Spatially-resolved humidity sensing by capacitive-CMOS arrays

Humidity sensing is critical across environmental, agricultural, industrial and healthcare sectors, yet existing sensors lack the ability to resolve transient, spatially detailed humidity variations required for accurate, real-time process monitoring and control. Here we introduce monolithically integrated capacitive sensor arrays fabricated in standard 40 nm complementary metal-oxide-semiconductor (CMOS) technology and subsequently functionalized for humidity sensing with hygroscopic carbon black ink dispensed via a custom-built low-cost inkjet printer. The picoliter-scale ink loading (~6.3 pL) enables rapid response and recovery (~0.6 s), facilitating real-time tracking of human-exhaled breath. Further, the dense pixel pitch (10 µm) in the arrays delivers ultrahigh spatial resolution, allowing precise mapping of localized humidity gradients from wet surfaces as well as visualization of stomatal structures on plant leaves. Altogether, this CMOS platform empowers microscale humidity imaging, opening new avenues for integrated environmental sensing.