Acoustophoresis Physics — Acoustic Radiation Force
When a standing acoustic wave is established in a microchannel resonator, particles experience a primary acoustic radiation force (ARF) directed toward pressure nodes or antinodes depending on the acoustic contrast factor Φ = (5ρ_p/2ρ_f − 1) − (β_p/β_f)/2. For red blood cells (density 1,100 kg/m³, compressibility 3.4×10⁻¹⁰ Pa⁻¹) in plasma (density 1,025 kg/m³, compressibility 4.5×10⁻¹⁰ Pa⁻¹), Φ = +0.17 — positive, directing RBCs toward pressure nodes (channel centre). Plasma, being the continuous fluid phase, exits at the channel edges. Platelets (density 1,040 kg/m³) have Φ = +0.01 — minimal lateral displacement, requiring secondary DEP (dielectrophoresis) capture for complete depletion.
Resonator Design & Fabrication
The BAW resonator uses a 200 µm × 150 µm cross-section silicon channel bonded to PDMS. A 3 MHz PZT transducer (Physik Instrumente P-143.10A) bonded to the silicon substrate generates a half-wavelength resonance mode (λ/2 ≈ 250 µm at 3.06 MHz in silicon). Input power: 50–200 mW. Channel length: 35 mm. Flow rate optimised at 10 µL/min — yielding an acoustic exposure time of 5.25 seconds per RBC traversal through the separation zone. Outlet bifurcation splits the central (cell-rich) and lateral (plasma) streams at a 1:3 volume ratio.
Experimental Results & Current Limitations
At 150 mW acoustic power and 10 µL/min flow rate: RBC removal efficiency 98.4%, platelet removal 92.1%, plasma purity 85.3% (vs. 99.2% for 400×g centrifuge reference). Plasma protein recovery: albumin 97.8%, CRP antibody epitope intact (Western blot confirmed). Key current limitations: (i) acoustic streaming at >100 mW causes temperature rise of 2.8°C — near the 3°C threshold for enzyme denaturation with our thermolabile conjugates; (ii) platelet removal is incomplete at 92% vs. clinical requirement of >99.9% for fibrinogen-based coagulation assays; (iii) haematocrit above 52% causes inter-RBC hydrodynamic coupling that disrupts separation efficiency by ~18%.
Next Steps: Cascaded Two-Stage BAW + DEP Hybrid
A two-stage cascade is now in design: Stage 1 — BAW (3 MHz, 120 mW) for bulk RBC removal at 90–95% efficiency. Stage 2 — Travelling wave DEP (TWD) electrodes (200 kHz, 5 Vpp) for residual platelet capture by positive dielectrophoresis onto electrode edges before the assay interface. COMSOL simulation predicts the cascade achieves >99.5% total particulate removal. Power budget: 170 mW total — compatible with standard USB-C delivery on portable instruments. Target study completion: Q2 2026.