Small amounts of adhesive ensure that every contact point and every block segment is reliably secured before the device is sealed with a potting compound. This prevents components from shifting during potting. It also prevents gaps from opening that would allow the compound to penetrate uncontrollably.

Potting produces a homogeneous protective layer over all the components to ensure long-term biocompatibility, as well as resistance to vibration and shock. Because body fluids tend to migrate into the gap between the primary and secondary barriers, an adhesion promoter is often applied between components to provide even greater protection and a long-term stable seal.

Precise adhesive application is also essential for assembling stents, particularly those made from materials with low radiopacity, such as nitinol. Such stents are difficult to see in X-ray images. To enable precise positioning in the body, tiny radiopaque markers are applied to the stents. For this purpose, gaps of just 15 to 30 microns are filled with minute quantities of adhesive. These micro-doses preserve the stent’s mobility, maintain its flexibility, and ensure long-term biocompatibility.

Another typical field of application for microdispensing is catheters. Here, a variety of components, such as connectors, must be joined within tight tolerances. Tiny dots of adhesive must be applied to secure the assembly without impairing the catheter’s flexibility or restricting flow in its lumens. Overdosing could lead to constrictions, blockages or even particle detachment. Underdosing could create weak mechanical points.

Some medical device assembly applications require dots of adhesive that are less than 2.5 millimeters in diameter. Photo courtesy ViscoTec.

Technological Challenges and Solutions

Microdispensing places high demands on dispensing systems, since even the smallest deviations can significantly affect the result. The axis accuracy of the dispensing platform must be precise and repeatable.

Among the greatest challenges, however, are material variations caused by temperature differences, entrapped air, or manufacturer-related viscosity deviations. Time-pressure dispensing systems reach their limits here because their results depend heavily on the fill level of the pressurized cartridge. Full, half-full or nearly empty cartridges can produce different application volumes, since air compresses differently than liquid.

Rehm Thermal Systems of Blaubeuren, Germany, is familiar with these challenges. The company makes automated systems for dispensing, coating, soldering, drying, metallizing and testing electronics, solar panels and medical devices. To ensure precise assembly, Rehm relies on microdispensing technology from ViscoTec.

Rehm Thermal Systems can provide a complete automated system for dispensing and curing adhesives on medical devices. Photo courtesy Rehm Thermal Systems.

A key advantage of ViscoTec’s technology is its progressive cavity design. A rotating screw transports material uniformly and without pulsation. This ensures high repeatability and makes the process independent of viscosity fluctuations. In contrast to time-pressure systems, dispensing is purely volumetric. The screw displaces the same volume per revolution, regardless of the cartridge fill level.

A key advantage of ViscoTec’s technology is that material variations, such as temperature changes or fillers, have little effect on dispensing consistency, because the screw conveys even highly viscous or filled media gently and at a constant delivery rate. Fillers remain in suspension, and there is no air entrapment or phase separation.

ViscoTec offers several options for microdispensing applications in medical device assembly. For example, the company’s new Vipro-Head dispensing valve is designed specifically for medical device assembly. All contact parts are made of stainless steel or FDA-certified plastics and elastomers. An autoclavable variant can withstand temperatures of up to 121 C.

Another option is the Preeflow Eco-Pen XS 180. It is designed for ultra-small dispensing volumes. Its compact design facilitates integration into automated assembly systems and enables parallel operation of multiple valves.

The Preeflow Eco-Pen XS was developed specifically for the smallest dispensing volumes. Photo courtesy ViscoTec.

With dispensing accuracy of up to ±1 percent, adhesive quantities in the microliter range can be applied precisely and reproducibly.

Dispensing is just the first step in the process. The material must also be cured. That’s where Rehm’s technology comes in.

To avoid air entrapment between the substrate and the dispensed material, the curing and drying steps must be carefully controlled. With a precisely coordinated temperature profile, the materials cure completely and form a smooth, homogeneous bond. Depending on the chemical composition of the materials, Rehm’s curing and drying systems are configured individually. Both infrared and ultraviolet technologies are available.

Quality Assurance Through Maintenance and Calibration

In validated medical device assembly processes, regular maintenance and calibration are crucial to ensure accurate and reproducible dispensing. A clean applicator is particularly important. ViscoTec’s technology solves that problem. The progressive-cavity screw can rotate forwards and backwards. The amount of retraction is programmable, so engineers can set a defined string break without dripping. This prevents unwanted material discharge.

In addition, Rehm equips its dispensing and coating systems with a tape that automatically cleans the dispensing needle before each application. The machine’s controller enables engineers to set every parameter, including material type, nozzle diameter, dispensing volume and flow behavior. Material output is monitored via precision load cells, so deviations can be detected and prevented at an early stage.

To determine location and position, even for soft or complex-shaped medical products, the systems feature automatic needle measurement, which checks the applicators at defined intervals and automatically readjusts them if necessary. An integrated fiducial camera detects alignment marks, corrects the zero point, and ensures precise operation, even if the assembly is not positioned accurately in the fixture. In addition, a 3D height sensor automatically compensates for any warpage or instability, which can happen with thin substrates.

An integrated 3D height sensor enables this automated system to dispense adhesives on parts with complex geometries. Photo courtesy Rehm Thermal Systems.

Flexibility and Reliability

If flexibility is important, Rehm’s dispensing systems can adapt to new applications “on the fly” with minimal changeover. Dispensing technology can accommodate various materials, including viscous silicones, watery acrylates and filled epoxy resins. Rehm’s technology automatically adjusts to accommodate different nozzle types, needle lengths, application patterns, and process parameters. Control software manages material parameters and dispensing paths centrally and automatically loads programs depending on the product, minimizing changeover times and increasing system availability.

To meet regulatory requirements, each assembly line is validated for a specific product; changes to the process, geometry or material are only permissible with extensive revalidation. In these cases, process stability takes priority over flexibility, and dispensing systems are usually tailored precisely to one product. In fact, EU medical device regulations stipulate that operator intervention must be minimized as much as possible in high-volume production.

Regardless of production volume, cleanliness is important in medical device assembly. Rehm’s systems are designed for ISO 7 clean room environments.

Material compatibility is also important. Every part of the dispensing system must be compatible with the material to be dispensed. This is particularly important for metal components. To ensure biocompatibility, aluminum surfaces are often replaced with stainless-steel variants.

Data collection and analysis are essential for medical device assembly for both quality assurance and regulatory compliance. Production must be traceable for many years. If a problem occurs, it is not sufficient to narrow the problem down to a rough time period. Systems must be able to determine exactly which serial numbers are affected, which process parameters were present during dispensing, and under which conditions the product was manufactured. This enables targeted recall actions without unnecessarily removing safe products from the market.

This is made possible by consistent traceability with individually configurable interfaces and continuous data acquisition. Temperatures, material batches, serial numbers, and process histories can be captured and synchronized horizontally between machines and reported vertically to higher-level MES systems.

Material and process interlocks provide extra safety. Before each start, the system checks whether the current program matches the intended product and material. If anything does not match, the process is not released and is automatically locked. These measures achieve seamless traceability and enable immediate response to process deviations.

Design for Automation

A particular challenge lies in scaling prototypes up to series production. New products go through lengthy development and validation phases, and only once all processes are qualified may they be transferred to series production. To shorten these timeframes, it is crucial to consider future automation during product development.

Both Rehm and ViscoTec support customers early in the development phase by carrying out practical trials using production and prototype parts. Under realistic conditions, optimum process parameters can be identified and potential challenges addressed early. This close collaboration upfront makes it possible to accelerate the transition from development to production and avoid later adjustments or revalidation.

The Vipro-Head dispensing valve is designed specifically for medical device assembly. All contact parts are made of stainless steel or FDA-certified plastics and elastomers. Photo courtesy ViscoTec.