Atomic layer deposition (ALD) has emerged as a critical enabling technology for flexible and stretchable electronics, offering precise nanoscale control over thin-film growth while accommodating the unique requirements of polymer-based substrates. Unlike conventional deposition techniques, ALD operates through self-limiting surface reactions, allowing for conformal coatings even on complex, three-dimensional geometries. This characteristic is particularly advantageous for flexible electronics, where device performance must be maintained under mechanical deformation. The compatibility of ALD with low-temperature processing, specialized precursors, and mechanically robust materials makes it indispensable for next-generation wearable sensors and conformal electronic systems.

A key advantage of ALD in flexible electronics is its ability to deposit high-quality films at temperatures compatible with polymer substrates. Many polymers used in flexible electronics, such as polyethylene terephthalate (PET) and polyimide, degrade or deform at elevated temperatures. Low-temperature ALD processes, typically operating below 150°C, enable the integration of functional inorganic layers without compromising substrate integrity. For example, aluminum oxide (Al₂O₃) films deposited via plasma-enhanced ALD at 80°C exhibit excellent barrier properties, with water vapor transmission rates below 10⁻⁶ g/m²/day, making them suitable for encapsulating organic light-emitting diodes and other moisture-sensitive components. Similarly, zinc oxide (ZnO) and tin oxide (SnO₂) can be deposited at similarly low temperatures, providing conductive and semiconductive functionalities essential for active devices.

The development of polymer-compatible precursors has further expanded ALD’s applicability in flexible electronics. Traditional precursors often require high activation energies or produce reactive byproducts that damage organic substrates. To address this, researchers have introduced metalorganic precursors with lower decomposition temperatures and reduced reactivity. For instance, trimethylaluminum (TMA) and ozone-based processes enable Al₂O₃ growth at room temperature, while avoiding the plasma-induced damage associated with oxygen radicals. Additionally, novel precursors such as diethylzinc (DEZ) and tetrakis(dimethylamido)tin (TDMASn) facilitate the low-temperature deposition of ZnO and SnO₂, respectively, without compromising film quality. These advancements ensure that ALD can be seamlessly integrated into existing flexible electronics fabrication workflows.

Mechanical durability is a critical consideration for ALD films in stretchable electronics, where repeated deformation can lead to cracking or delamination. The nanolaminate approach, which alternates layers of different materials, has proven effective in enhancing flexibility. For example, alternating Al₂O₃ and ZnO layers creates a composite structure that redistributes strain and prevents crack propagation, maintaining electrical conductivity even under 5% tensile strain. Another strategy involves the use of inherently flexible materials such as indium tin oxide (ITO) or graphene, which can be deposited or seeded via ALD to form conductive networks that withstand bending cycles exceeding 10,000 repetitions. The conformal nature of ALD also ensures uniform coverage over rough or textured surfaces, further improving mechanical resilience.

Wearable sensors represent one of the most promising applications of ALD in flexible electronics, particularly in healthcare and environmental monitoring. ALD-enabled thin-film transistors (TFTs) on polyimide substrates have been demonstrated for epidermal electrophysiological sensors, capable of conforming to skin while maintaining stable operation under motion. These sensors leverage ALD-deposited gate dielectrics, such as hafnium oxide (HfO₂), which provide high capacitance and low leakage currents essential for sensitive signal detection. Similarly, ALD-grown metal oxides like tungsten trioxide (WO₃) serve as active layers in gas sensors, detecting volatile organic compounds with parts-per-billion sensitivity. The ability to precisely control film thickness and composition at the atomic level allows for tailored sensor responses, optimizing selectivity and sensitivity for specific analytes.

Another emerging application is in flexible energy storage devices, where ALD plays a dual role in enhancing performance and durability. Thin-film batteries and supercapacitors benefit from ALD-deposited solid-state electrolytes, such as lithium phosphate (Li₃PO₄), which exhibit ionic conductivities comparable to liquid electrolytes while being mechanically robust. Similarly, ALD-coated carbon nanotube electrodes show improved cycling stability in lithium-ion batteries, with capacity retention exceeding 90% after 500 cycles. These developments are particularly relevant for self-powered wearable systems, where energy storage must remain functional under bending and stretching.

The scalability of ALD further enhances its suitability for industrial adoption in flexible electronics. Roll-to-roll ALD systems have been developed to accommodate large-area polymer substrates, enabling high-throughput production of barrier films and conductive coatings. This scalability, combined with the technique’s inherent precision, positions ALD as a cornerstone technology for the mass fabrication of next-generation wearable and conformal electronic devices. As the demand for flexible electronics grows, continued advancements in low-temperature processes, novel precursors, and mechanically durable films will further solidify ALD’s role in this transformative field.