As smartphone screens become rollable, smartwatch bands can monitor ECG, and AR glasses fit seamlessly to the face, flexible electronics are transitioning from concept to everyday reality. Amidst this wave of human-machine integration, devices not only require "bendable" screens and circuits but also a material that can safely contact the skin, cushion impacts, provide sealing protection, and offer optical transparency as a "soft interface" for human-machine interaction. Silicone rubber, especially high-transparency liquid silicone rubber (LSR), stands out due to its unique comprehensive properties, becoming a crucial material for flexible display backplanes, wearable sensor encapsulation, and device structural components.

I. Optical Transparency and Flexible Support

Flexible OLED or Micro-LED displays, despite being bendable, remain vulnerable to scratches and impacts on their ultra-thin glass or PI substrates. To address this, manufacturers often cover the back or edges with a layer of high-transparency silicone rubber film (transmittance > 92%):

It provides impact resistance and prevents cracking upon drops.

The surface can be treated with anti-fingerprint (AF) or anti-reflective (AR) coatings.

With a bending radius <5 mm, it shows no whitening or stress marks, outperforming TPU or PET materials.

For instance, some foldable phones use transparent silicone gel in hinge areas to both conceal mechanical structures and maintain continuous visual experiences.

II. "Electronic Skin" for Wearable Sensors

Smart bracelets and health patches need to continuously adhere to the skin to collect physiological signals such as ECG, PPG, and EMG. Silicone rubber plays three roles here:

Biocompatible substrate: Certified by ISO 10993, it is non-allergenic and suitable for sensitive skin.

Elastic encapsulation layer: Wraps around flexible circuits and chips, providing waterproofing (IP68) while allowing stretchability.

Functional integration platform: Incorporating conductive fillers (e.g., liquid metals) to create strain sensors or embedding microfluidic channels for sweat analysis.

The Apple Watch's heart rate sensor uses medical-grade silicone gaskets around the back to ensure close contact between the optical window and the skin, reducing motion artifacts.

III. Sealing and Bonding: Reliable Miniaturization

Given the limited internal space in wearables, traditional screws and adhesives are impractical. Silicone rubber achieves reliable sealing through selective dispensing or in-mold decoration (IMD):

Acoustic sealing for miniature microphones/speakers.

Creating waterproof barriers around battery compartments and charging contacts.

Stress-relieving bonding of flexible FPC cables to the motherboard.

Its low modulus of elasticity (0.5–2 MPa) absorbs repeated deformations during daily activities, preventing solder joint fatigue and failure.

IV. Touch Sensation and Aesthetic Design

Consumer expectations for the tactile experience of wearable devices are increasingly demanding. Silicone rubber can be adjusted to match the softness of human skin (30A–50A) and supports various surface treatments:

Matte, glossy, or textured finishes.

Multi-color co-injection molding for gradient effects or functional zones (e.g., anti-slip areas on sports bands).

Seamless bonding with hard materials like metal or ceramics to enhance premium aesthetics.

V. Challenges and Future Directions

Improved dirt resistance: Developing oil- and water-repellent surfaces to reduce sweat residue.

Self-healing capabilities: Introducing dynamic bonding networks for heat-repairable scratches.

Biological integration: Combining with living cells or enzymes to achieve closed-loop biosensing.

Conclusion

In the realm of flexible electronics, silicone rubber serves as a silent "tactile translator," transforming cold circuits into warm wearing experiences and converting precise optical signals into dependable health data. Though it doesn't emit light, it makes screens more durable; though it doesn’t compute, it ensures accurate sensing. This soft, transparent interface adds a layer of reassurance and reduces the gap between humans and machines with every touch.


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