Soft, miniaturized, wireless olfactory interface for virtual reality

Human-machine interfaces (HMIs) are increasingly incorporating sensory feedback beyond vision and audio, with haptics and olfaction gaining importance. Olfaction, or the sense of smell, significantly impacts human perception and experience, influencing both physiological and psychological responses. Current olfaction-generating technologies suffer from limitations such as bulky size, wired connections, slow response times, and limited odor selection, hindering their integration into VR/AR systems. Wearable odor generators (OGs) offer a solution by creating personalized and localized olfactory environments, overcoming the limitations of existing systems. This research focuses on developing a new generation of wearable olfaction interfaces that address these shortcomings by integrating arrays of miniaturized, flexible OGs into soft, wearable electronics. The system aims for a skin-integrated or face-mask-based design, offering wireless and programmable operation with numerous odor options and accurate concentration control. The use of biocompatible materials and rapid response times are also key design considerations for seamless integration into VR/AR applications.

Existing research on odor detection, using gas sensors, has advanced significantly, especially in the realm of flexible sensing electronics. However, odor generation technologies for HMIs remain underdeveloped. Most current systems either involve large instruments for generating smells in enclosed spaces or are incorporated into bulky VR headsets with wired connections, slow response times, and limited odor functionality. This has limited the widespread adoption of olfactory feedback in VR/AR applications. Previous attempts at wearable OGs have relied on liquid atomizers, presenting issues with clumsy mechanics, bulky packaging, and maintenance requirements, limiting their miniaturization and flexibility. The need for a soft, miniaturized, wireless, and programmable olfactory interface with high-channel odor generation, adjustable concentrations, and long operational duration has thus far remained largely unmet.

This study presents a novel design and fabrication process for wirelessly controlled olfactory interfaces consisting of millimeter-scale OGs integrated into thin, soft, and flexible electronic sheets. Two device architectures are developed: a skin-integrated device (Device 1) with two OGs mounted directly on the upper lip, and a face-mask-based device (Device 2) with a 3x3 array of OGs. The OGs themselves are based on a multilayer structure: a food-grade paraffin wax layer infused with various liquid perfumes serves as the odor source; a polyimide (PI)-supported gold (Au) trace with a thermistor acts as a temperature-controlled heater; and a cantilever-structured mechanical actuator based on electromagnetic induction provides fast on/off switching and heat dissipation. A soft silicone frame encapsulates the OG, ensuring flexibility and skin compatibility. The electronic control panel for both devices utilizes a flexible printed circuit board (FPCB) incorporating a microcontroller unit (MCU), Bluetooth module, battery, resistors, capacitors, and connections to the OGs. PDMS is used for encapsulation and protection. Device 1 uses a substitutable adhesive layer for secure skin attachment, while Device 2 is integrated into a 3D-printed TPU face mask. The control panel features a power management system with two batteries and voltage converters to provide various voltage levels for different circuit components. The MCU controls MOSFETs for heating the OGs, monitors temperature via thermistors and ADCs, and utilizes H-bridges for controlling the mechanical actuators. Bluetooth enables wireless communication with a computer for programmable operation. Device 2 requires additional shift registers and a multiplexer to manage the 9 independent OGs. Extensive material characterization and optimization studies, including finite element analysis (FEA), were conducted to optimize thermistor selection, heating power, and mechanical actuator parameters for optimal response time, temperature stability, and power consumption. The performance of the OGs was evaluated based on response time, temperature stability under airflow, continuous odor stimulation duration, and mechanical stability under various deformations and vibrations. Odor generation was characterized using both ethanol sensors and human sensory tests to assess concentration and recovery times. Safety assessments were also performed using infrared thermal imaging to ensure safe operating temperatures.

The optimized OGs exhibited high performance in various aspects: a fast response time of 1.44 s at a heating temperature of 50 °C, accurate temperature control, and long-term continuous operation (up to 52 hrs). The thermistor's B value significantly impacted temperature fluctuation, with higher B values resulting in better stability. Heating power influenced both response time and temperature fluctuation, with 0.25 W being selected as the optimal balance. The mechanical actuator's lift height also affected response time, with 0.99 mm achieving a good balance between speed and power consumption. The OGs demonstrated excellent mechanical robustness and stability under various bending, twisting, and vibration tests, with minimal impact on performance. The generated odor concentration was adjustable and above human olfactory thresholds, as verified by both ethanol sensor measurements and human sensory tests. Human breathing significantly reduced odor recovery times. Both Device 1 and Device 2 demonstrated wireless operation with sufficient range for practical applications. Device 1 was shown to deliver precisely timed and controlled odor releases that matched the progression of a virtual event in a movie-watching demonstration. Device 2 was effective in facilitating smell-based communication for visually and auditorily impaired individuals after appropriate training. It also showed promise in emotion modulation and memory recall. Finally, integration with VR systems and motion-capture devices enabled interactive experiences in virtual environments like virtual reality-based online botany lessons.

This work successfully demonstrates a novel soft, miniaturized, wireless olfactory interface for VR applications. The optimized design and fabrication of the OGs, along with the integrated electronic control panel, addresses many shortcomings of previous olfaction systems. The two device formats, skin-integrated and face-mask-based, offer versatile options tailored to different applications. The demonstrated capabilities in various application scenarios, including immersive media experiences, alternative communication methods, emotion regulation, memory recall, and interactive VR/AR environments, showcase the broad potential of this technology. The findings highlight the feasibility of integrating olfaction into existing VR/AR systems to enhance user experience and open new avenues for HMI development. The successful integration with a motion-capture system further illustrates the potential for interactive applications and opens avenues for innovative uses in education, healthcare, and human-computer interaction.

This research presents a significant advancement in olfactory interface technology for VR/AR applications. The developed soft, miniaturized, wireless devices offer high-performance odor generation, accurate concentration control, and excellent stability. Demonstrations across diverse applications, including immersive media, communication aids, and interactive VR environments, show the technology's wide-ranging potential. Future research should focus on miniaturizing OGs further to increase channel density and reduce odor switching delays, enhancing the system's overall responsiveness and capabilities.

While the developed olfactory interfaces demonstrate significant potential, several limitations should be noted. The current system’s odor recovery time, particularly with less volatile compounds, could be improved. Minimizing OG size and improving airtightness would help address this. The relatively limited number of odor channels in the devices could also restrict the complexity of olfactory experiences. Future work could explore expanding the range of odors and improving the accuracy of odor concentration control. Finally, extensive user studies are needed to fully assess the long-term usability and impact of olfactory feedback in various applications.