Stretchable conductors and interconnects are critical components in flexible electronics, enabling devices to maintain electrical functionality under mechanical deformation. These materials must exhibit high conductivity while accommodating strains from bending, twisting, or stretching. Three primary material classes dominate this field: liquid metals, conductive polymers, and carbon-based nanomaterials. Each offers distinct advantages and challenges in terms of fabrication, strain tolerance, and electrical stability.

Liquid metals, particularly eutectic gallium-indium (EGaIn) and gallium-indium-tin (Galinstan), are widely used due to their inherent deformability and high conductivity. These materials remain conductive even under extreme strains because their liquid state allows them to flow and reconfigure without fracture. Fabrication techniques include direct printing, injection into elastomeric channels, and patterning via stencil lithography. A key advantage is their self-healing capability; if ruptured, the liquid metal can reconnect, restoring conductivity. However, challenges include oxidation, which forms a surface oxide layer that can hinder flow, and encapsulation requirements to prevent leakage. Strain tolerance typically exceeds 500%, with minimal resistance change under cyclic loading. Integration with other components often requires careful interfacial engineering to ensure adhesion and prevent delamination.

Conductive polymers, such as poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), offer mechanical compliance and moderate conductivity. These materials are solution-processable, enabling deposition via spin-coating, inkjet printing, or spray coating. To enhance stretchability, conductive polymers are often blended with elastomers like polyurethane or polystyrene-butadiene-styrene (SBS). The resulting composites can withstand strains of up to 200% while maintaining conductivity. Electrical stability under deformation depends on the percolation network of conductive fillers; excessive strain can disrupt this network, increasing resistance. Fatigue resistance is a concern, as repeated stretching can lead to microcracks and irreversible conductivity loss. Strategies to mitigate this include incorporating self-healing agents or designing hierarchical structures that distribute strain more evenly.

Carbon-based nanomaterials, including graphene and carbon nanotubes (CNTs), are prized for their exceptional electrical and mechanical properties. Graphene’s high carrier mobility and flexibility make it suitable for stretchable interconnects, while CNTs form conductive networks that remain stable under strain. Fabrication methods include chemical vapor deposition (CVD) for graphene, followed by transfer onto elastomeric substrates, and solution processing for CNT-based inks. Strain tolerance varies; graphene can sustain strains of up to 25% before cracking, while CNT networks can endure over 100% strain due to their entangled structure. To improve performance, hybrid approaches combine these materials with elastomers or liquid metals. For example, embedding CNTs in a silicone matrix enhances both conductivity and stretchability. Challenges include achieving uniform dispersion of nanomaterials and minimizing contact resistance at junctions. Fatigue resistance is generally better than conductive polymers but remains inferior to liquid metals.

Fabrication techniques for stretchable conductors must balance precision with scalability. Direct writing methods, such as screen printing or aerosol jet printing, enable patterned deposition of conductive inks. Laser ablation can create stretchable meshes by selectively removing material to form serpentine or fractal designs, which accommodate strain by out-of-plane deformation. Transfer printing allows pre-fabricated conductive layers to be bonded onto stretchable substrates. Each method has trade-offs in resolution, throughput, and compatibility with other materials.

Electrical stability under deformation is a key metric for stretchable conductors. Resistance changes during stretching are influenced by material properties and geometric design. Serpentine or horseshoe-shaped traces reduce strain concentration, minimizing resistance fluctuations. For liquid metals, resistance remains nearly constant due to volume conservation. Conductive polymers and carbon-based materials exhibit more significant resistance changes, often following a power-law relationship with strain. Cyclic loading tests reveal degradation mechanisms; for instance, PEDOT:PSS films may develop cracks after 1,000 cycles at 50% strain, while CNT networks show gradual increases in resistance due to nanotube reorientation.

Integration with other components poses additional challenges. Stretchable conductors must interface with rigid or semi-flexible elements, such as sensors or transistors, without causing mechanical mismatch. Strategies include using gradient stiffness designs or island-bridge architectures, where rigid islands are connected by stretchable interconnects. Adhesion promoters, such as silane coupling agents, improve bonding between dissimilar materials. Encapsulation is often necessary to protect conductors from environmental factors like moisture or abrasion.

Fatigue resistance is critical for long-term reliability. Liquid metals excel in this regard, as they do not suffer from fatigue in the traditional sense. Conductive polymers and carbon-based materials, however, are prone to cumulative damage. Self-healing materials, such as polymers with dynamic bonds or embedded microcapsules of healing agents, offer potential solutions. Alternatively, designing conductors with redundant pathways ensures continued functionality even if some portions fail.

Emerging trends focus on multifunctional stretchable conductors that combine conductivity with other properties, such as transparency or biocompatibility. For example, transparent conductive films of silver nanowires or graphene are sought for wearable displays. Biocompatible materials like gold-coated elastomers are explored for implantable devices. Advances in computational modeling aid in optimizing material compositions and geometries for specific applications.

In summary, stretchable conductors and interconnects rely on liquid metals, conductive polymers, and carbon-based nanomaterials, each with unique benefits and limitations. Fabrication techniques must address strain tolerance, electrical stability, and integration challenges. While liquid metals offer unparalleled deformability, conductive polymers and carbon-based materials provide complementary advantages in processing and functionality. Future progress hinges on improving fatigue resistance, scalability, and multifunctional integration to meet the demands of next-generation flexible electronics.