Recent Advances in 3D Printing of Biomedical Sensing Devices.

Additive manufacturing, also called 3D printing, is a rapidly evolving technique that allows for the fabrication of functional materials with complex architectures, controlled microstructures, and material combinations. This capability has influenced the field of biomedical sensing devices by enabling the trends of device miniaturization, customization, and elasticity (i.e., having mechanical properties that match with the biological tissue). In this paper, the current state-of-the-art knowledge of biomedical sensors with the unique and unusual properties enabled by 3D printing is reviewed. The review encompasses clinically important areas involving the quantification of biomarkers (neurotransmitters, metabolites, and proteins), soft and implantable sensors, microfluidic biosensors, and wearable haptic sensors. In addition, the rapid sensing of pathogens and pathogen biomarkers enabled by 3D printing, an area of significant interest considering the recent worldwide pandemic caused by the novel coronavirus, is also discussed. It is also described how 3D printing enables critical sensor advantages including lower limit-of-detection, sensitivity, greater sensing range, and the ability for point-of-care diagnostics. Further, manufacturing itself benefits from 3D printing via rapid prototyping, improved resolution, and lower cost. This review provides researchers in academia and industry a comprehensive summary of the novel possibilities opened by the progress in 3D printing technology for a variety of biomedical applications.

N protein-based ultrasensitive SARS-CoV-2 antibody detection in seconds via 3D nanoprinted, microarchitected array electrodes.

Rapid detection of antibodies to SARS-CoV-2 is critical for COVID-19 diagnostics, epidemiological research, and studies related to vaccine evaluation. It is known that the nucleocapsid (N) is the most abundant protein of SARS-CoV-2 and can serve as an excellent biomarker due to its strong immunogenicity. This paper reports a rapid and ultrasensitive 3D biosensor for quantification of COVID-19 antibodies in seconds via electrochemical transduction. This sensor consists of an array of three-dimensional micro-length-scale electrode architecture that is fabricated by aerosol jet 3D printing, which is an additive manufacturing technique. The micropillar array is coated with N proteins via an intermediate layer of nano-graphene and is integrated into a microfluidic channel to complete an electrochemical cell that uses antibody-antigen interaction to detect the antibodies to the N protein. Due to the structural innovation in the electrode geometry, the sensing is achieved in seconds, and the sensor shows an excellent limit of detection of 13 fm and an optimal detection range of 100 fm to 1 nm. Furthermore, the sensor can be regenerated at least 10 times, which reduces the cost per test. This work provides a powerful platform for rapid screening of antibodies to SARS-CoV-2 after infection or vaccination.

Ultrarapid and ultrasensitive detection of SARS-CoV-2 antibodies in COVID-19 patients via a 3D-printed nanomaterial-based biosensing platform.

Rapid detection of antibodies during infection and after vaccination is critical for the control of infectious outbreaks, understanding immune response, and evaluating vaccine efficacy. In this manuscript, we evaluate a simple ultrarapid test for SARS-CoV-2 antibodies in COVID-19 patients, which gives quantitative results (i.e., antibody concentration) in 10-12 s using a previously reported nanomaterial-based three-dimensional (3D)-printed biosensing platform. This platform consists of a micropillar array electrode fabricated via 3D printing of aerosolized gold nanoparticles and coated with nanoflakes of graphene and specific SARS-CoV-2 antigens, including spike S1, S1 receptor-binding domain (RBD) and nucleocapsid (N). The sensor works on the principle of electrochemical transduction, where the change of sensor impedance is realized by the interactions between the viral proteins attached to the sensor electrode surface and the antibodies. The three sensors were used to test samples from 17 COVID-19 patients and 3 patients without COVID-19. Unlike other serological tests, the 3D sensors quantitatively detected antibodies at a concentration as low as picomole within 10-12 s in human plasma samples. We found that the studied COVID-19 patients had higher concentrations of antibodies to spike proteins (RBD and S1) than to the N protein. These results demonstrate the enormous potential of the rapid antibody test platform for understanding patients' immunity, disease epidemiology and vaccine efficacy, and facilitating the control and prevention of infectious epidemics.

3-D Printed Adjustable Microelectrode Arrays for Electrochemical Sensing and Biosensing.

Printed Electronics has emerged as an important fabrication technique that overcomes several shortcomings of conventional lithography and provides custom rapid prototyping for various sensor applications. In this work, silver microelectrode arrays (MEA) with three different electrode spacing were fabricated using 3-D printing by the aerosol jet technology. The microelectrodes were printed at a length scale of about 15 µm, with the space between the electrodes accurately controlled to about 2 times (30 µm, MEA30), 6.6 times (100 µm, MEA100) and 12 times (180 µm, MEA180) the trace width, respectively. Hydrogen peroxide and glucose were chosen as model analytes to demonstrate the performance of the MEA for sensor applications. The electrodes are shown to reduce hydrogen peroxide with a reduction current proportional to the concentration of hydrogen peroxide for certain concentration ranges. Further, the sensitivity of the current for the three electrode configurations was shown to decrease with an increase in the microelectrode spacing (sensitivity of MEA30: MEA100: MEA180 was in the ratio of 3.7: 2.8: 1), demonstrating optimal MEA geometry for such applications. The noise of the different electrode configurations is also characterized and shows a dramatic reduction from MEA30 to MEA100 and MEA180 electrodes. Further, it is shown that the response current is proportional to MEA100 and MEA180 electrode areas, but not for the area of MEA30 electrode (the current density of MEA30 : MEA100 : MEA180 is 0.25 : 1 : 1), indicating that the MEA30 electrodes suffer from diffusion overlap from neighboring electrodes. The work thus establishes the lower limit of microelectrode spacing for our geometry. The lowest detection limit of the MEAs was calculated (with S/N = 3) to be 0.45 µM. Glucose oxidase was immobilized on MEA100 microelectrodes to demonstrate a glucose biosensor application. The sensitivity of glucose biosensor was 1.73 µAmM-1 and the calculated value of detection limit (S/N = 3) was 1.7 µM. The electrochemical response characteristics of the MEAs were in agreement with the predictions of existing models. The current work opens up the possibility of additive manufacturing as a fabrication technique for low cost custom-shaped MEA structures that can be used as electrochemical platforms for a wide range of sensor applications.

3D Assembly of MXene Networks using a Ceramic Backbone with Controlled Porosity.

Transition metal carbides (MXenes) are novel 2D nanomaterials with exceptional properties, promising significant impact in applications such as energy storage, catalysis, and energy conversion. A major barrier preventing the widespread use of MXenes is the lack of methods for assembling MXene in 3D space without significant restacking, which degrades their performance. Here, this challenge is successfully overcome by introducing a novel material system: a 3D network of MXene formed on a porous ceramic backbone. The backbone dictates the network's 3D architecture while providing mechanical strength, gas/liquid permeability, and other beneficial properties. Freeze casting is used to fabricate a silica backbone with open pores and controlled porosity. Next, capilary flow is used to infiltrate MXene into the backbone from a dispersion. The system is then dried to conformally coat the pore walls with MXene, creating an interconnected 3D-MXene network. The fabrication approach is reproducible, and the MXene-infiltrated porous silica (MX-PS) system is highly conductive (e.g., 340 S m-1 ). The electrical conductivity of MX-PS is controlled by the porosity distribution, MXene concentration, and the number of infiltration cycles. Sandwich-type supercapacitors with MX-PS electrodes are shown to produce excellent areal capacitance (7.24 F cm-2 ) and energy density (0.32 mWh cm-2 ) with only 6% added MXene mass. This approach of creating 3D architectures of 2D nanomaterials will significantly impact many engineering applications.

Shape distortion in sintering results from nonhomogeneous temperature activating a long-range mass transport.

Sintering theory predicts no long-range mass transport or distortion for uniformly heated particles during particle coalescence. However, in sintering-based manufacturing processes, permanent part distortion is often observed. The driving forces and mechanisms leading to this phenomenon are not understood, and efforts to reduce distortion are largely limited to a trial-and-error approach. In this paper, we demonstrate that distortion during sintering results from mass-transport driven by nonhomogeneous temperature distribution. We then show that hitherto unknown mass transport mechanisms, working in the direction opposite to temperature gradient are the likely cause of distortion. The experimental setup, designed for this purpose, enables the quantification of distortion during sintering. Two possible mass transport mechanisms are defined, and the continuum model applicable to both is formulated. The model accurately predicts the transient and permanent distortion observed during experiments, including their size dependence. Methods to control distortion that can give rise to 4D printing are discussed.

CMU Array: A 3D nanoprinted, fully customizable high-density microelectrode array platform.

Microelectrode arrays provide the means to record electrophysiological activity critical to brain research. Despite its fundamental role, there are no means to customize electrode layouts to address specific experimental or clinical needs. Moreover, current electrodes demonstrate substantial limitations in coverage, fragility, and expense. Using a 3D nanoparticle printing approach that overcomes these limitations, we demonstrate the first in vivo recordings from electrodes that make use of the flexibility of the 3D printing process. The customizable and physically robust 3D multi-electrode devices feature high electrode densities (2600 channels/cm2 of footprint) with minimal gross tissue damage and excellent signal-to-noise ratio. This fabrication methodology also allows flexible reconfiguration consisting of different individual shank lengths and layouts, with low overall channel impedances. This is achieved, in part, via custom 3D printed multilayer circuit boards, a fabrication advancement itself that can support several biomedical device possibilities. This effective device design enables both targeted and large-scale recording of electrical signals throughout the brain.

Sensing of COVID-19 Antibodies in Seconds via Aerosol Jet Nanoprinted Reduced-Graphene-Oxide-Coated 3D Electrodes.

Rapid diagnosis is critical for the treatment and prevention of diseases. An advanced nanomaterial-based biosensing platform that detects COVID-19 antibodies within seconds is reported. The biosensing platform is created by 3D nanoprinting of three-dimensional electrodes, coating the electrodes by nanoflakes of reduced-graphene-oxide (rGO), and immobilizing specific viral antigens on the rGO nanoflakes. The electrode is then integrated with a microfluidic device and used in a standard electrochemical cell. When antibodies are introduced on the electrode surface, they selectively bind with the antigens, changing the impedance of the electrical circuit which is detected via impedance spectroscopy. Antibodies to SARS-CoV-2 spike S1 protein and its receptor-binding-domain (RBD) are detected at a limit-of-detection of 2.8 × 10-15 and 16.9 × 10-15 m, respectively, and read by a smartphone-based user interface. The sensor can be regenerated within a minute by introducing a low-pH chemistry that elutes the antibodies from the antigens, allowing successive sensing of test samples using the same sensor. Sensing of S1 and RBD antibodies is specific, which cross-reacts neither with other antibodies such as RBD, S1, and nucleocapsid antibody nor with proteins such as interleukin-6. The proposed sensing platform could also be useful to detect biomarkers for other infectious agents such as Ebola, HIV, and Zika.

Breaking the barrier to biomolecule limit-of-detection via 3D printed multi-length-scale graphene-coated electrodes.

Sensing of clinically relevant biomolecules such as neurotransmitters at low concentrations can enable an early detection and treatment of a range of diseases. Several nanostructures are being explored by researchers to detect biomolecules at sensitivities beyond the picomolar range. It is recognized, however, that nanostructuring of surfaces alone is not sufficient to enhance sensor sensitivities down to the femtomolar level. In this paper, we break this barrier/limit by introducing a sensing platform that uses a multi-length-scale electrode architecture consisting of 3D printed silver micropillars decorated with graphene nanoflakes and use it to demonstrate the detection of dopamine at a limit-of-detection of 500 attomoles. The graphene provides a high surface area at nanoscale, while micropillar array accelerates the interaction of diffusing analyte molecules with the electrode at low concentrations. The hierarchical electrode architecture introduced in this work opens the possibility of detecting biomolecules at ultralow concentrations.

Three-dimensional microarchitected materials and devices using nanoparticle assembly by pointwise spatial printing.

Three-dimensional (3D) hierarchical materials are important to a wide range of emerging technological applications. We report a method to synthesize complex 3D microengineered materials, such as microlattices, with nearly fully dense truss elements with a minimum diameter of approximately 20 µm and having high aspect ratios (up to 20:1) without using any templating or supporting materials. By varying the postprocessing conditions, we have also introduced an additional control over the internal porosity of the truss elements to demonstrate a hierarchical porous structure with an overall void size and feature size control of over five orders of magnitudes in length scale. The method uses direct printing of nanoparticle dispersions using the Aerosol Jet technology in 3D space without templating or supporting materials followed by binder removal and sintering. In addition to 3D microlattices, we have also demonstrated directly printed stretchable interconnects, spirals, and pillars. This assembly method could be implemented by a variety of microdroplet generation methods for fast and large-scale fabrication of the hierarchical materials for applications in tissue engineering, ultralight or multifunctional materials, microfluidics, and micro-optoelectronics.

Origami lithium-ion batteries.

There are significant challenges in developing deformable devices at the system level that contain integrated, deformable energy storage devices. Here we demonstrate an origami lithium-ion battery that can be deformed at an unprecedented high level, including folding, bending and twisting. Deformability at the system level is enabled using rigid origami, which prescribes a crease pattern such that the materials making the origami pattern do not experience large strain. The origami battery is fabricated through slurry coating of electrodes onto paper current collectors and packaging in standard materials, followed by folding using the Miura pattern. The resulting origami battery achieves significant linear and areal deformability, large twistability and bendability. The strategy described here represents the fusion of the art of origami, materials science and functional energy storage devices, and could provide a paradigm shift for architecture and design of flexible and curvilinear electronics with exceptional mechanical characteristics and functionalities.