The challenge of designing and developing an underwater heartrate monitor for swimmers

The product in question is Instabeat, a heart rate monitor intended to help swimmers monitor their training schedules. It was the brainchild of Hind Hobeika, a Lebanese mechanical engineer and former competitive swimmer, who worked closely with the Aavid Thermalloy design centre to realise this concept and start prototyping the design for production. Her idea was to provide swimmers with the sort of training information that runners were accustomed to. "It was based on the idea that it had to be frictionless for the swimmer. There are a lot of watches on the market that can measure heart rate in swimming, but swimmers do not wear them because they do not provide real-time feedback and they create friction," said Hobeika. "We wanted a device that would be integrated into an accessory that swimmers already wear, and swimming goggles were the best option because they are great place to provide feedback for the swimmer too!" It was decided to design a module that would mount on most goggles, rather than build electronics into the goggles, because goggles are both very personal and disposable items. However, while the technology is in principle the same as for a running or cycling heart monitor, there are a different set of challenges because of the water and the location on the head. "The artery location can differ between one person and another so we had to add multiple sensors to make sure all cases are covered," observed Hobeika. "The blood flow in this area of the head is really little and the signal needs to be much more amplified than if it were on the wrist or on the finger. The amplification creates a lot of noise and makes it harder to detect a 'clean' signal, thus a good accuracy." Also, if water goes over the sensor it impacts the heart rate reading, and this had to be accounted for in the mechanical design - to have it in such a way that water cannot interfere with it. To accommodate the unique design needs of this new device, Aavid Design, a branch of Aavid Thermalloy, was employed to help adapt the product design to accommodate the rigorous and highly specific requirements for the Instabeat. For the Instabeat, the Aavid team worked to refine the design so that it would be more reliable and compact. Product considerations at this stage of the design were that it was to have no connections that would be exposed to water and that information transfer was through USB. Power is supplied by an internal rechargeable Li-ion battery. Prototype phase It was the underwater operation of this product that made it particularly difficult to prototype, as Andy Grunes, director at Aavid, described: "The initial challenges were with material selection – selecting the correct materials that would bond well to each other and thus create water tight seals. Effectively the product is electronics moulded into low hardness plastic. In addition to being water tight the product had to be flexible to adjust to any swimming goggles, but rigid enough to hold firm against the swimmers head for good heart rate measurement. So the hardness range of the plastic further limited the material options." Other factors to consider when choosing the materials were compatibility with the long term use in environments such as sea water, chlorine, and direct sunlight. Aavid worked with a number of plastic manufacturers and moulding specialists before determining the best combination of plastics to use. Grunes added: "It was determined that to get good seals between the plastic and the 'guts' in our initial prototypes we had to employ a production-type process, injection moulding." Design process The mechanical design team took a 3D surface file from the industrial design team and laid the surfaces into an assembly with the electronics.  There was some back and forth to adjust surfaces slightly to allow space for the internal components. Once the outside surfaces were set the mechanical team repaired any surface irregularities and ultimately created a virtual solid around the electronics in 3D CAD. Since the silicon casting is low temperature and pressure, all the electronics could be moulded over without risk of damage. Grunes said: "Before moving to injection moulding the prototype design was fairly simple – design the electronics and cast silicon around them. A soft tool was built to hold the electronics while silicon was hand poured and cast in place to complete the first product prototypes.  The handmade prototypes were an important step in the development and gave a platform for refining some algorithms and heart rate measurement. Unfortunately the handmade prototypes did not stay completely water tight and could not be used easily in the swimming pool, which is where they needed to be for further refinement. The decision was made to injection mould the next set of prototypes." Injection moulding introduced a series of new challenges that ultimately had to be faced anyway, but the design team had to be wary that facing these production challenges in the prototype stages was not disrupting to the core technology development such as heart rate accuracy, usability, and comfort. Because injection moulding is a high temperature and high pressure process the design team simulated the process to be certain that components were properly protected. Through thermal modelling of the moulding process it was determined that the battery would exceed its recommended temperature during the injection moulding without adding significant insulation, which there was not space for. The team decided to design the product such that the battery would be installed after moulding. In addition, to protect the PCB components from the pressures of injection moulding, a composite shell was designed. "The material compatibility between the outside rubber skin and the internal PCB shell was critical to the cosmetic and functional results," said Grunes. "If the rubber skin did not adhere to the internal 'guts' there would be separation, air pockets would form, and water could ingress." Aavid Design brought the project from concept to a working manufacturable product, with the primary concern being protection of the electronics through the high temperature and pressure experienced during injection moulding. Unsurprisingly, Instabeat's designs did have to evolve. Grunes explained: "To position the electronics accurately within the injection moulding tool required support pins. This was especially challenging because the heads-up display portion of the electronics is on a flexible PCB. The support pins create physical holes through the outside skin of the device. To minimise the impact the pins have on the industrial design, we kept them all on one side and arranged them in an aesthetically pleasing as possible way, and designed custom plugs to fill the support pin holes afterwards." The product was launched in August 2014, but development has not stopped there. Could the product be used to supply more 'in-swim' information or even music and headphones? "Absolutely," said Hobeika. "Lots of exciting features coming in the next versions, so stay tuned!"