My wrists were naked from 2005 to 2019. It was 2005 that I got my first cell phone, a flip phone with a secondary display which displayed the time on the outside. For me, the phone was a better clock than my watch had ever been, so the watch came off. Fast forward to 2019, when I ran my first marathon. In the process of training, I bought a running watch. The Garmin product tracks myriad things while you run including pace, heart rate, stride length, and importantly to me: distance. With the distance tracking I no longer had to plan my runs nor find the landmark which I’d use to determine my distance. With less planning it eliminated one of my excuses not to run. The pace tracking was also beneficial for me. If I ran hard and went fast for a part of the run I knew it and could quantify it. If I was finding my run challenging and was going slow, I could see that on my watch and could understand how my run was affected by the condition of my body. The running watch was a game changer.
Perhaps I was slower to the smartwatch game than most. To me though, the smartwatch is the archetype of wearable technology which has exploded in the last decade. When people discuss wearable tech, they’re usually referring to worn electronics. Smart watches may be the archetype but, hearing aids, Virtual Reality headsets, and some blood glucose monitors, are all considered wearables. The definitions tend to exclude clothing and cosmetics. Technical fabrics are not typically considered wearables, and as much technology may be in cosmetic formulations like modern broad-spectrum sunscreens, these also are not wearables. Although related, pocket technology e.g., mp3 players and smartphones are excluded.
As a machine designer I see wearables differently than others. While most people see cool and fashionable tech, I also see the enormous effort that the design team put into those devices, because wearables bring many design challenges. Here I’d like to look at those challenges. The challenges with designing wearables include:
1. Industrial Design: getting the look, feel, ergonomics and human factors right
2. Miniaturization: there are challenges with miniaturization of electronic and mechanical components.
3. Robustness: it is challenging and critical to achieve the high degree of robustness wearables require.
While it can be complex and difficult to meet product goals for wearables, designers and engineers today have access to a variety of technologies which make it possible.
1. Industrial Design
With wearable products, the industrial design of the product is critical. Industrial design includes aspects of the design like appearance, ergonomics, feeling and usability human factors. These factors are challenging to get right. The device must be comfortable to wear and use for extended periods. Different users may wear the device different ways or use it in different body positions. The challenge is augmented by the subjective nature of comfort and usability. Integrated into these challenges is the use of flexible materials. Flexible foams, rubbers, and textiles give fit, wearability and comfort. However, it is difficult to model the drape, flex, stretch and compression of flexible materials in software, or predict it other ways. Getting the appearance right can pose a challenge as well. Success in industrial design factors takes iteration. Even before the machine inside is working, design teams will build many non-functional cosmetic models. These models allow comparison of different concepts and development of targeted concepts for the shape, look, and feel, the distortion of rubber parts and the accessibility of buttons. An iterative process of trial and modification creates the best products, but it can consume a lot of schedule time.
Fortunately, modern manufacturing technology and organizations make rapid iteration in design easier than ever. Quick-turn prototyping techniques like 3d printing, urethane casting, and CNC machining make it possible to get samples of rigid parts fast. Also, many companies now do cosmetic finishing of prototypes. Sanded and painted 3d-prints look as good as injection-molded parts and are extremely useful in scrutinizing industrial design factors of rigid components. Integrating these with samples of the flexible materials yields cosmetic prototypes, which form the basis of industrial design iterations. However, this is still before all the technology is squeezed into the available space.
In wearables, smaller tech is almost always better. Smaller, lighter objects are less noticeable and usually more comfortable to wear. Usually, wearables require miniaturization of both the electronic and mechanical systems. Fortunately, there are ways of miniaturizing these systems.
Electronics can be miniaturized by using both small components and efficiently using the space available. Often the battery is the single largest and heaviest component in a wearable. The new lithium and lithium-polymer pouch-style of batteries offer high energy density which help a lot with miniaturization. These types of batteries power almost all rechargeable wearables on the market. Circuit boards must also be made compact. Electrical components are packed tightly on circuit boards while still placing buttons, connectors and displays in the required positions. One challenge with many small components packed tightly together is to make the necessary electrical connections between them all. In wearables, often the answer is in many-layer circuit bords. Eight-, twelve-, even twenty-four-layer boards are sometimes required. Other designs that do not have a flat space for a circuit board, must divide the board into two pieces, or the space for the board might distort or change in use. In these cases, flex circuits are very useful. Flex circuits are boards or portions of boards formed from a thin layer of polyimide film rather than a thicker sheet of rigid fiberglass. The thinner polyimide board can bend and distort allowing for flexible or bent circuitry. Flex circuits are used in a lot of wearables for their many advantages.
It isn’t just the electronics that need to shrink but the mechanical components as well. Efficient use of space is critical for the mechanical design of any compact device. To do this, designers take advantage of a number of technologies which help with miniaturization. Perhaps the most important technology is thin-wall injection molding. What constitutes thin walls depends on the overall size of the part as well as the type of plastic used. In general, if the wall thickness is less than about 1mm (0.040 inch) or if the plastics has to flow a distance more than 150-200 times the wall thickness during injection, it might be considered thin-walled. Conventionally, it can be challenging to injection-mold these kinds of parts, however, there are new molding techniques for thin parts. These involve high injection pressures, and limits on the types of plastics used. Thin-wall plastics can be used to create attractive exterior cases, internal supports and frameworks and a variety of other parts in wearable technology. However, sometimes thin-wall plastics are not strong enough for frames or mechanisms in some wearables.
Whether it’s for a frame, a hinge, a latch, an adjustment or a moving mechanism, many wearables require high strength parts. Often metals are required. Sometimes, even the case of the device must be metal to meet product requirements. Fortunately, there are metal manufacturing processes which allow creation of small mechanical parts. These technologies include MIMs, thixomolding, micromachining and EDM. MIMs or metal injection molding is a new process where metal powders are mixed with a binder and formed using injection molding. The green part is then heated to sinter the metal powders into a single part. It allows for small, detailed, inexpensive metal components. Thixomolding is a method of casting magnesium alloys without fully melting them. It creates lightweight metal parts with excellent dimensional accuracy and stability. Micromachining is like conventional machining but using very small cutters. It requires small mills with very high spindle speeds. Finally, EDM or electro-discharge machining is an expensive but extremely accurate method of cutting metals. It comes in two forms, sinker EDM and wire EDM. Both of these are slow and precise and can cut metals at any hardness. Using these techniques, it’s possible to create tiny, robust parts for wearables. Compact, usable, ergonomic designs, however, aren’t enough for a successful wearable product. Wearables need to be robust to impact and moisture.
A good wearable will not break when dropped, but designing in drop resistance is also a challenge for the design team. Impact resistance comes from a combination of strength and deflection. Parts that don’t deflect can chip or shatter, but too much deflection can mean loads get transmitted to fragile parts. In wearables, designers must carefully balance deflection of impact absorbing components to absorb impact energy without transmitting forces to fragile components. Even with a properly designed amount of deflection, only testing of functional prototypes can verify the design survives drops. Test results can motivate design improvements. Usually designs take multiple iterations of prototyping, testing, and improving to achieve good drop performance.
Physically dropping the wearable isn’t the only thing which can break wearables, it isn’t even the only kind of drops which can break them. Drops of liquids can also damage electronics. Humans are moist and moisture resistance is an important challenge for wearable design. Water can damage electronics in essentially two ways. First it can short electrical connections. Water usually contains conductive electrolytes, and water on a circuit board can connect electrical contacts which should not be connected. The second mechanism of moisture-induced failures is more insidious. Exposure to water can cause metals on electrical circuits to corrode. Corrosion is slow to form and may take days or weeks before it causes electronics to fail. People’s sweat is especially corrosive because it has sodium chloride salt as well as other electrolytes. These chemicals speed up corrosion. However, there are solutions to mitigate potential moisture damage.
Design teams achieve moisture resistance by sealing an enclosure or by directly protecting sensitive surfaces. Sealing and enclosure prevents contact between moisture and sensitive components. If the design calls for a sealed enclosure of dissimilar materials, usually a mechanical seal like an o-ring or over-molded elastomer works well. Ultrasonic welding and laser welding can be used to join similar plastics together. Welds like this can provide both a strong, permanent mechanical joint as well as a positive seal. The most complex sealed joints may require a liquid adhesive. Sealing is not the only way to provide moisture resistance: some designs simply coat all circuit boards, connectors, and other sensitive elements with a conformal coating or high-tech fluoropolymer thin-film coating. These coatings protect the components from moisture when it’s challenging to seal the case parts. As with drop resistance, good moisture resistance comes from good initial design concept followed by testing and design iteration.
As a product development engineer, when I see people with wearables in public, not only do I think about the capabilities those devices give the wearers, I also think about the enormous effort that went into creating those devices. I think about the iterations with the design team and working through the fit and feel of the soft goods. I think about the fight to find small electrical components and pack them tightly. How much effort goes into layout of the many-layered boards or flex circuits used. I think about the manufacturing methods which enable the mechanical design. And I also think about the effort the design team expended developing their initial non-functional prototypes up through a robust and reliable enough product for people to wear every day. Wearable design is hard, but with collaboration, iteration, and use of modern manufacturing techniques, product designers around the world are creating wearable technology for people everywhere.