Multifunctional medical devices continue to evolve and grow, bringing challenges in determining the best methods for introducing novel features. Addition of sensing electronics to an endotracheal tube, stimulation electrodes to a neural implant, or radiopaque markings to a catheter involve critical design decisions influencing the device value and efficacy.
There are a limited number of methods for printing traces directly to flexible three-dimensional surfaces. Pad printing, screen printing, inkjet, or thermal transfer can be used effectively for planar surfaces, but they are limited in their application to irregular or flexible articles. A class of direct writing techniques that exhibits satisfactory control and manipulation of three-dimensional, irregular substrates is based on flow-based micro-dispensing techniques, in which printed materials are extruded with a high degree of control through a syringe and a precision pen tip. Micropenning is such a technique, and enables the precise application of fine, conformal traces of functional materials directly onto medical devices with hard or soft surfaces, and onto three-dimensional geometries such as medical balloons, catheters, and surgical instruments.
Many factors influence the effectiveness of the technology. The composition of the ink, the cost of materials, and trace and feature dimensions are all important elements for consideration.
Electrical conductivity requirements are defined to achieve a specific effect, such as localized heating (e.g. RF ablation probes), or signal detection (e.g. sensing applications). Higher conductivity applications may utilize fillers such as silver, gold, or carbon. High-aspect-ratio particles—including flakes, platelets, and needles—yield the best conductivity at the lowest loading levels. Other particles which are not available in such morphologies, such as titanium nitride, may also be formulated into Micropennable inks and are suitable for many electrically conductive applications.
Radiopaque marking is particularly useful in several x-ray-based medical diagnostic procedures. X-ray absorption enables the position of the medical device to be clearly visible during procedures or after implantation. Platinum, tungsten, and barium sulfate are often used in medical devices because of their excellent biocompatibility and wide availability. These materials can be obtained in a fine particle form that is suitable for designing inks compatible with the Micropenning process.
If biocompatibility issues arise, ink traces can be covered with a biocompatible polymeric overcoat, which protects the written material and provides a physical and electrical barrier layer between the ink materials and the patient. Alternatively, they can be formulated from individual components of known biocompatibility.
Critical parameters such as adhesion and flexibility are influenced by proper ink selection. Many polymers favored by medical device manufacturers, such as silicone elastomers or polytetrafluorethylene, pose difficulties for achieving ink adhesion. The interaction between the ink and substrate can be optimized, however, by careful formulation and matching of the substrate and ink components. In some cases, an intermediate layer material can be identified which adheres well to both the ink and the substrate, and can be printed or coated beneath the ink layer. Alternatively, surface modification of the substrate material by corona, plasma or flame treatment can be useful in solving adhesion problems.
In conclusion, several key factors related to ink and substrate selection should be considered when choosing to apply flow-based Micropenning technology to enhance the functionality of medical devices. Sensors, ablation and denervation electrodes, electrosurgical devices, cauterization probes, discrete heaters, radiopaque markings, monitoring devices, and others applications have been demonstrated through careful optimization of the manufacturing process and materials selection.