its durability and repeatability. The results demonstrate that the SkinCell has an average spatial resolution of 0.827 mm, an average sensitivity of 0.328 $\mathsf{m}\mathsf{\Omega }/\mathsf{\Omega }/\mathsf{N}$, expressed as the change in resistance per force in Newtons, an average sensitivity of 1.795 µV/N at a loading pressure of 2.365 PSI, and a dynamic response time constant of 63 ms which make it suitable for both large area skins and fingertip human–robot interaction applications.

In this study, we propose an alternative, low-cost, packaging method using both inkjet and aerosol jet printing (AJP) to fabricate functional interconnects for custom die-level packages assembled on printed circuit boards (PCBs). Our process involves the manufacturing of a stacked structure with insulating and conducting layers, fabricated using inkjet and aerosol jet printing, respectively. In the first part of the study, we characterized UV adhesive deposition on the standard resin-coated PCB to acquire the optimal parameters necessary for the fabrication of the insulating layer with thicknesses around 100 μm. In the second part of our study, we developed a precise method for printing silver conducting lines using an aerosol jet printer, enabling the realization of structures with features as small as 50 μm on nonplanar surfaces. The insulating part of the microelectromechanical systems (MEMS)/PCB structure serves as a ramp ensuring continuity of the aerosol jet printed connection between the PCB and the Si die. A 1 cm × 1 cm Si chip with a custom MEMS microrobot was used to demonstrate the feasibility and flexibility of our approach in practice. After fabrication, we evaluated the interconnects for conductivity and repeatability. The packaging and inspection process was carried out using Nexus, a unique robotic system integrating additive manufacturing, robotic transport, and metrology. The results show that we successfully fabricated printed interconnects between copper PCB pads and Si die cleanroom fabricated gold pads, while electrical characterization revealed resistances in the range of 1–10 Ω. Our approach can be utilized in the manufacturing of electrical interconnects for custom devices on different substrates, including traditional and flexible PCBs. Furthermore, the applied printing techniques enable the use of other insulating or conducting inks and the formation of structures of custom geometries across a wide range of scales—20 μm to 1 mm.

This paper demonstrates the NeXus system, a multiscale robotic additive manufacturing platform developed at the Louisville Automation and Robotics Research Institute, as a rapid prototyping tool through additively manufacturing a multilayer flexible printed circuit board (FPC) with a printed strain sensor and soldered surface mounted devices (SMD). Manufacturing of the demonstrator requires the application and curing of multiple materials with specialized properties, tools for automated assembly, and software advances to streamline the process enabling the use of industry-standardized programs to command the NeXus system. Additive manufacturing processes supported by the NeXus include aerosol jet printing (AJP) for fine feature silver conducting lines, direct write ink-jet printing for insulating materials, and intense pulsed light (IPL) for curing materials between depositions. The NeXus system transports and manipulates parts using a six-degree-of-freedom (DOF) high-precision positioner. Solder paste deposition and pick-and-place (PnP) procedures are performed by a 4DOF Selective Compliance Articulated Robot Arm (SCARA). Conversion methods between traditional printed circuit board (PCB) design software and production-ready command scripts were developed to translate basic drawings into command scripts. Multilayer structures with AJP 50-μm wide lines, an insulating bridge with a thickness of around 100 μm, and SMDs soldered to silver AJP pads were integrated within the demonstrator. An operational amplifier and other SMDs reduce the complexity of the accompanying control circuit and amplify the sensor's response by 1830 times. The successful fabrication of the demonstrator FPC highlights the rapid prototyping ability of the NeXus system.