Design and Optimisation of Elliptical‐Shaped Planar Hall Sensor for Biomedical Applications

The magnetic beads detection‐based immunoassay, also called magneto‐immunoassay, has potential applications in point‐of‐care testing (POCT) due to its unique advantage of minimal background interference from the biological sample and associated reagents. While magnetic field detection technologies are well established for numerous applications in the military, as well as in geology, archaeology, mining, spacecraft, and mobile phones, adaptation into magneto‐immunoassay is yet to be explored. The magnetic field biosensors under development tend to be multilayered and require an expensive fabrication process. A low‐cost and affordable biosensing platform is required for an effective point‐of‐care diagnosis in a resource‐limited environment. Therefore, we evaluated a single‐layered magnetic biosensor in this study to overcome this limitation. The shapeinduced magnetic anisotropy‐based planar hall effect sensor was recently developed to detect a lowlevel magnetic field, but was not explored for medical application. In this study, the ellipticalshaped planar hall effect (EPHE) sensor was designed, fabricated, characterized, and optimized for the magneto‐immunoassay, specifically. Nine sensor variants were designed and fabricated. A customized measurement setup incorporating a lock‐in amplifier was used to quantify 4.5 μm magnetic beads in a droplet. The result indicated that the single‐domain behaviour of the magnetic film and larger sensing area with a thinner magnetic film had the highest sensitivity. The developed sensor was tested with a range of magnetic bead concentrations, demonstrating a limit of detection of 200 beads/μL. The sensor performance encourages employing magneto‐immunoassay towards developing a low‐cost POCT device in the future.