Feature Article

Flow-based micro-dispensing techniques apply patterns of functional materials to the outside or inside of medical devices. The factors to consider to benefit from using this technology are discussed together with application examples that illustrate its capabilities.

By: L. Shaw-Klein, Micropen Technologies, Honeoye Falls, New York, USA

FIGURE 1: Conductive flowable materials create sensing electrodes on this endotracheal tube.

The use of materials deposition processes with medical devices has become increasingly important. However, there are a limited number of methods for applying traces directly to flexible three-dimensional (3D) surfaces. Although pad printing, screen printing, inkjet and thermal transfer can be used effectively for planar surfaces, with irregular or flexible items their application is restricted.

 

A novel class of direct writing techniques that exhibits satisfactory control and manipulation of 3D, irregular substrates is derived from 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.¹ This technology has the ability to add features and functions to medical devices when standard electronic components or alternative printing methods prove to be limiting or ineffective.

Three-dimensional printing capability, coupled with the ability to accommodate a range of printing inks and pastes, makes flow-based micro-dispensing a particularly attractive option. This technology enables the precise application of fine, conformal traces of functional materials directly onto medical devices with hard or soft surfaces, and onto 3D geometries such as medical balloons, catheters and surgical instruments. Integrating the trace directly onto the surface of medical devices also solves several problems that mounting or wrapping techniques can create.

 

The function of the trace is the first consideration in the ink selection process. Inks should be biocompatible and adhesive, and can be radiopaque, conductive or dielectric. The factors influencing conductivity, radiopacity and biocompatibility are
discussed below.

Depending on the application, conductivity requirements can vary widely. Electrical conductivity can be optimised to achieve a specific effect such as localised heating on frequency ablation probes or signal transmission in sensing applications. High conductivity applications may utilise 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.

 

Silver and carbon are frequently chosen for electrical conductivity because they are readily available in these morphologies. Other materials include copper, nickel, platinum, palladium and conductive polymers. Nano-silver, graphene and carbon nanotubes are novel technologies that may be employed. These are relatively newly developed materials and their applications are still under investigation, but they are all known to be extremely conductive. Distinctive combinations of materials such as silver-coated copper particles are commercially available that provide cost benefits. Copper is less expensive than silver and some of the surface electrical properties of the silver are obtained with particles that are partially copper.

Radiopaque marking is particularly useful in several X-ray-based medical diagnostic procedures, from fluoroscopy and angiography to computed tomography. Measurement standards for radiopacity often require that an implanted device is tested in its final form in a body or body mimic.²

 

FIGURE 2: Patterns on medical balloons and other medical devices can be made on a variety of shapes, sizes and materials.

The mass attenuation coefficient at the radiation wavelength of interest is a useful comparative value for materials selection purposes. This value is dependent on atomic number. Materials that have atoms with high atomic numbers are typically chosen for radiopaque markers. Platinum, tungsten and barium sulphate are most often used in medical devices because of their excellent biocompatibility and wide availability. Several bismuth compounds, including bismuth trioxide, bismuth oxychloride and bismuth bicarbonate, have also been successfully used as radiopaque markers in medical devices.³ These materials can be obtained in a fine particle form that is suitable for micro-dispensing.

 

Radiopaque inks are often made at relatively high filler particle loadings to maximise the visibility, however, this may lead to poor mechanical properties in the printed traces as the polymer binder level decreases. This can be circumvented somewhat by re-balancing the ratio of binder and filler and printing multiple layers of ink. This is only viable if the increased height of the resulting trace can be accommodated.

The trace dimensions and cross-section can be critical to the function of the device. In direct flow micro-dispensing writing techniques, the ink is dispensed from a syringe and extruded through a pen tip. Tips are selected from an extensive range of sizes, based on the width of the desired trace.

 

For some applications such as sensing, maximum conductivity is required and wide lines of 2 mm or more are preferred. In other conductive applications such as radio frequency ablation, the lines and line spacing must be extremely fine and closely controlled. A pen tip with an inner diameter of 25 to 50 microns works well in this scenario. Ink selection is important here to ensure a smooth flow through the small orifice. Particles in the ink should be in a well-dispersed, stable suspension. The particle size should be relatively small and the distribution reasonably narrow to avoid blocking the pen tip.

 

Together with control over the dispersed phase of the ink, rheological properties also deserve careful consideration. The viscosity may be relatively low at the higher shear rates at the pen tip, but relatively high at rest. This can minimise uncontrolled spreading once the ink is deposited on the surface.⁴ Thixotropic inks, often used in screen-printing applications, are specifically designed to provide this combination of properties.

 

Surface tension of the ink and surface energy of the substrate can also play important roles in ink spreading. These
can be adjusted through judicious selection of ink additives or substrate surface treatments.

 

Other critical parameters, including adhesion and flexibility, are markedly influenced by correct ink selection. Many polymers favoured by medical device manufacturers are found to be notoriously difficult for achieving good ink adhesion. Silicone elastomers and fluorinated polymers such as polytetrafluoroethylene are recognised for their non-stick properties. These materials are used in tubing, catheters, guidewires and many other devices; they can be applied as a surface coating over another material or they can make up the entire device substrate. These yield excellent results for biocompatibility, but pose serious restrictions on ink selection.

 

High filler loading can disrupt adhesion or cause brittleness in the printed trace. The more dominant characteristics are typically determined by the nature of the polymeric matrix, and whether it can provide adequate adhesion and flexibility.

 

In summary, the interaction between the ink and substrate is optimised if the solvent and polymer are chosen judiciously. In some cases, an adhesion-promoting layer can be identified that sticks well to both the ink and the substrate and can be printed or coated beneath the ink layer. A partial list of the compatible substrates evaluated to date is available.⁵

 

Another consideration is the specific gravity of the filler. Because most physical phenomena in a dried ink trace, like other composite materials, are governed by the volume fraction of the filler rather than the weight fraction, it is beneficial to consider filler materials with lower specific gravities. Blends of materials also can be considered for some applications.

Several factors must be considered when choosing to apply flow-based micro-dispensing technology to enhance the functionality of a diverse set of medical devices. These devices include sensors, ablation catheters, electronic surgical devices, cauterisation probes, discrete heaters, radiopaque markings, monitoring devices and others. The selection of ink is the most critical factor, because it creates functional elements designed to meet application requirements. Within this decision, trace and feature dimensions, substrate material compatibility and the cost of materials also require careful consideration to create the most innovative medical devices that satisfy demanding applications. 

is Senior Research Scientist at
Micropen Technologies, 93 Paper Mill Street Honeoye Falls, New York, 14472 USA
tel. +1 585 624 2610

e-mail: lshawklein@micropen.com