3D-printed microneedle acoustofluidic mixer for rapid on-chip biochemical assays

Efficient mixing in microfluidic systems is bottlenecked by laminar flow and diffusion-dominated transport, limiting the performance of biochemical assays such as conversion rates of enzymatic processes. Here, we present an acoustofluidic micromixer based on two-photon polymerization (2PP)-printed three-dimensional microneedles that act as ultrasonic microresonators to generate localized acoustic streaming. The freeform arrangement of the microneedles enables integration into diverse microfluidic channel layouts and produces strong, tunable streaming vortices that enhance transport. We characterize the streaming flow fields as a function of actuation frequency, and demonstrate rapid and voltage-dependent mixing with a 2.5-fold enhancement. When applied to the alkaline phosphatase-fluorescein diphosphate (ALP-FDP) reaction, acoustic mixing doubles the fluorescein signal under optimal conditions and significantly reduces the assay time and channel footprint required in continuous flow. These resultsdemonstrate that 3D microneedle-based acoustofluidics is a compact, versatile, and integrable solution for enhancing transport-limited biochemical processes in lab-on-a-chip systems.