Compass Technology Reimagined: Quantitative, High-Sensitivity Biosensing with a Smartphone Magnetometer

As biosensing technologies evolve, a tradeoff to consider is that complexity and hardware requirements can lead to advanced capabilities, but often with higher costs and greater demands for operator training. These traits can be barriers to widespread adoption, particularly in resource-limited areas. However, the ubiquity of smartphones, which contain a suite of embedded sensors and computational power in a user-friendly package, presents an opportunity to provide accessible bioanalysis tools to billions of people worldwide. But what are the minimal adaptions required to transform a smartphone into a bioanalysis platform? Prior efforts have focused on additions to smartphones to utilize the camera for mobile microscopy and other modes of optical analysis. We introduce here a platform that leverages the smartphone's compass as a signal transducer to bring magnetics to the world of smartphone-based analysis for the first time, along with benefits such as inherent quantifiability and operation in optically unclear samples. The platform consists of a small plastic attachment piece containing an analyte well and cheap "test strips" made from size-changing smart hydrogel structures. The test strips transduce analyte concentrations from a test solution into displacement of magnetic particles away from the embedded smartphone compass, producing a quantifiable change in field strength that is recorded by the phone. Careful design of the hydrogel structure and geometry maximizes particle displacement, leveraging the rapid spatial decay of magnetic fields to achieve sensitivities comparable to complex laboratory instruments. Initial demonstrations with glucose- and pH-responsive hydrogels achieved detection limits on the order of single-digit micromolar glucose and 50 μpH. Extension of the platform to other biomarkers are possible by modifying the hydrogel chemistry and are the focus of ongoing work. This smartphone compass-based platform represents a cost-effective, portable, and accessible solution for biosensing, with significant potential for deployment in resource-limited settings.