Nail Guard: a Wear OS app that helps you stop biting your nails

Nail biting mostly happens without you noticing, and that's exactly where a watch can help. Nail Guard is a Wear OS app we built for the Xiaomi Watch 5 (Wear OS 6). It learns how you hold your hand when you bite your nails, and gives you a nudge as soon as your hand stays at your mouth too long.

No expensive shock wristband, no subscription: just a smart app on the watch you already wear. Fully in the Waligoo style, in Dutch and English.

The challenge

People who bite their nails often only notice once it has already happened. Existing aids are wristbands that vibrate or even give an electric shock. They easily cost more than €100 and aren't always accurate. So the question was: can an ordinary smartwatch do this itself, accurately enough to be useful and efficiently enough to run all day?

Why Nail Guard works

01 — A calm alert, not a shock (Approach)

A vibration and a calm navy screen with a short replacement exercise — instead of an electric shock.

02 — Trained on your own habit (Personalization)

You record your own biting pose, in different situations, for recognition that fits you.

03 — Posture plus micro-motion (Sensors)

Accelerometer and gyroscope work together to tell biting apart from an innocent pose, even in the same position.

04 — Built to always work (Reliability)

Foreground service, watchdog and automatic recovery keep the app running, day after day.

The solution

A calm alert, not a shock

When your hand stays in your nail-biting pose for about 2 seconds, the watch vibrates and shows a calm navy screen: Hands off. A countdown ring guides you through a short exercise: squeeze your fist for 30 seconds. A replacement action like this helps break the habit. A false alarm? One tap on I wasn't biting and the app learns that pose straight away.

Trained on your own habit

Everyone bites differently. On first start you record your biting pose three times, 10 seconds each, ideally in different situations: at your desk, standing, on the couch. You can also record poses that are fine, such as your chin on your hand, drinking or a phone call.

Posture plus micro-motion

Detection combines two sensors:

  • Accelerometer: always on. It measures how your wrist and forearm are held, and compares that with your trained poses.
  • Gyroscope: only switches on once your hand is already at your mouth. It feels the small, restless movements of biting. That's how the app tells biting apart from calmly resting your chin on your hand, even in the same pose.

Using the gyroscope only when needed keeps battery use down.

Progress and control

  • A start screen you read at a glance: today's count, the comparison with yesterday and how long you've been bite-free
  • A last-7-days overview, your weekly trend and your longest bite-free stretch
  • One-tap snooze: in the app, from the tile next to your watch face, with the NG Snooze shortcut on the watch button, or with a finger gesture
  • A complication that shows today's count on your watch face
  • Automatic pauses: at night (adjustable), while charging and when you take the watch off

Under the hood

A habit tool only works if it always works, so reliability was a key focus:

  • Foreground service that keeps running, also with the screen off and in deep sleep (Doze)
  • Watchdog that checks every 15 minutes whether everything is still running, and restarts the sensors or the app when needed
  • Automatic recovery after a crash, a watch reboot or an app update
  • Check-up screen that shows right away what could get in the way: battery restrictions, notifications, vibration, sensor data
  • Waligoo design system on the watch: navy #003049, sharp corners, outlined buttons with an arrow, Sora and Inter, and a black background that saves battery on an OLED screen

Tested: 62 automated logic tests (detection, storage, time rules) and an end-to-end suite that builds, installs and operates the app on a Wear OS 6 emulator. Using simulated hand movements, it tests onboarding, detection, snooze, pauses, deep sleep, crash recovery and reboots, among other things. All images on this page are real screenshots from that emulator.

Technology: Wear OS 6 (Android 16) · Java · Android Sensor Framework (accelerometer, gyroscope, off-body detection) · Foreground Service · Wear OS Tiles · Complications · Android Studio · PowerShell/ADB test automation

Gallery

Ten real screens from the Wear OS emulator.

Interested in Nail Guard? Request the install!