Invisible Light Touch: Standing Balance Improvement by Mid-Air Haptic Feedback

Research Background and Issues

• Identified Problems or Challenges:
  The increasing aging population heightens the risk of falls, making the improvement of standing balance crucial for preventing falls and maintaining the well-being of older adults. However, existing physical and wearable device-assisted solutions either rely on external supports (e.g., handrails or canes) or require prolonged usage, limiting user acceptance and flexibility.

• Importance of the Research:
  Fall prevention not only reduces related injuries and medical costs but also improves quality of life. Exploring non-contact technologies without wearable devices to provide flexible and effective balance improvement solutions for a broader user group is of great significance.

• Research Motivation and Related Work:
  This study is based on the "light touch effect," where an individual's standing balance improves when they touch a stable surface with minimal force (typically less than 1 N). Researchers use ultrasound focusing technology to present hollow tactile points, combining this medical observation with advanced human-computer interaction technology to support balance training without physical contact.

Solution

• Proposed Method or Solution:
  The Invisible Light Touch (ILT) technology is proposed, which generates hollow tactile points using ultrasound focusing devices to induce the light touch effect for balance improvement without requiring wearable devices.

• Innovations:

  1. Ultrasound technology precisely generates tactile hotspots in three-dimensional space, eliminating the need for physical supports or wearable devices.
  2. Provides adjustable tactile point positions, supporting various standing postures and hand positions.
  3. Maintains a light touch state instead of physical support, making it particularly suitable for balance training.

• Implementation Steps:

  1. Use the Ultrahaptics STRATOS Explore ultrasound device to generate tactile points.
  2. Adjust output parameters (e.g., tactile point diameter, intensity, and frequency) to ensure tactile force remains below 1 N.
  3. Design comparative experiments (including conditions such as eyes open/closed and with/without ILT) to verify the effectiveness of the technology.
  4. Measure the center of pressure (COP) trajectory during standing using a force plate to evaluate quantitative indicators of balance improvement.

Research Outcomes

• Specific Results:

  1. ILT significantly reduced postural sway during standing, especially under closed-eye conditions.
  2. Follow-up studies with older adults demonstrated that ILT is more effective for groups with impaired balance abilities.
  3. Showed that the light touch effect can be achieved without physical contact, validating the practical application potential of the technology.

• Comparison with Existing Solutions:

  1. Compared to wearable device-based balance support, ILT eliminates physical dependency, enhancing convenience and applicability.
  2. Although ILT's balance improvement effect is slightly lower than traditional physical light touch, its non-contact nature and psychological advantages (such as high flexibility and user sense of safety) result in higher subjective satisfaction.
  3. When tactile points are dynamically moved (e.g., oscillating), the balance improvement effect diminishes, highlighting the necessity of fixed tactile points.

• Experimental Results:

  1. Under closed-eye conditions, COP indicators (path length, root mean square, and rectangular area) significantly decreased compared to when ILT was not enabled, indicating more stable postures.
  2. Initial experiments showed that users subjectively preferred ILT over physical touch.
  3. Statistical analysis emphasized the effectiveness of hollow tactile sensations in reproducing the light touch effect.

• Limitations and Future Directions:

  1. ILT's tactile force is only 16 mN, far below the approximately 1 N of physical light touch, which may affect balance improvement. Future research should explore methods to enhance ultrasound force output.
  2. The current device size is relatively large, and fixed installation affects portability. Future work could focus on miniaturized devices or mobile array systems.
  3. Research subjects primarily consist of healthy individuals; subsequent studies should validate the technology in patient groups with impaired balance abilities.
  4. Long-term effects of ILT on balance training and its application in dynamic activities (e.g., walking or dancing) require further investigation.

Conclusion

Invisible Light Touch (ILT) utilizes ultrasound focusing devices to induce the light touch effect for the first time, providing a non-contact, flexible solution for improving standing balance. Experiments validated the effectiveness of the technology, particularly in significantly reducing postural sway under closed-eye conditions. However, further exploration is needed to enhance force feedback and improve learning outcomes. ILT is expected to have broad application prospects, including rehabilitation training, home balance improvement, and emerging fields such as auxiliary support in VR environments.