**4.2 MR sensors in droplet-based microfluidics**

The combination of MR sensors and droplet-based microfluidics is most commonly based on integration of MR sensors into the channels to achieve highest sensitivity. For microfluidic integration, MR sensors have to be as thin as possible not to disturb the laminar flow present in the channel. Practically, overall thickness of MR sensors (magnetically responsive layer, metallic supply and measurement electrodes as well as electrical isolation) should be optimally located in the nanometer regime, making these sensors composed either of 1D (nanowires) or of 2D (thin film) materials while general widths and lengths of individual components are located in the micrometer range. This

#### **Figure 8.**

*Tracking and characterization of droplets in millifluidics using MR sensors (a) theoretical concept of ferrofluid droplet analyzer using GMR sensors enabling multiparametric analysis for (b) various droplet volumes (left) and ferrofluid concentrations (right) [32]. (c) Ferrofluid droplet generation in a commercial T-junction. (d) Detection of a ferrofluid droplet chain using PHE sensorics. (e) Investigations of limit of detection of the PHE sensor. Adapted with permission from ref. [81].*
