Devices built for the medical marketplace must be developed quickly without compromising device operation. Many of the technical specifications that must be met detail requirements for accuracy and durability.

Fast development and attention to detail were the challenges we faced in the design of our Diagnostic Lateral Flow Cartridge. The device uses a lateral flow test strip to evaluate, within a matter of minutes at the point-of-care, whether or not a patient is infected with a virus. In conducting such a test, a fluid sample is collected from the patient and mixed with a buffer, after which several drops of the combined sample and buffer are added to a sample well. The sample is applied, test strips run, and a clinician then places the device into an instrument that reads the test result.

The final design of the Diagnostic Lateral Flow Cartridge shows the industrial and mechanical design aspects.

In parallel with the development of the internal cartridge features, we needed to develop an exterior industrial design that includes a user interface, features to aid in handling and insertion into the instrument, and aesthetic factors that give the product a unique appearance. The needed turnaround time for this project was four months.

Our first goal was to ensure that our vision and expertise aligned with the client’s vision and expectations. This required input from our designers, engineers and administrators, but also the experience of professional resources such as DSM Somos.

While we could machine prototypes, it would take too much time. We had to test proper device function using actual biological samples. Specifically: the sample is spiked with various quantities of a designated virus to evaluate the detection ability of the assay—a molecular biology procedure that tests and measures the activity of a drug or organic sample. The chemistry of the assay is then revised and evolved as necessary to achieve the required sensitivity.

Based on our analysis, we determined that working prototypes would best be produced through stereolithography. The material we chose was WaterShed® XC 11122, an optically clear rapid prototyping resin developed by DSM Somos with ABS-like properties and good temperature resistance. It is a fast-curing, low-viscosity resin, that produces clear, functional, accurate parts that simulate acrylic in appearance, and shows better water resistance than alternative resins.

We produced several iterations of the cartridge design while fine-tuning the geometry to optimize medical assay performance. These iterations were quickly evaluated to establish precise parameters for the final design. Our plan revolved around short prototyping cycles usually characterized by our ability to fabricate prototypes via stereolithography in one day, followed by one to two days of testing, leading to the design and fabrication of the next iteration on the fourth day.

We used two types of engineering methods in a parallel path to complete the device. On one path we used industrial design to make the product appealing to the user with human factors, branding, and ergonomics. In this case industrial design involved the grip features at the end of the device that enable a user to hold it in a specific way for proper insertion into the instrument that reads the test result. The pointed end is another industrial design cue that points the user to install the device in the correct orientation.

The industrial design also involved a two-color scheme for visual appeal. The overall size and shape are intended to be ergonomic, easy to handle, and clear to read. We made a variety of industrial design blanks through stereolithography for evaluation. Based on feedback we would iterate the industrial design as necessary until we achieved a final one.

The initial cartridge design was intended to allow rapid prototyping iteration and development of the critical internal features that interface with the test strip and that will maximize medical assay performance.

On the parallel path, we developed the mechanical design that includes features that assemble the product and ensure that the device functions properly. The mechanical design required an understanding of fluid mechanics, and the prototyping material allowed us to visualize the flow through the strip as it ran during this mechanical design phase.

Additionally, inside the cartridge we designed and built several features that engaged the strip in key areas. The degree of compression was critical for proper strip function, therefore accuracy and repeatability of the internal cartridge features were important. Hence, the mechanical stereolithography models we produced gave us the parts we needed and were as close to actual molded components as we could possibly get.

Once we had achieved our final industrial and mechanical designs they were merged and used to create a prototype mold. The fact that we were able to use stereolithography for both mechanical design and industrial design was critical to rapid turnaround. The WaterShed® material performed as promised, with accuracy and repeatability with each prototyped iteration. Some iterations required a tolerance of 0.002 in. And we completed the project in less than four months!

Symbient Product Development
www.symbientpd.com

DSM Somos
www.dsmsomos.com

MPF