Assembling medical devices can be a difficult task when dimensions are very delicate and when exact bonding is the prerequisite to a functioning part. The innovative test chip from GBO Greiner Bio-One GmbH, called Genspeed, is such a part. Used to detect MRSA (methicillin-resistant Staphylococcus aureus) infections, one of the world’s most frequently identified hospital pathogens, it’s made up of two parts: a microfluidic upper part of polystyrene (PS) and a PS film as a bottom component, both of which have to bonded in the assembly process.

GBO decided on ultrasonic welding as the method for joining the two halves. Ultrasonic welding provides the fine parameterization to weld the PS microfluidic test chip in such a way that its function is assured. The cross-section in the test channel has to remain constant at all times in order to achieve the required capillary effect. An energy director only 0.1 mm in height on the part allows dimensionally precise joining without the expulsion of melt.

The bonding requirements which ultrasonic welding has to meet are as follows:

• The bonded parts have to be absolutely leak-tight in order to function as test chips.
• A visually perfect appearance is indispensable in medical technology and crucial for subsequent opto-electronic test evaluation.
• A dimensionally perfect fit is of elementary importance, otherwise the flow behavior inside the reaction channel necessary for the sensitivity of the Genspeed test is impaired.

Focusing the Ultrasonic Energy

In order to ensure the transport of liquid via the microfluidic channel into the “waste reservoir”—the collecting vessel for the reaction fluids—by capillary action, the cross-section of the reaction channel has to be reproducibly identical. Only a sensitive joining method such as ultrasonic welding with exact parameterization can satisfy this demand. The parts are very delicate, with a thickness of 1.5 mm for the upper part and 0.15 mm for the film.

The upper and lower parts are joined by welding a 0.1-mm high energy director along the microfluidic structure. A reaction channel measuring 30 x 2 x 0.1 mm is created in which the individual test steps are performed by capillary action. A further energy director runs around the outer contour of the upper part to fix the complete film. In the waste reservoir there are four small spacer domes which ensure that the film is positioned flat under the waste area. (During the application test series, it was also discovered that the gentle ultrasonic vibrations did not affect the device’s capture molecules.)

One advantage of the ultrasonic technique is the targeted energy input which permits controlled and dimensionally stable melting. Thanks to the special joint design, the ultrasonic waves are focused onto the joint zone without exerting a thermal burden on the part. The joint design for injection molded parts consists of appropriate welding geometries with peaks or edges in the joint zone—these energy directors define where the melting begins. During the course of the process optimization, the welding tool or “sonotrode” surface is partially milled away outside the seam geometries; this improves the focus of the ultrasonic waves, prevents coupling at the surface, and results in lower mechanical impact on the film. 

Weld Depth: A Process Criterion

The welding process can be defined and optimized using a variety of weld modes. This is expedient, as materials react differently, and part forms can be more demanding. For the Genspeed test chip, a miniature part, the melt distance measured from a “reference zero point” is defined as a fixed shutdown parameter—that is, the ultrasonic vibrations are switched off when the programmed weld depth is reached.

By zeroing the reference point, the exact point at which the weld process is to start is calculated individually for each new welding. The weld depth is then always the same and the melt volume remains constant. With a 0.1-mm high energy director, the weld depth is less than 0.1 mm. This determines the energy and time required for each weld process.

The machine control displays the weld process as a graph. After analysis of the graph, the process is further optimized by defining tight process windows with minimum and maximum values. The curve of the joining velocity, in particular, is important; it is a unique “fingerprint” and should ideally have a constant linear gradient, because under normal circumstances a constant joining velocity guarantees a stable weld process with reproducible weld strength.

Biotechnology Demands: Traceability

Important for Greiner Bio-One for the production of the test chips are the software validations for the welding process demanded by regulatory requirements in the in vitro diagnostics sector—validations which the Herrmann HiQ Dialog ultrasonic welding machine offers. The requirements are that all production steps are protected by passwords, that authorization packages and logbooks are created, and that extensive software audit possibilities are available.

The new FDA Software Component (FSC) from Herrmann Ultrasonics offers extensive user authentication and monitoring of the authorizations. All parameter changes and user actions are recorded in electronic audit trails. If a parameter is changed, the previous values can be called up again at any time. This type of traceability of production steps has already been important in certain branches for many years, such as in the pharmaceuticals and food industries. The FSC component satisfies the demands of the U.S. Food and Drug Administration’s Directive CFR 21 Part 11, which are also important in Europe.

About the author… Astrid Herrmann’s role in Herrmann Ultrasonics covers public relations and strategic marketing. She says she’s “infected with the ultrasonic virus” and is always looking for ways to transfer ultrasonic technology benefits for the customer into meaningful best-practice application articles (for example, visit www.herrmannultraschall.com/en/newsroom00).