Behind the Design: How iRhythm Built Its New Zio Monitor

iRhythm Technologies recently launched its next-generation Zio monitor and enhanced Zio long-term continuous monitoring service in the United States. The San Francisco, CA-based company touts an improved form factor with the new wearable heart monitoring device, which is 23% thinner, 62% lighter, and 72% smaller compared to previous generations of the technology.

But Mark Day, the chief technology officer at iRhythm, told MD+DI that the goal in designing the Zio monitor wasn’t just to make it smaller, but to deliver a better patient wear experience. Below is a Q&A based on our conversation with Day, in which he explains why the company made these design changes, and how it accomplished it from an engineering perspective. The responses have been edited for length.

MD+DI: What was motivation behind the design considerations for the new Zio monitor?

Day: It’s not just about making it smaller, it’s about the impact. The journey was really that we took a lot of the information that we've had and feedback from both patients and providers that we've received from the over 6 million Zio XT devices that we've manufactured, distributed, and serviced. And we took a really hard look at how we can improve that experience. So, that was the impetus. It is still, despite its fairly long tenure on the market now approaching the better part of 10 years, it’s still an industry-leading diagnostic device with respect to its wear time and patient compliance, but we saw an opportunity to really improve it.

MD+DI: What design and engineering challenges had to be overcome to make the device so much smaller and yet still manufacturable?

Day: From a technical viewpoint what we did was really to leverage a lot of advances not just in microprocessor efficiency, which is certainly part of it in terms of miniaturization as well as energy efficiency, but also we really challenged ourselves to figure out how to effectively fit in the smallest possible form factor, which for us was motivated a lot by the size of our battery, which is just a standard, easily available, and fortunately easily recyclable coin cell battery. We challenged ourselves to figure out how to miniaturize the device down to the level of one of those coin cells instead of the two that the Zio XT device uses. And that became feasible by the advances in microelectronic microprocessor efficiency and size as well through its miniaturization. But it also required a lot of really complicated dynamics from an electrical engineering perspective to figure out how to get all the components of a wearable medical device and a biosensor onto that small form factor. So, miniaturization was focused on very clever ways of handling some of the larger components, which for example involved moving some of the big resistors that are required on the device to the more flexible part of the patch instead of on the actual printed circuit board in the housing. And this is a patented feature that we've already received a patent on it and we're pursuing more around it, but it was the idea that these three big resistors are not too big in terms of the homage that they have but also in terms of the size that they are. So, if you put them on the circuit board, they are these pretty big components, so we realized pretty early on that we couldn't really do that and still fit the design constraints. So, we innovated a different way, which was to integrate it into the ECG tracings themselves.

MD+DI: That's really cool. Were there any other design changes or unique features added to this device compared to previous generations?

Day: I'd say we also learned a lot from our experience of how to attach circuit boards to a flexible housing. That was certainly another area that we’d already explored with the Zio XT device but could yet improve based on the feedback and what we saw both in our manufacturing as well as our intake experience from the device.

But the other thing is to really talk more about the adhesive structure, which we call the dermal adhesive assembly. It's really the part of the patch that secures itself to the patient, which is obviously critical. For that it wasn't so much that we went back and reimagined a different adhesive, we do value the place that we've gone, which is the custom adhesive formulation. And we still have a lot of trust and belief that is effectively an appropriately balanced approach where you kind of have to have an aggressive enough adhesive to be able to work well, but not so aggressive that it's uncomfortable or irritating to a meaningful portion of population. So, it wasn't so much on the adhesive itself but rather on our observation that over time certainly one of the things that we've seen is that when patients sweat ... that sweat gets absorbed in the hydrocolloid adhesive, and one component of that adhesive in particular. And normally, like on the Zio XT patch, which is basically a fixed surface, it doesn't really have anywhere to go. So, it goes into the adhesive, and it changes the structure of the adhesive a little bit, and it has to basically get reabsorbed in the skin, which it will do if the patient stops sweating over time, but that's not necessarily an ideal approach.

So, we put perforations in the dermal adhesive assembly itself to allow that moisture to transpire in one direction to be able to handle that moisture creation more efficiently and frankly, from a performance perspective, more effectively, and more comfortably from a patient's perspective, which is that you don't have this kind of moist mass against you for a long period of time, it figures out how to kind of passively get the moisture away from the skin. And we think all of that is going to result in a more comfortable experience for the patient.

MD+DI: I noticed the press release about the Zio monitor launch mentioned the device has a waterproof housing, but then in the footnotes it basically said that doesn’t mean the device should be submerged in water, so patients still have to be careful if they’re in a bathtub or whatnot.

Day: In general, water and adhesives don't mix very well together. But the main reason [the device shouldn’t be submerged in water] is because water conducts electricity, so it will actually short the electrodes when you are swimming or you otherwise submerge it, you will also impact the performance over time. You can do those things, but we don't recommend them, you won't get any ECG that's readable of that. And it can’t do good things to the adhesive, it will probably shorten your wear time.

MD+DI: What other unique takeaways or design and engineering lessons can you take from this project that you might be able to apply to a future project?

Day: The other lesson that we put into this as well was to prepare for the future, in a lot of ways. In the time frame of the Zio XT device being commercialized we also introduced the Zio AT device, which looks similar to the Zio XT device (same form factor and size) but underneath the hood, the circuit board underneath that device between those two devices is different, one having Bluetooth the other not.

And really, as we look forward to what we think the future looks like for biosensors of all kinds is that they have a connected feature in them. And we'll have to work through the appropriate regulatory and reimbursement processes to see this fully change, but I would strongly suspect that in five to 10 years the idea of a wearable device that doesn't communicate with some type of back-end infrastructure during, say, a wear time of a cardiac arrhythmia patient, will not be competitive.

...All of this gets put into the engineering consideration of effectively looking at the landscape around you and trying to figure out what kind of technology you need to put into a hardware platform, because you don't want to be changing your hardware platform continuously, it's extremely costly for a business to invest in a new hardware platform ... So, you want to take an approach of making sure that you can design and develop a circuit board and frankly, an entire platform, that is future proof from the perspective of being connectable not just to our own infrastructure of say a gateway, a cellular gateway, but also a patient’s phone. That just opens up all kinds of possibilities and optionality of the platform in the future so, we really designed this to not just perform well from a patients perspective, but also to enable this platform to be a true platform in the sense that the Zio monitor device that we just introduced and we're rolling out right now is really the first of a number of different device services, effectively, that we deliver off this platform.