The advantages of joining technologies for flow management devices

Kun Wang, business development manager, medical Branson welding and assembly at Emerson discusses the advantage of joining technologies for single-use medical flow management devices.

To achieve optimal effectiveness, medical tests and treatments need to be simple and easy for healthcare professionals to administer. Patient accessibility and ease of use are also key factors. With these objectives in mind, medical manufacturers are creating innovations in medical flow management devices ranging from microfluidic “tests on a chip” to in-vitro home tests, enhanced oxygen therapies, and patient-friendly peritoneal dialysis equipment for home use. 

Making diagnostic tests and therapies more user-friendly for nonprofessionals creates novel design and manufacturing challenges. Home-based test results must be just as reliable as those generated in a laboratory, with consistent and dependable fluid and gas flows. The tests must also be lightweight, easy to ship and affordable. To protect patients from adverse outcomes, such as peritoneal infection due to nonsterile fluid connections, they must be designed only for single use. To meet these complex demands in mass production, medical manufacturers produce flow management devices made of engineered polymers. These are typically assembled from multiple injection-moulded parts and assembled using plastic welding. 

Plastic welding options 

Single-use assemblies, like automated peritoneal dialysis (APD) cassettes, can benefit from ultrasonic plastic welding. This widely used assembly technique combines vibratory motion, heat and gentle compressive force to create strong welds between plastic components. Components are secured in position and then subjected to high-frequency (10–70 kilohertz), low-amplitude (1–250 µm) mechanical vibrations, which generate friction to melt mating surfaces and create a strong molecular bond. This process suits high-volume production due to its short cycle times - often less than a second - and the ability to integrate with automated production lines. Innovations in ultrasonic welding allow precise control over downforce and weld depth, enhancing accuracy of the weld. 

Ultrasonic welding, like other plastic welding methods, joins assemblies without the need for expensive consumable items such as adhesives or fasteners. However, parts must meet specific geometric and material criteria for this method. 

Certain flow-management products are sometimes assembled using a different process: hot-plate welding. Here, mating components are pressed against a heated plate until the edges soften, and then they are joined under pressure as the plastic cools. This method offers a high level of control over heating and weld quality. 

For devices requiring smaller microfluidic channels, like single-use disks, cartridges, or cards used in diagnostic tests, the added precision of laser welding is often preferred. There are two different approaches: Simultaneous Through-Transmission Infrared (STTIr) welding and a Quasi-Simultaneous method. Both employ laser energy to heat surfaces to be welded with unsurpassed precision. In these processes, two components - one made of a laser-transparent plastic and the other made of a laser-absorbing polymer - are pre-assembled and held together while a light beam is directed through the transmissive component to the absorptive surface. Heat is generated at this interface, melting the plastic so bonding can occur.  

In STTIr welding, laser energy is directed through specially shaped fibre-optic waveguides so the entire weld joint is heated simultaneously. This results in a lower power density and shorter cycle times, which are ideal for mass production. Quasi-Simultaneous welding uses moving mirrors to trace a laser beam along the weld joint. This requires higher power density and longer cycles but offers precise control over energy application. 

Single-use fluid management components  

At-home peritoneal dialysis systems, blood separation units, and “wearable kidney” devices are examples of systems that incorporate disposable fluid management components. These components, such as injection-moulded cassettes and cartridges, are assembled using different plastic welding techniques depending on the application requirements. Here are three applications, each involving the use of a different welding technology.  

APD Fluid Management Cassettes: The at-home peritoneal dialysis market heavily relies on single-use products, as patients must use a fresh, sterile set of fluid connections to perform dialysis regularly and prevent peritonitis. To ensure home-based patients can correctly make essential fluid connections - from their peritoneal catheter to PD fluid bags and the APD machine (cycler) - manufacturers have developed precise yet cost-effective single-use plastic cassettes. These cassettes provide an easy interface to the APD cycler with external connection ports for flexible tubes. Internally, they feature various fluid paths, chambers and valves. 

Typical APD cassettes are assembled by ultrasonically welding two injection-moulded halves. This assembly process is challenging because each part must be made of transparent, biocompatible plastic, and their internal features must join cleanly and consistently to form fluid channels or circuits connected to external tubes for different fluid flows during the APD process. For these reasons, ultrasonic welding is the process of choice.  

Wearable Kidney: The “wearable kidney,” currently in clinical trials, is a portable device consisting of a small infusion machine and a special disposable adsorbing cartridge. It aims to enable continuous, tidal peritoneal dialysis using just two litres or less of dialysate solution. After the dialysate fluid is infused, small amounts of dwelling dialysate fluid are passed through the adsorbing cartridge, where thin, absorptive membranes capture and remove uremic toxins from the spent fluid. The infusion machine then reconstitutes the fluid by infusing it with minerals and glucose to desired levels, allowing it to be reinfused. This process of continuously draining, cleansing, reconstituting and reinfusing small amounts of fluid makes continuous “tidal” PD possible. 

The wearable kidney relies on a laser-welded plastic assembly. Laser welding is chosen because it can hermetically seal the cassette while providing control over heat inputs. 

Blood-separator Cassettes: Cassettes from a blood separator unit are assembled by joining matching injection-moulded parts using hot-plate welding. While this disposable part is associated with a hospital-grade blood separation unit, its single-use cassette design reflects the same design imperatives as point-of-care type products. The cassette accepts several flexible tubes that, together with the separator machine, process fluid flows to separate various blood components.  

In each of these applications, the design of the specific device dictates which of the various plastic-assembly options will be chosen for use. To ensure success, developers of flow-management devices should work closely with the experts from their welding-equipment supplier.