Disposable Design: Consider All Interfaces

An increasing number of disposable components are being used in healthcare settings today. The biggest drivers behind this are costs, efforts to reduce infections from cross-contamination, and the demand for body-worn therapeutics/drug delivery. Technology is also driving more electronics into these components, often to reduce risk, while increasing accuracy and functionality.

At the same time, the disposable component is becoming more challenging as products get smaller and more complex. For example, advances in microfluidics and lab-on-a-chip, genetic-based assays have increased the number of functions performed on a single disposable. This, in turn, complicates the interface between the disposable and the durable component. In some cases, we are seeing hundreds of physical and electrical connections between the disposable and durable parts within a single device, all of which must work seamlessly every time. This is forcing designers and manufacturers to approach these connections with great care, recognizing the probability of an error is proportional to the number of connections.

Together, we’re seeing an environment where more and more complex disposables are being used, and the designers and engineers of these products need to develop methods to meet the new demands. Our strong suggestion is to start with the interfaces between the disposable and the durable component.

Disposable Design Interfaces
We have seen ample cases where product designers start with the durable component, thinking this will support the timely development of the disposable. This is often not the best approach. The unique features and functions of a complex, highly functional disposable often defines how it will interface with the durable part, so it’s usually best to start here (i.e., understanding the details of each feature and function needed in the disposable). This will often allow the rest of the product design to fall into place.

Another caution is to recognize not all disposable interfaces are physical features. For example, designers sometimes limit their focus to the electrical or fluid connections between the disposable and the durable component. Be sensitive to the less obvious design interfaces that can affect reliable function and performance; several of these common interfaces are noted in the following sections.

User Interface
The goal of designers is to create intuitive products, with a smooth, seamless workflow, and no critical-use errors. Designers who think they know how the end-user is going to interface with a device have never participated in the early stages of a formative user study. It can be humbling to watch (through a two-way mirror) as a nurse or technician takes a desiccant cartridge and tries to jam it into the slot where an air tubing cartridge fits, or cringe as roughly half of the end-users put an air mask on upside down (as in the CoolStat nasal mask for CoolTech Medical). This is not the fault of the users; it’s incumbent on the designers to ensure the product will be used properly, with an appropriate level of instructions or training. Designing and running user interface studies is an art in and of itself, so don’t try to go it alone if you’re not experienced. These studies are vital to understanding how end-users will interact with the product. This is especially true when the product has a disposable component. It will almost certainly take several iterations (formative studies) to get it right, and it becomes more costly and painful the longer you wait to do it. At the highest level, these studies should simulate the real-use environment, using the actual end-user (nurse, doctor, technician, etc.), lighting, physical space, time restrictions (if applicable), etc.

Outcomes from these studies will generate some surprises, but the solutions don’t have to be complicated. For example, interfaces for different connection points can be physically and visibly different to avoid confusion. Use poke-yoke elements and color-coding. The solution that solved the problem of users from putting the CoolStat mask on upside down was a simple “This Side Up” sticker.

Electronics
Electronics are increasingly used in disposable medical devices, especially those designed for point-of-care and sample-to-answer applications. One example is sensors that record key patient data, such as vital signs. Once prohibitively expensive, it is becoming cost-effective to make accurate sensors that can be embedded directly into the disposable set. The market for disposable sensors is booming. Data Bridge Market Research estimates growth in the disposable sensors market of nearly 10 percent annually between 2020 and 2027. So don’t discount the option to use these kinds of sensors prematurely.

Data storage is another area that has expanded to include the disposable component. The ability to place data storage directly on the disposable can speed up workflow in the end-user environment and minimize the need for costly training or reliance on labeling/instructions. This is especially valuable for disposable configurations that look similar but have different workflows. In some cases, this can even be extrapolated to using simple PCBs on the disposable component, with onboard memory functionality and connectors that can be manufactured for well under $10 in bulk. In higher-end disposable components, for example, with DNA-based functionality, a PCB in this cost range is a fraction of the cost of the packaged disposable unit. While cost is a significant consideration for use of disposable PCBs, and they add a step to the assembly process, including memory in disposable elements can increase reliability and reduce reliance on users, and therefore, reduce risk, making it well worth it in a growing number of cases.

Electrical measurements of and through fluids (e.g., capacitive fluid level sensing) are being used more and more in disposables that involve fluidics to ensure reliability of the assay operation. There are unique design constraints when the electronics are not only functional printed circuit boards, but also fluid interfaces. For example, flatness of the fluid interfacing elements is often more important than it would be on a typical PCB. Electronic fluid manipulation in microfluidic chips is another new and novel interface application with a slew of important design considerations. Designers are developing entire devices around these new methods of electronic manipulation. The key to success in this space is to use high-end, experienced fabricators able to produce high volume boards.

One side note regarding disposables and some medical facilities, especially in Europe, is the increasing requirement to follow strict guidelines for disposal of electronics waste (e-waste). In many cases, providers are being charged for disposal by quantity and type of material waste. If you decide to embed electronics in your disposable, design it so the electronic component can be separated from other disposable waste, to save added cost that may be imposed for e-waste. Separation of e-waste from traditional consumable elements can be achieved by the instrument or a user, facilitated by designing them as discrete elements.

The Physical Interface
This interface is the most familiar to designers and engineers, but we need to note it here for completeness. This, of course, is the method used to physically dock or connect the disposable component to the durable device. Placement confirmation is always a good method. For example, whenever practical, design an interface with feedback, such as electrical resistance, pressure differential, light sensors, or limit switches. This is extremely valuable to confirm both the presence as well as the proper connection and placement of critical connections of the disposable to the device. One suggestion for your early alpha prototypes is to create visible access to the disposable so you can see what’s happening in the disposable component and how it’s interacting with the durable parts. Without it, you are blind to what is going wrong inside the device during operation. The end user should be provided feedback at each critical operating step to confirm it was completed properly. Visual and audible cues are great for this.

It is also important to design interfaces to eliminate wetting or any cross-contamination of the capital equipment. For example, diaphragms, magnetic beads, or peristaltic pumps are great for fluid manipulation and mixing to avoid wetting the durable parts. Similarly, blister packs for reagent injection are great for mixing in lab-on-a-chip applications. Use compliant interface parts such as grommets, films, and gaskets whenever possible to minimize reliance on tolerances.

Disposable Design Needs to Optimize Performance
We are seeing more and more complex disposables being used today as new assays and technologies are influencing design options. The designers and engineers of these products need to develop methods to meet the new demands. The growing interfaces between the disposable and durable parts of these products are essential for reliable, safe operation of devices, and can be affected by many factors including the end user, workflow, assay complexity, cost, intuitiveness of tasks, fit, etc. Every project will have its own set of unique challenges. 

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Will DeMore is a mechanical engineer and project manager at Key Technologies Inc. He has roughly six years of experience in new medical product development and has managed several interdisciplinary projects with electrical, computer, and mechanical engineers and industrial designers. Prior to Key Technologies, he received his bachelor’s degree in mechanical engineering from the University of Maryland, College Park, and is currently in the process of earning a master’s degree in engineering management from the Johns Hopkins University.