CARWatch App

Get CARWatch in the Play Store:

CARWatch is an open-source Android research application designed to support accurate and standardized saliva sampling in psychoneuroendocrinological studies. The app enables objective recording of sampling times using barcode scans and automated reminders, reducing reliance on self-reports and improving data validity.

Originally developed for assessing the cortisol awakening response (CAR), CARWatch has evolved into a flexible framework for a range of saliva sampling protocols, including wake-up-based sampling, fixed clock-time sampling, multi-day protocols, optional evening samples, and full diurnal cortisol profiling.

CARWatch is a research-study companion app for invited participants. It is not a general consumer app and cannot be used without a valid study configuration QR code provided by a study team.

Objective

The CAR is the natural increase in cortisol levels that occurs within 30-45 minutes after waking up and can be an important indicator for overall health and well-being.

However, many CAR studies suffer from low adherence to the study protocol as well as from the lack of precise and objective methods for assessing the awakening and saliva sampling times which leads to measurement bias on CAR quantification. This issue can be solved by CARWatch, an Android application that allows for objective saliva sampling times during CAR studies.

Features

The main purpose of the application is to reduce saliva sampling inaccuracies by objectively assessing saliva sampling times during CAR assessment.

CARWatch includes the following features:

• Awakening time assessment: Awakening can be assessed either by setting an alarm by the user or by pressing a button in the application in the case of spontaneous awakening.
• Saliva sampling time assessment: Saliva sampling times are recorded by setting alarms for each saliva sample which can only be turned off by scanning a barcode on the saliva sampling tube.
• Data export: All data collected by CARWatch can be exported which can be used for further analysis.
• QR code-based study configuration for participant setup.
• Flexible alarm scheduling relative to awakening or at fixed clock times.
• Support for multi-day and multi-sample study designs.
• Daily overview of completed, pending, and missed samples.
• Optional evening sample reminders and bedtime sampling workflow.
• Built-in tutorial for participant guidance.
• Privacy-preserving local data handling with no backend data storage.

CARWatch is intended for use in scientific studies. It is not a medical device and does not provide medical advice, diagnostics, or treatment recommendations.

Getting Started

The easiest way to use CARWatch is to install it from the Google Play Store.

To build the app from source:

1. Clone this repository.
2. Open the project in Android Studio.
3. Let Android Studio sync the Gradle project.
4. Connect an Android device or start an emulator.
5. Build and install the debug variant, or run:

./gradlew assembleDebug

CARWatch requires camera access for QR and tube barcode scanning. On newer Android versions it also requires notification permissions so that wake-up, saliva sample, and evening sample reminders can be shown reliably.

Tutorial and store-listing screenshots are created with Android's Device Art Generator.

Usage

CARWatch is designed for studies in which invited participants collect saliva samples according to a configured sampling schedule. The app guides participants through setup, wake-up reporting, sample reminders, barcode scanning, bedtime reporting, and log export.

CARWatch itself is not the sponsor, operator, or investigator of any specific study. It is a research-enablement tool used by external research teams and institutions to support the collection of saliva sampling data.

Study setup

Before a participant starts using the app, the study team should prepare:

1. A study configuration QR code.
2. Barcode labels for all saliva tubes used in the study.
3. Participant instructions that explain when to use the Wakeup, Alarm, and Bedtime screens.
4. A contact email address for log export, if logs should be sent by email.

Study configurations can be created with the CARWatch Study Manager. The Study Manager helps researchers define the study metadata, sampling schedule, participant settings, evening sample requirement, and log-sharing contact email, then provides the QR code that participants scan during onboarding.

On first launch, CARWatch opens the setup flow:

1. Welcome screen and study information.
2. Camera and notification permission request.
3. Study QR code scan.
4. Participant ID entry, if the participant ID is not already included in the QR code.
5. Study configuration confirmation.
6. Short tutorial.
7. Start of the active study.

The study QR code configures the app for one specific study. Without this QR code, the app cannot be used because the QR code provides the study schedule and study metadata.

Daily participant workflow

The main app has three primary screens:

1. Wakeup: report spontaneous awakening and start the morning sampling sequence.
2. Alarm: set the wake-up alarm, review scheduled saliva sample alarms, and set the evening sample reminder if required.
3. Bedtime: report going to bed and record the evening sample if the study requires one.

Recommended daily workflow:

1. Before going to sleep, open the Alarm screen and set the desired wake-up alarm time for the next morning.
2. If the study includes an evening sample, use the Bedtime screen before sleep and scan the evening sample tube when prompted.
3. If the study includes an evening reminder, configure the reminder in the Alarm screen. The default suggested time is 21:00 unless the participant has selected another time before.
4. In the morning, either wake up through the CARWatch alarm or open the Wakeup screen and confirm that you just woke up.
5. If the wake-up sample is required immediately, scan the wake-up sample tube. The wake-up event is only recorded after the barcode scan succeeds.
6. CARWatch schedules the remaining saliva sample alarms according to the study configuration.
7. When a saliva sample alarm fires, take the sample and scan the matching tube barcode to record it.
8. Use the Alarm screen to see which samples are scheduled, due, overdue, or already recorded.
9. After the last required sample for the day, follow the in-app message. If the study is finished, export the logs and send them to the study team.

Menu actions

The floating menu in the main app provides additional actions:

• Share Logs: creates a ZIP archive of the collected log files and opens Android's share sheet.
• Study Information: shows the loaded study configuration, including participant ID, study days, sampling plan, and evening sample setting.
• Reregister: clears the current study configuration and returns to QR setup. Existing log files are kept.
• Show Tutorial: opens the tutorial screens again.
• Finish Study Day: cancels remaining samples for the current day and marks the day as finished.
• Delete Logs: deletes locally stored log files after confirmation. This cannot be undone.
• Kill Alarms: cancels scheduled alarms and timers after confirmation. This should only be used for troubleshooting.
• App Info: shows the installed app version.

Study configuration QR code

The setup QR code contains semicolon-separated key-value pairs. It must start with CARWATCH. Values are separated from keys with :.

Researchers can generate this QR code with the CARWatch Study Manager.

Supported fields:

Field	Meaning	Example
N	Study name	CARWatchTest
D	Number of study days	3
NP	Number of participants covered by the tube barcode set	20
T	Relative saliva sample offsets in minutes after wake-up	0,15,15,15
A	Fixed saliva sample times in HHmm format, comma-separated; can be empty	0800,1200
SS	First sample label prefix/index	S0
E	Evening sample required, 1 for yes and 0 for no	1
M	Contact email address for log export	study@example.org
FD	Strict barcode checking, 1 to reject duplicates and enforce the expected barcode ID, 0 to accept any scanned barcode ID	1
PID	Optional participant ID stored directly from the QR code	VP_001
V	Optional web/config generator version	1.0.0

Example:

CARWATCH;N:CARWatchTest;D:3;NP:20;T:0,15,15,15;A:;SS:S0;E:1;M:study@example.org;FD:1;PID:VP_001;V:1.0.0

Notes:

• A first T value of 0 means that the wake-up sample is due immediately after wake-up.
• Further T values schedule relative samples after the previous sample time.
• A can be used for fixed-time samples in addition to relative samples.
• If E is 1, CARWatch shows evening sample controls and prompts the participant to record the evening sample before bed.
• The QR code is generated and distributed by the study team, for example through the CARWatch Study Manager or another study-specific configuration workflow.

Saliva tube barcode scanning

CARWatch uses the camera to scan the barcode printed on each saliva tube. With strict barcode checking enabled through FD:1, CARWatch rejects duplicate barcode values and validates that the encoded participant number is within the configured NP participant range, the encoded day and sample values are within the study configuration, and the encoded day/sample matches the expected sample. The encoded barcode participant number is not compared against the optional PID field. With FD:0, CARWatch accepts any scanned barcode ID and does not reject duplicate scans.

When a barcode scan succeeds, CARWatch:

• logs the scan event,
• marks the corresponding sample as recorded,
• cancels the related reminder or alarm,
• updates the sample status in the Alarm screen.

Example study flow

For a study configured with one immediate wake-up sample, three additional wake-up-based samples, two fixed-time samples, and one evening sample, a participant completes seven samples per study day:

• wake-up sample at 0 minutes,
• wake-up-based samples at +15, +30, and +45 minutes,
• fixed-time samples at 12:00 and 15:00,
• evening sample before going to bed.

Example review or demonstration flow:

1. Launch the app.
2. Read the onboarding information.
3. Grant camera and notification permissions.
4. Scan the study QR code.
5. Confirm the displayed study configuration.
6. Proceed through the tutorial.
7. Use the Wakeup tab to report waking up.
8. Scan the wake-up sample barcode when prompted.
9. Use the Alarm tab to monitor scheduled and fixed-time samples.
10. Scan each saliva tube barcode when the corresponding sample is collected.
11. Use the Bedtime tab to record the evening sample, if required.
12. Open the menu and use Share Logs to export the locally stored study logs.

Log export

CARWatch stores study logs locally on the device. Use Share Logs from the menu to create and share a ZIP archive. The share dialog is prefilled with the configured study contact email address when one was provided in the QR code.

Use Delete Logs only after the logs have been exported and received by the study team.

Privacy and data handling

CARWatch stores study configuration, participant ID, scanned barcode history, and log files locally on the device so the participant can complete the study. The app does not upload study data to a backend server.

Camera access is used only for QR code and saliva tube barcode scanning. Notification permission is used only to remind the participant when samples are due. Logs are shared only when the participant explicitly selects Share Logs.

Important:

• The app cannot be used without a valid study QR code.
• Camera permission is required for QR code and barcode scanning.
• Notification permission is required for sample reminders.
• Data stays on the device unless the user explicitly shares logs.

How to cite

If you use CARWatch in your research, please cite our Psychoneuroendocrinology article:

Richer, R., Abel, L., Küderle, A., Eskofier, B. M., & Rohleder, N. (2023). 
CARWatch – A smartphone application for improving the accuracy of cortisol awakening response 
sampling. Psychoneuroendocrinology, 151, 106073. https://doi.org/10.1016/j.psyneuen.2023.106073

Studies using CARWatch

The following papers used CARWatch in their research:

• Richer, R., Abel, L., Küderle, A., Eskofier, B. M., & Rohleder, N. (2023). CARWatch – A smartphone application for improving the accuracy of cortisol awakening response sampling. Psychoneuroendocrinology, 151, 106073. https://doi.org/10.1016/j.psyneuen.2023.106073
• Richer, R., Küderle, A., Dörr, J., Rohleder, N., & Eskofier, B. M. (2021). Assessing the Influence of the Inner Clock on the Cortisol Awakening Response and Pre-Awakening Movement. 2021 IEEE EMBS International Conference on Biomedical and Health Informatics (BHI), 1–4. https://doi.org/10.1109/BHI50953.2021.9508529
• further papers to be added - let us know if you used CARWatch in your research!

Compatibility

CARWatch requires an Android device running Android 6.0 (Marshmallow) or higher.

Contributing

We welcome contributions to CARWatch! To contribute, please fork this repository and submit a pull request with your changes.

License

CARWatch is licensed under the MIT License. See the LICENSE file for more information.

Contact

If you have any questions or feedback about CARWatch, please contact Robert Richer.