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Design That Clicks: The Art and Science of Tactile Feedback

“Stamp, stamp. Clickety click. The machinery runs with a quick sharp tick…” begins the line describing a post office in Margaret Wise Brown’s 1952 children’s poem “Seven Little Postmen.”

More than half a century later, stamp canceling machines still click. So do buttons on computer keyboards, automobile gas caps turned to lock, and hundreds of other devices. It’s one of those things you don’t notice until you notice it. This type of tactile feedback is intentional. A firm, audible click is one of the best signals we have to tell the user of a product that an action is completed.

There’s nothing really novel about making something click. But there’s an art and a science to doing it right. When a client came to us and requested that we make a scalpel with a guarded blade that could be passed from nurse to surgeon safely, even in the dark, we decided to dive deep into the click hole. We came back with good data on just what makes a click tick.

Anatomy of a Click

Our first stop on our quest to comprehend the click was a stationery store. We were looking for the simplest clicking devices we could think of: pen caps. The satisfying click you can feel and hear when you push the cap of a pen or marker tells you the cap is on securely. It’s simple, universally recognized, and almost impossible not to fidget with during a meeting.

We purchased several pens and took them back to the lab to try to quantify the sound and force of each snap. We rated each pen on how it felt to click, how much force it took, how sharp or mushy it was. Then we set up an auditory test in an echo-free chamber and analyzed the volume and shape of the sound waves each click generated (see pens and waveforms charted, above and below). Instron force-testing also revealed the characteristic shape of an effective click, with its sudden rise and drop-off. Once we’d rated the pens for sound and feel, we looked at what it was about each pen design that gave its click its unique, specific auditory and tactile qualities.

The basic pen is a male cylinder with discrete bumps or ridges around the perimeter, near the nib. The cap is a female cylinder with ridges that rub across the ridges on the pen. The quality of the click depends on the height and ramp angles of the ridges, the space between them, the type of plastic from which they are made, and the stretch of the cap as it passes over them.

We learned:

  • A ridge with a more gradual ramp makes a softer click. The height of each ridge determines the sound of the click, and a steeper drop-off creates a more noticeable click.
  • The material, height of ridges, and space between ridges determines how hard it is to engage the click.
  • Mechanisms made of ABS plastic make a sharper click than ones made of polypropylene. Metal makes the click sharper still.
  • Manufacturing a click takes some experience. In molded plastic parts, undercuts and ramp angles must be designed in a way that the mold that will release the plastic cap as it slides out of the tool.

Pen caps may be the simplest clicking devices, but the basic technology stays the same no matter what product we looked at. The placement of the ridges depends on the shape of the device (a razor cartridge would have them in a different location than a pen, obviously) but the micro details, how you design the bumps and angles to produce the click, never changes.

Click for Safety

To make sure we got the click right on the Alcon Safety Knife¬†(shown below), we interviewed nurses, ophthalmic surgeons and surgical technicians to understand the environment in which the safety knife would be used. A key point was that eye surgeons operate in very dimly lit rooms, with light shining only on the patient’s eye, and often with music playing in the background.

The Center for Disease Control estimates that healthcare workers suffer 600,00 to 800,000 needle sticks and accidental cuts every year in the U.S. Surgeons, nurses, and technicians are handling extraordinarily sharp scalpels and can easily cut themselves if they grasp the wrong part of the device.

To prevent these types of injuries, the Alcon Safety Knife is retractable and has a guard. It also has a click. To get the right sound and feel, we designed a whole line of safety knifes that differed only in click. Some were louder, some quieter, some harder, and some softer. Our client made the final choice.

When the eye surgeon asks for the knife, the technician picks it up and clicks it. The click, loud enough to be heard over background music, signals the blade is now guarded. The surgeon takes the knife from the tech, and can feel from position of the click mechanism that the blade is indeed guarded, a reassuring tactile cue.

The click of the Alcon Safety knife uses the same basic idea as the pen cap. That wasn’t the only technology we considered, though. Pens that have a retractable nib, for example, use a hysteresis model. When you push once, the nib follows a track in one direction to go down. Push again, and it follows a different track to come back up. Fancy. But making that work for a surgical scalpel would have required the surgeon to change their hand position to retract or extend the blade. And our user analysis showed they didn’t want to do that.

The Click That Satisfies

Most products that click would work just as well if they were silent, but the user would lose an important sense of security.

How do you know your razor blade is secured on your shaver if it doesn’t click? How do you know a cartridge has been successfully inserted? How do you know your car trunk won’t fly open on the highway if it didn’t click when you closed it?

Seeing might be believing, but feeling is knowing. That click tells the surgeon that the scalpel is extended the same way the click tells a writer the pen is ready. And there’s nothing quite as satisfying a clicking a cap back on when you’re done with a project.

Happy clicking.