Electrospinning Machine| Skin- and hair- inspired nanofibrious hydrogel composite as multimodal sensor for polysomnographic monitoring in sleep apnea syndrome

Flexible sensors, particularly biomimetic hair-like sensors, offer significant advantages in real-time monitoring of sleep apnea syndrome (SAS) due to their exceptional sensitivity in detecting subtle respiratory airflow. However, most current hair-inspired designs focus solely on single-signal-response hair structure simulation, neglecting the critical role of skin-hair functional integration in biomechanical sensing.

Recently, a research team led by Professor Cheng Si from Soochow University, in collaboration with Dr. Cheng Suhang (Associate Chief Physician) from Suzhou Kowloon Hospital, published their findings titled 'Skin- and hair-inspired nanofibrous hydrogel composite as multimodal sensor for polysomnographic monitoring in sleep apnea syndrome' in the Journal of Colloid and Interface Science. The first author is Shen Meng, an undergraduate student from Soochow University, with corresponding authors being Professor Cheng Si from Soochow University and Dr. Cheng Suhang from Suzhou Kowloon Hospital.This work developed a biomimetic skin-hair structured sensor (Figure 1) through alkaline-hydrolyzed polyacrylonitrile/ionic liquid/electrostatically flocked carbon fiber (aPAN/IL/CFs) nanofibrous hydrogel. This hydrogel combines conductive aPAN/IL hydrogel (simulating skin) with vertically aligned carbon fibers fabricated via electrostatic flocking (simulating hair), enabling multimodal airflow and strain detection for comprehensive polysomnographic SAS monitoring.The aPAN/IL/CFs nanofibrous hydrogel demonstrates exceptional sensitivity for both airflow and strain sensing, including: rapid airflow response time of 0.10 s, minimum detectable airflow velocity of 0.033 m s-1, fast tensile response time of 0.6 s, and maximum strain sensitivity (GF) of 76.1. Furthermore, the lightweight and self-adhesive aPAN/IL/CFs nanofibrous hydrogel facilitates seamless integration with medical devices to monitor physiological signals in SAS patients, including respiratory airflow, abdominal breathing, and pulse detection. Beyond respiratory monitoring, this sensor also supports gesture recognition for critically ill patients with impaired speech ability.Previously, Professor Cheng Si's team at Soochow University published a review on electrostatic flocking titled 'Electrostatic Flocking: Reborn to Embrace Multifunctional Applications' (Small Struct. 2025, 2500143) in Small Structures. The current work builds upon this electrostatic flocking preparation technology, leveraging the advantage of electrostatic flocking structures in responding to minute forces to fabricate this sensor."

Fig. 1: a) Fabrication schematic; b) Applications (respiration/pulse/gesture).

The core innovation of this sensor lies in its biomimetic structural design: The electrostatic flocking technique is employed to create vertically aligned carbon fiber arrays (aPAN/IL/CFs) on the surface of aPAN/IL nanofibrous hydrogel substrate. In this configuration:

1. The vertically aligned carbon fibers mimic human hair follicles, achieving highly sensitive external signal transmission through their high aspect ratio and dynamic contact-separation mechanism between fibers.

2. The aPAN/IL nanofibrous hydrogel simulates both skin and subcutaneous neural networks, not only establishing interconnected conductive pathways to reduce sensor response time, but also ensuring conformal adhesion to biological surfaces through its intrinsic adhesiveness.

The fabrication process involves:

• First, alkaline hydrolysis converts PAN/IL electrospun fiber membranes into highly adhesive nanofibrous hydrogels.

• Subsequently, electrostatic flocking technology distributes carbon fibers into uniform vertical arrays on the sticky hydrogel surface.

• This binder-free process simplifies the electrostatic flocking preparation while maintaining fiber array stability.

Material characterization reveals:

The 90-minute alkaline-hydrolyzed hydrogel achieves approximately 50% elongation at break, significantly exceeding the original PAN/IL electrospun fiber membrane's performance.This represents a remarkable improvement in toughness (Figure 2).

Fig. 2: Material characterization (SEM, mechanics, adhesion).

The aPAN/IL/CFs nanofibrous hydrogel demonstrates exceptional performance in both airflow and strain sensing:

For airflow detection:

• Ultra-fast response time of 0.10 seconds

• Exceptionally low detection limit of 0.033 m s⁻¹

• High sensitivity of 38.0% s m⁻¹ in low airflow regime (<1 m s⁻¹)

• Significantly outperforms both aPAN/IL (0.046% s m⁻¹) and aPAN/CFs (6.5% s m⁻¹) hydrogels (Figure 3)

For strain sensing:Remarkable gauge factor (GF) of 76.1 within 13%-20% strain rangeRapid response time of 0.6 secondsMaintains stable response over 250 stretching cyclesExhibits outstanding resolution for minute deformations (Figure 4)

All numerical values, units, material designations, and performance characteristics are rendered precisely as in the original Chinese text while conforming to English scientific writing conventions.

Fig. 3: Airflow sensing performance.

Fig. 4: Strain sensing performance.

In sleep apnea syndrome (SAS) monitoring, this sensor demonstrates multidimensional clinical value:

1. Nasal Cannula Integration:

• Precisely discriminates between:
  • Deep breathing
  • Shallow respiration
  • Rapid breathing
  • Coughing episodes
  • Apnea events

2. Abdominal Monitoring:

• Tracks respiratory-induced abdominal deformation

• Eliminates motion artifacts associated with conventional abdominal belts

3. Wrist-worn Application:

• Captures characteristic pulse waveform components:
  a) Percussion wave (P)
  b) Tidal wave (T)
  c) Dicrotic wave (D)

• Provides supplementary diagnostic data for SAS evaluation

4. Gesture Recognition (Novel Application):

• Enables American Sign Language (ASL) translation when attached to finger joints:
  • Numeric signs (1-4)
  • Functional commands ("Error," "Yes," "Forget")

• Establishes critical communication channel for SAS patients with speech impairments (Figure 5)

Fig. 5: SAS monitoring and ASL translation demonstrations.

In summary, the research team has developed a flexible, self-adhesive aPAN/IL nanofibrous hydrogel integrated with vertically aligned CF arrays via electrostatic flocking, creating an ultralight, ultrasensitive sensor for SAS monitoring.

Key innovations:

1. Bioinspired Design:

• aPAN/IL hydrogel mimics the subcutaneous neural network

• Flocked CFs replicate the hair layer

• Synergistic combination enables biomimetic high sensitivity

2. Material Advantages:

• Intrinsic hydrogel adhesiveness enables binder-free CF fixation

• Resultant aPAN/IL/CFs composite shows exceptional responsiveness to:
  • Airflow at varying angles
  • Different flow velocities

3. Clinical Performance:

• Successfully deployed in SAS respiratory monitoring systems

• Accurately discriminates between:
  • Deep breathing
  • Shallow respiration
  • Other abnormal breathing patterns

• Provides real-time feedback on sleep-disordered breathing

• Enables gesture recognition for critically ill SAS patients with communication impairments

4. Broader Impact:

• The biomimetic design philosophy

• Novel material innovations
  May inspire advancements in respiratory monitoring and related fields

Paper link: https://doi.org/10.1016/j.jcis.2025.138406