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In addition, if devices have probes that are metallic and need to touch the skin, these invariably need special gels to ensure good contact, and the materials also need to be safe for use.

One of the frequently faced problems is water ingress inside the wearable devices. In many cases, initially, the product remains waterproof but sometimes seals that were used for prevention of water ingress wear out over a period of time, in turn, losing the ability to prevent the water from entering. Designers need to factor this as well and ensure that proper material is selected.

Fig. 4: A conventional test pad and a bead probe test pad
Fig. 4: A conventional test pad and a bead probe test pad

Designers can define the product usage period (useful life) so it can be discarded once the prescribed time is over. This works well when the product cost is low, but if the product is expensive, a proper mechanism of inspection and preventive check is a must if the design has aging issues. This is especially true in the case of a wearable device used for medical applications, where the product cost is high and customers tend to use it for a longer period.

Finally, sometimes wearable devices may have consumables such as gels, pads and wipes. These have to be consistent and their tolerances have to be within the specified limits. Many a time, as time passes, cost pressure takes precedence and companies switch over to a low-cost material, which impacts the product, leading to failures. This happens also when some critical inputs are forgotten or missed out. This must be kept in mind.

Support phase. Most wearable devices are use-and-throw types at the end of their life or when gone faulty. This is due to the low cost of the products and high repair costs. Product manufacturing process in this case uses a specialised procedure that cannot be replicated in the repair centre.

However, when the product is expensive (like some medical equipment), it must be supported and would need to be repaired. If this is a requirement from the beginning, designers have to plan and implement this in the design phase itself. They need to clearly define a process, which will achieve the same type of assembly that is done at the manufacturing line, ensuring product integrity and reliability.

An important support issue is software upgrade; most wearable devices when launched do not have the full complement of the features and may also have bugs. Designers need to make provisions for the following two important aspects:
1. Provide a reliable communication mechanism through which the software can be upgraded.
2. Provide adequate internal memory so that if the upgrade process fails, the product can still work with an earlier version of the software.

Both of these provisions should be planned in the design phase and implemented.

Another way to tackle this issue is to keep the software functionality to bare minimum and carry out all complex operations either on the central application or on a smartphone, depending on where the application is running. This also ensures that cost of hardware is kept low and the software is simple, so that it can be tested and does not need an upgrade, if properly tested. With the option of software upgrade being done in the application, large-volume device software upgrade related issues can be reduced.

Wearable system software
Software for wearable devices is completely dependent on design consideration, functionality of the device and controllers used. However, there are certain generic elements that software developers need to keep in mind when deciding the software architecture of wearable devices. Following guidelines should be helpful when deciding on software architecture:
1. Use a standard-platform based architecture so that maintaining the software is easy.
2. Even when a simple two to three tasks functionality is needed, use of a real-time scheduler (kernel) allows predictable performance and ensures consistency in data, when data needs to be collected for a longer duration and data volume is high, along with time stamp. This particular aspect is very successful in a wearable device design, especially when healthcare functionality is implemented.
3. For complex functionalities, a Linux based platform is advantageous. Customising the Linux kernel by removing unwanted drivers creates a compact footprint as well as tighter and verifiable codes.
4. Implement the power management function specifically for your hardware by measuring the power consumed and understand the battery that is being used. Many a time, using a generic power management function results in sub-optimal performance. Knowledge of processor core and the memory used is important for implementing the power management function. A wearable device’s success mostly depends on its battery life.

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