TFS: Tablet Fingertip Syndrome
Tablet Fingertip Syndrome
Tablet computers have become very popular with the iPad and Androids from a dozen or more manufacturers. These devices center on a graphical user interface with the user’s fingertip gestures providing the commands and navigation.
The primary user interactions are the ‘tap’ and ‘slide’ or ‘swipe’ gestures. These are almost exclusively applied to the top surface of the tablet which senses the user commands through a capacitive glass sensor that is mounted atop the display. Tablet computer displays and capacitive sensors range in size from 6 to 10 inches (diagonal measurement).
The same touch gestures and input device configurations are used in hundreds of millions of smartphones, including the iPhone series, Motorola’s Droids and dozens of other Android-based mobile phones from numerous manufacturers. The display size on these devices ranges from approximately 3.5 inches to 5 inches.
I am an early adopter and have been using a Samsung Android Tablet for over 6 months now. I have noticed that after using the tablet for extended periods of time (two hours or more) my index fingertip becomes sensitive – not sore or exceedingly painful, but definitely sensitive. So much so that I begin to swipe the tablet with the side of my fingertip to avoid aggravating the sensitive tip. Tapping appears to be far less injurious to the fingertip than sliding or swiping.
Why should this happen? ‘As smooth as glass’ is an oft used expression in our everyday language. Yet, excessive swiping of the same fingertip over a very smooth glass surface can cause discomfort to the user.
Glass is indeed smooth on a macroscopic scale, but not so smooth on a microscopic scale, as can be seen in the Atomic Force Microscope image below (Figure 1).
Figure 1: Atomic Force Micrograph of a 20×20 micron glass surface (from Wikipedia article on Atomic Force Microscopy http://upload.wikimedia.org/wikipedia/commons/e/e8/AFMimageRoughGlass20x20.JPG)
The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, made of about 75% silica (SiO2) plus Na2O, CaO, and several minor additives. Glass is quite hard, measuring 5.5 on the Mohs scale, the same as a knife blade. (The Mohs scale ranges from 1.0 for talc to 10 for diamond). The abrupt sharp silicate edges in the glass are like shards of glass that one finds when a drinking glass is shattered on concrete. A large array of such shards on the surface of a smooth glass tablet display have similar cutting properties to sandpaper.
Sandpaper is graded by a grit factor, based on the size of the silica particles on the surface of the paper. So-called “superfine” sandpaper (rated 500 grit) uses particles averaging 20 microns in diameter. Ultra Fine sandpaper (1000 grit) uses particles approximately 10 microns in diameter. As can be seen from the Figure 1, smooth glass has ridges and edges of with feature sizes of approximately 0.1 micron. If we were to think of “smooth” glass as Super-Ultra Fine sandpaper, it would equate to 100,000 Grit, with average silicate particles of 0.1 microns in feature size.
So much for the hardness and sandpaper roughness of smooth glass. But surely the amount of sliding and swiping of a fingertip across its surface is pretty minimal, one is tempted to think. The range of a typical swipe across a tablet computer or smartphone screen is only an inch or two. This motion is repeated hundreds or thousands of times per hour of use. That can add up.
How far does a fingertip travel across the glass sensor surface of a smartphone or tablet? It depends on the activity and the display size of the device. For example, reading e-mails or searching through tabular lists frequently involves a sliding motion up and down the display page as each segment of the document is read by the user. For a typical use-case of reading, say, 20 e-mails, with an average of four displays per e-mail document, the user’s fingertip may slide across 320 inches of glass on a 4” smartphone or 800 inches on a 10” tablet. This task may typically be done in less than 30 minutes. In another use case, a game of Angry Birds requires about 4-5 slingshot slides, of about 1-2 inches plus 3-5 full-screen screen swipes per level. For a typical game of 30 levels in 30 minutes (including level replays) the finger might slide across 660 inches on a 4-inch smartphone screen or 1080 inches on a 10-inch tablet screen.
Let’s make the assumption that these 2 activities represent a typical daily usage, then the user’s forefinger will rub across 80+ feet of glass on a 4” smartphone or 150+ feet on a 10” tablet! You could therefore swipe a mile or more of 100,000 Grit sandpaper on your tablet of smartphone every month. That can certainly explain the onset of what I am calling “Tablet Fingertip Syndrome” or TFS.
It is not likely that tablet computers or smartphones will disappear in the near future. After all, keyboards did not go away when we recognized their uncomfortable or painful effects on users’ wrists (carpal tunnel syndrome). But can anything be done to head off or alleviate a potential TFS epidemic?
Here are a few possibilities:
- Manufacturers can specify glass that has fewer or smaller microscopic variations/edges.
- Manufacturers can coat the touch surface with a softer material or a lubricant to reduce the sandpaper effect on the users fingertip.
- Manufacturers can incorporate other types of user interface devices such as trackballs for use with most common navigation functions. The first Android smartphone, the G1 by HTC included a mini-trackball.
- New sensor technologies can be developed that do not require actual touching. For example, optical sensing can determine the location, tapping or “pressure” of the user’s fingertip without the need to actually touch the glass surface.
- Software developers can define alternative gestures for navigation that require less (or no) fingertip travel across the glass surface. Perhaps joystick style taps or ‘page up” and “page down” buttons that eliminate the need for full page swipes.
- Fingertip protection that can be worn without impeding the utility of the tablet or smartphone.
This paper has been written to expose the potential for more widespread user injuries as tablets and smartphones become more prevalent. Now is the time for manufacturers and developers to devise better user interfaces and experiences.
June 16, 2011