During the last years, scientific research in biotechnology has been reporting a considerable boost forward due to many advances marked in different technological areas. Researchers working in the field of regenerative medicine, mechanobiology and pharmacology have been constantly looking for non-invasive methods able to track tissue development, monitor biological processes and check effectiveness in treatments. The possibility to control cell cultures and quantify their products represents indeed one of the most promising and exciting hurdles. In this perspective, the use of conductive materials able to map cell activity in a three-dimensional environment represents the most interesting approach. The greatest potential of this strategy relies on the possibility to correlate measurable changes in electrical parameters with specific cell cycle events, without affecting their maturation process and considering a physiological-like setting. Up to now, several conductive materials has been identified and validated as possible solutions in scaffold development, but still few works have stressed the possibility to use conductive scaffolds for non-invasive electrical cell monitoring. In this picture, the main objective of this review was to define the state-of-the-art concerning conductive biomaterials to provide researchers with practical guidelines for developing specific applications addressing cell growth and differentiation monitoring. Therefore, a comprehensive review of all the available conductive biomaterials (polymers, carbon-based, and metals) was given in terms of their main electric characteristics and range of applications.

The use of electrochemical sensors for the analysis of biological samples is nowadays widespread and highly demanded from diagnostic and pharmaceutical research, but the reliability and repeatability still remain debated issues. In the expanding field of printed electronics, Aerosol Jet Printing (AJP) appears promising to bring an improvement in resolution, miniaturization, and flexibility. In this paper, the use of AJP is proposed to design and fabricate customized electrochemical sensors in term of geometry, materials and 3D liquid sample confinement, reducing variability in the functionalization process. After an analysis of geometrical, electrical and surface features, the optimal layout has been selected. An electrochemical test has been then performed quantifying Interleukin-8, selected as reference protein, by means of Anodic Stripping Voltammetry. AJP sensors have been compared with standard screen-printed electrodes in terms of current density and relative standard deviation. Results from AJP sensors with Ag-based Anodic Stripping Voltammetry confirmed nanostructures capability to reduce the limit of detection (from 2.1 to 0.3 ng/mL). Furthermore, AJP appeared to bring an improvement in term of relative standard deviation from 50 to 10%, if compared to screen-printed sensors. This is promising to improve reliability and repeatability of measurement techniques integrable in several biotechnological applications.

Printed electronics have led to new possibilities in the detection and quantification of a wide range of molecules important for medical, biotechnological, and environmental fields. The integration with microfluidics is often adopted to avoid hand-deposition of little volumes of reagents and samples on miniaturized electrodes that strongly depend on operator’s skills. Here we report design, fabrication and test of an easy-to-use electrochemical sensor platform with microfluidics entirely realized with Aerosol Jet Printing (AJP). We printed a six-electrochemical-sensors platform with AJP and we explored the possibility to aerosol jet print directly on it a microfluidic structure without any support material. Thus, the sacrificial material removal and/or the assembly with sensors steps are avoided. The repeatability observed when printing both conductive and ultraviolet (UV)-curable polymer inks can be supported from the values of relative standard deviation of maximum 5% for thickness and 9% for line width. We designed the whole microfluidic platform to make the sample deposition (20 μL) independent from the operator. To validate the platform, we quantified glucose at different concentrations using a standard enzyme-mediated procedure. Both mediator and enzyme were directly aerosol jet printed on working electrodes (WEs), thus the proposed platform is entirely fabricated by AJP and ready to use. The chronoamperometric tests show limit of detection (LOD) = 2.4 mM and sensitivity = 2.2 ± 0.08 µA/mM confirming the effectiveness of mediator and enzyme directly aerosol jet printed to provide sensing in a clinically relevant range (3–10 mM). The average relative standard inter-platform deviation is about 8%. AJP technique can be used for fabricating a ready-to-use microfluidic device that does not need further processing after fabrication, but is promptly available for electrochemical sample analysis.

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One of the main hurdles to improving scaffolds for regenerative medicine is the development of non-invasive methods to monitor cell proliferation within three-dimensional environments. Recently, an electrical impedance-based approach has been identified as promising for three-dimensional proliferation assays. A low-cost impedance-based solution, easily integrable with multi-well plates, is here presented. Sensors were developed using biocompatible carbon-based ink on foldable polyimide substrates by means of a novel aerosol jet printing technique. The setup was tested to monitor the proliferation of human mesenchymal stromal cells into previously validated gelatin-chitosan hybrid hydrogel scaffolds. Reliability of the methodology was assessed comparing variations of the electrical impedance parameters with the outcomes of enzymatic proliferation assay. Results obtained showed a magnitude increase and a phase angle decrease at 4 kHz (maximum of 2.5 kΩ and −9 degrees) and an exponential increase of the modeled resistance and capacitance components due to the cell proliferation (maximum of 1.5 kΩ and 200 nF). A statistically significant relationship with enzymatic assay outcomes could be detected for both phase angle and electric model parameters. Overall, these findings support the potentiality of this non-invasive approach for continuous monitoring of scaffold-based cultures, being also promising in the perspective of optimizing the scaffold-culture system.

Printed electronics is an expanding research field that can reach the goal of reducing the environmental impact on electronics exploiting renewable and biodegradable materials, like paper. In our work, we designed and tested a new method for fabricating hybrid smart devices on cellulose substrates by aerosol jet printing (AJP) and photonic curing, also known as flash lamp annealing (FLA), capable to cure low temperature materials without any damage. Three different cellulose-based materials (chromatographic paper, photopaper, cardboard) were tested. Multilayer capability and SMDs (surface mount devices) interconnections are possible permitting high flexibility in the fabrication process. Electrical and geometrical tests were performed to analyze the behavior of printed samples. Resulted resistivities are 26.3 × 10−8 Ω⋅m on chromatographic paper, 22.3 × 10−8 Ω⋅m on photopaper and 13.1 × 10−8 Ω⋅m on cardboard. Profilometer and optical microscope evaluations were performed to state deposition quality and penetration of the ink in cellulose materials (thicknesses equal to 24.9, 28.5, and 51 μm respectively for chromatographic paper, photopaper, and cardboard). Furthermore, bending (only chromatographic paper did not reach the break-up) and damp environment tests (no significant variations in resistance) where performed. A final prototype of a complete functioning multilayer smart devices on cellulose 3D-substrate is shown, characterized by multilayers, capacitive sensors, SMDs interconnections.

Industry 4.0 has radically been transforming the production processes and systems with the adoption of enabling technologies, such as Internet of things (IoT), big data, additive manufacturing (AM), and cloud computing. In this context, sensors are essential to extract information about production, spare parts, equipment health, and environmental conditions necessary for improving many aspects of industrial processes (flexibility, efficiency, costs, etc.). Sensors should be placed everywhere (on machines, smart devices, objects, and tools) inside the factory to monitor in real-time physical quantities such as temperature, vibrations, deformations that could affect the production. Printed electronics (PE) offers techniques to produce unconventional sensor and systems or to make conventional objects “smart”. This work aims to analyze innovative PE technologies—inkjet printing and aerosol jet printing in combination with photonic curing—as manufacturing technologies for electronics and sensors to be integrated into objects, showing a series of sensors fabricated by PE as applications that will be adopted for smart objects and Industry 4.0.

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