Why Do LFI Cassettes Give Problems?

By June 6, 2017Articles
Rapid diagnostic test, lateral flow assay, lateral flow test, point-of-care diagnostic

– Keith Kopitzke and Brendan O’Farrell, DCN Diagnostics

The standard approach to lateral flow cassette design is for the strip developer to transfer the specifications for a strip to an industrial design house or an engineering firm that handles the design of the cassette. Often times, this happens towards the end of the development phase. The cassette design process is then handled in an iterative fashion, moving from rapid prototype parts through low cost molds with inserts and on into a production mold. As test developers, DCN long ago realized that this element of the development process can lead to annoying delays and cost drift when it is handled separately from the test. This comes from a variety of factors, including starting the design and integration of the cassette too late in the process, the need for multiple suppliers to develop the assay and the cassette, and the typical process that OEM cassette developers tend to follow, where each cassette tends to be designed from scratch using “industry standard” design features, low cost molding and assembly methods.

DCN has never been happy with that approach, as we know from experience that when it comes to product performance the cassette is an essential part of the product. It’s the key to functionality of the ultimate product, to usability, to user satisfaction, as well as to branding and value proposition. This component should not be an afterthought.

So while investing in the development of a high performance lateral flow test, why design another piece of cheap white plastic that is poorly molded and prone to failure? Why follow the “industry standard process”? At DCN Diagnostics, we don’t. That’s why we’ve designed a process for cassette design and development that supports our clients’ needs for cost, quality and performance that is a hassle-free and effective.

Let’s examine the common issues and talk about some potential solutions:

1. Cassettes tend to be an afterthought

One of the most common – and biggest – mistakes made in lateral flow device design and development is that the cassette is considered as an afterthought. The focus is placed on making the test work and it is assumed that one can drop the assay into a cassette, possibly even a generic cassette that is taken “off the shelf”, and everything will work fine. After all, the cassette is just a holder for the strip….Not!

The cassette and the test are intertwined in their design and functionality. A good cassette design will rarely overcome problems in the test architecture or design (the test does need to work independently of the cassette first!).  A bad cassette design will definitely exacerbate those problems.

Typical problems seen when developers try to drop a strip into a cassette that has not been designed for the strip architecture include:

  • Sample leakage and flooding of the strip
  • Improper or uneven flow patterns
  • Incomplete flow (tests don’t run)
  • Uneven line development
  • Bad background development
  • Red blood cell leakage if it’s a whole blood-based test

Most of these failures are catastrophic to the overall performance of the product, and all of them can be fixed relatively simply by designing the cassette to match the strip architecture and the needs of the sample.

2. Standard Practice does not yield well designed or well molded cassettes (sorry to say!)

In the lateral flow OEM segment, generation of cassette designs for years has depended on a relatively standard process. Assay developers engage an engineering firm to design a cassette once the test has been developed to a certain point. Initial conceptual and functional designs result in the generation of SLA’s (stereolithographic, rapid prototype parts) that are used to “dial in” the key pressure points and closure pressures in a lateral flow cassette. The next step is typically to move into a cheap mold insert (or MUD) that is used to produce parts for testing. This mold is modified, sometimes repeatedly, until the parts it produces work adequately. This is a slow, tedious, iterative process that eats up laboratory, engineering and machine shop time and it costs money.

Also, this process doesn’t work very well in many cases.  SLA parts don’t allow for good approximations of closure of molded parts and the materials used are different and behave differently to the final molded parts.  In addition, this process is often performed by contracted engineering firms who do not have intimate knowledge of the test and how lateral flow actually works.  Finally, the design of the part is often considered separately to the rest of the manufacturing process including cassette closure methods. This is a potentially disastrous mistake (see #4 below).

DCN’s engineering and assay development teams work hand-in-hand all the way through the process. Our industrial designers produce designs that address functionality and user needs. SLA parts are produced to assess look and feel and initial functionality. We then move very quickly into the use of Protomold parts that behave like the final molded part. Crucially, our process starts with a proven base cassette design that within two or three iterations at most will typically generate the final part. This yields a highly cost-effective and very fast process. Additionally, the design is done in conjunction with the development of the assembly process, so that closure issues are considered from the beginning. We’ll talk more about that later.

3. The focus historically has been on cost vs quality

This can affect the design of the part and the design of the molding process. Cost pressure tends to force molders to skimp on measures that can produce better quality parts. Poor gating, poor mold flow, poor part ejection are all aspects that all lead to part failures in the final test. They are easily fixed for a penny or two per part, so it’s short sighted not to do it right.  In this day and age, for any except the very lowest quality qualitative tests, this is no longer an acceptable approach. Proper attention needs to be paid to mold design and molding process. Spend the money to do it right! Short term pain will yield serious long term gains.

4. Closure Design, Closure Process, Closure, Closure, Closure…….

Do we need to emphasize this more?!

Many issues happen because of what has become an industry standard cassette closure design and closure method. Old school pin and boss closures still predominate, often with simple round pins and round holes. There’s too much detail to go into here, but give us a call and ask us why this is a problem and how it’s easily fixed. Also, the closure process is often ignored during the design of the part. This is a CRITICAL mistake. The entire functionality of the device depends on the fidelity and reproducibility of the closure and the pressure points within the cassette. The cassettes created by DCN Diagnostics are always designed with the closure process in mind. If pins are used they are never round pins going into round holes. Pins and bosses are placed at locations that ensure that the strip can’t be over or under compressed. Guide features ensure that the pins go into the holes properly during closure and don’t shear or deform.

It’s simple stuff, but it’s got to be considered. Performance is always tested in our labs using the closure method for which the part is designed and that whole process is transferred to the manufacturer. Oh, and don’t ever rely on the pins to define the closure pressure of the cassette. Ask us why and how it can be done better.

Don’t waste the opportunity to design the cassette properly

The process at DCN Diagnostics yields high performance cassettes at the appropriate end unit cost using a cost effective, hassle free process with minimal iteration. We also design cassettes that are user and application centric. Appropriate user centric design ultimately means adding value to the product while removing complexity for the user. It does not necessarily mean adding cost or complexity to the product. Don’t stick to “industry standard.”  Talk to the team at DCN Diagnostics about getting more value from the cassette design and development process and building more performance and value into your product.