Hybrid microchannel-solid state micropore device for fast and optical cell detection

Abstract

This paper presents a methodology for cell detection and counting using a device that combines PDMS (polydimethylsiloxane) microfluidic multilayer channels with a single solid state micropore. Optimal conditions of solid-state micropore fabrication from crystalline silicon wafers are presented. Micropores of varying size can be obtained by directly etching using an etchant agent concentration of 50 wt% KOH, at varying temperatures (40, 60, 80 °C) and voltages (100, 500, 1000 mV). Scanning Electron Microscopy (SEM), and profilometry techniques have been used for the micropore characterization. In order to find optimal conditions for cell detection a COMSOL Multiphysics simulation was performed. Pressure drop, shear stress, fluid viscosities and flow rates parameters were evaluated. The potential viability of the device for cell detection and counting, avoiding cellular damage, is demonstrated.

Fig. 1. Fabrication of silicon micropores by electrochemical method. Schematic image of the track-etching setup.

Fig. 2. Setup fabrication for cell counting (a) microchannels design and PDMS replica bond on a glass slide, (b) micropore is bonded and aligned with PDMS block (5 mm thick), left down, piece of silicon with micropore dimensions. (c) Microfluidic device scheme and setup montage.

Fig. 3. SEM images of the micropores. (a) KOH20-HCl-80-1 (front side), (b) KOH50-HCl-80-1 (front side), (c) KOH50-HCOOH-80-0.1 (front side), (d) KOH50-HCOOH-80-0.1 (back side). The codes of the micropores correspond with the concentration of the KOH, followed by the acid, temperature and voltage applied during the electrochemical etching.

Fig. 4. SEM images from silicon surfaces treated with; (a) 50 wt% KOH, 1 V at 40 °C, (b) 20 wt% KOH, 1 V at 60 °C, (c) 20 wt% KOH, 1 V at 60 °C, (d) 20 wt% KOH, 1 V at 80 °C, (e) 20 wt% KOH, 1 V at 80 °C (f) 20 wt% KOH, 1 V at 80 °C.

Fig. 5. I–t plots recorded during the electrochemical etching in 50 wt% KOH for silicon wafers subjected at different conditions: (a) HCl 12 M at 80 °C; (b) HCl 12 M at 60 °C; (c) HCl 12 M at 40 °C; (d) HCOOH 12 M at 80 °C; (e) HCOOH 12 M at 60 °C; (f) HCOOH 12 M at 40 °C. Voltage of 1000 V, 500 V and 100 V were applied during the etching process. The red arrows indicate the pore opening time.

Fig. 6. Pressure distribution in the micropore and PDMS microchannel. Conditions: viscosity of 1.5 cP and a flow rate of 16.6 μL min⁻¹.

Fig. 7. Shear stress distribution on the micropore and PDMS microchannel. Conditions: viscosity of 1.5 cP and a flow rate of 16.6 μL min⁻¹.

Fig. 8. (a) Pressure drop and (b) shear stress calculated at varying flow rates and viscosities.

Fig. 9. Velocity profiles on the micropore cross-section. Conditions: viscosity of 1.5 cP and a flow rate of 16.6 μL min⁻¹.

Fig. 10. Identification of whole cell under a bright field of microscope. (a) Cell image, (b) nuclei cell image, (c) processing image of a cell by ImageJ-FIJI, (d) processing image of nuclei cell by ImageJ-FIJI. Micropore dimensions: ∼150 μm × 150 μm.

Fig. 11. HEK-293 cells measured at different flow rates.