Additive manufacturing (AM) is increasingly crucial for creating conformal electronics that seamlessly integrate with complex components. However, fabricating conformal electronics is challenging due to the need to adapt to substrate curvilinearity, topography, and material. This research utilizes aerosol jet (AJ) printing, an AM technique employing ink-based materials, combined with a custom-built lathe mechanism. This lathe-based AJ printing allows for conformal electronics deposition around the circumference of rotational bodies with 3D curved surfaces via cylindrical-coordinate motion. The study aims to characterize the capabilities of this LAJ printing method and demonstrate its versatility through the creation of flexible conformal electronics, including multilayer carbon nanotube transistors, and a graphene sensor integrated onto an inflated catheter balloon for real-time monitoring of temperature and inflation. The significance lies in overcoming the limitations of existing conformal electronics manufacturing techniques (soft lithography and direct-write printing methods) by providing a simpler, cost-effective, and adaptable additive manufacturing process.

Current methods for producing conformal electronics primarily involve soft lithography followed by transfer/stamping or direct-write (DW) printing. Soft lithography, while effective, is complex, expensive, and lacks the versatility of additive manufacturing. DW printing, particularly droplet-based techniques like aerosol jet (AJ) printing and inkjet printing, offer inherent 3D capabilities and cost-effectiveness. AJ printing, however, faces challenges when dealing with steeply curved substrates. Although existing advanced motion systems (e.g., 5-axis trunnion-based printers) can address this, their complexity increases costs. This research proposes a simpler, one-axis rotational motion system (LAJ printing) as a cost-effective alternative for conformal printing on curvilinear surfaces, focusing on applications such as catheters and inflatable catheter balloons which are inherently cylindrical and rotatable.

A custom attachment was designed for a commercial AJ printer to enable cylindrical-coordinate LAJ printing. This attachment translates linear motion in the cartesian x-axis to rotational motion in the cylindrical θ-axis while maintaining linear motion in the axial y/z-axis using a rack, pinion, and axle clamp mechanism. The system allows for conformal printing around the entire circumference of rotationally symmetric bodies (cylinders, cones, etc.) without manual intervention. The impact of the print incidence angle on conformal printing was investigated, demonstrating that LAJ printing maintains a consistent 90° print incidence unlike planar AJ printing, which results in significant variations as the substrate curvature changes. The diametric ratio (substrate diameter/pinion diameter) was also studied, showing its effect on the motion translation and print speed, and requiring adjustments to design files for optimal results. The resolution and repeatability of the LAJ system were tested by printing various patterns with varying pitches and speeds, demonstrating micron-scale accuracy and consistency. The fabrication of several devices—resistive traces, capacitors, and thin-film transistors—was demonstrated using LAJ printing, showcasing its capabilities for single and multilayer electronics on flexible substrates. The printing of graphene and AgNWs on concave and conical mandrels highlighted the LAJ system's adaptability to complex shapes, though limitations were acknowledged for substrates with multidirectional curvature. Finally, the integration of a graphene sensor onto an inflated catheter balloon involved a modified LAJ system with an on-axis inflation system, showcasing the applicability of the technique for medical applications. Detailed material specifications (graphene ink, CNC, CNT ink, AgNWs, etc.), aerosol jet printing parameters, and characterization techniques (optical microscopy, profilometry, resistance measurements, capacitance measurements, TFT characterization) are included in the supplementary information.

The LAJ printing system successfully translates cartesian motion to 3D cylindrical rotation, enabling high-resolution, conformal printing on 3D curved surfaces. Maintaining a consistent 90° print incidence angle was shown to be crucial for uniform printing on curved surfaces. LAJ printing demonstrated significant improvements over planar AJ printing in print quality on curved surfaces. The diametric ratio between the substrate and pinion significantly affected the print speed and trace thickness, requiring design adjustments. The LAJ system achieved micron-scale accuracy and repeatability in printing complex patterns. The fabrication of functional multilayer devices (capacitors and thin-film transistors) showcased the technology's versatility and accuracy. Conformal printing on concave and conical substrates demonstrated the system's adaptability, although limitations were noted for highly complex geometries. Finally, a functional graphene sensor was successfully printed on an inflated catheter balloon, indicating the potential for medical applications. The graphene sensor demonstrated a sensitive and predictable response to temperature changes, and also functioned as an inflation/deflation sensor. The sensor's sensitivity was calculated as -0.42 in the physiological temperature range and -1.48 for higher temperatures.

The developed LAJ printing method effectively addresses the limitations of conventional conformal electronics manufacturing. Its simplicity, cost-effectiveness, and scalability make it a promising approach for various applications. The successful fabrication of multilayer electronics and the integration of a sensor onto a catheter balloon highlight the technology's potential in flexible and bioelectronic devices. The findings support the importance of maintaining normal print incidence for consistent results on curved surfaces. While the current system has limitations with highly complex geometries, its modular design allows for future improvements. The integration of functional sensors onto medical devices opens new possibilities for advanced medical procedures.

This research successfully developed and characterized a novel lathe-based aerosol jet printing system for fabricating conformal electronics. The system demonstrates high-resolution, accurate, and repeatable printing on complex 3D surfaces, improving on existing methods. The successful creation of functional devices and the integration of a sensor onto a catheter highlight the technology's potential for various applications, particularly in flexible electronics and medical devices. Future research could focus on expanding the system's capabilities to handle more complex geometries and integrating more sophisticated sensor functionalities.

The current LAJ printing system is optimized for cylindrical and rotational geometries. Its ability to conform to complex, multidirectional curves is limited. While the system handles gradual curvature well, more complex substrates may require a more advanced multi-axis system. The graphene sensor integrated onto the catheter balloon demonstrated some drift in the inflated state resistance, and further optimization of the sensor design and measurement techniques is needed. The study primarily used graphene and other readily printable materials; further exploration with a wider range of materials is necessary.