## 1 Introduction
### Working Principle and Device Design  
The elution device (Figure 1a,b) consists of a rotating mechanism, ring magnet, a vial at the center with the magnetic particles, and the electrical control unit. It resembles a sun and planetary gear mechanism, with a planetary gear rotating around the teeth of an external fixed gear, the centre distance between them being 38.1 mm. The device is designed with 16 teeth on the planetary gear and a facewidth of 12 mm on both structures to prevent gear failure. The gear ratio between the mating gears is 2.9, with a module of 2.5 mm. A cylindrical vial (15 mm diameter and 45 mm height) is placed at the centre of the device, supported by clamps. Smaller magnetic particles (3.2 mm\({}^{3}\)) are placed in confinement (gap spacing \(\sim\)5 mm) inside the walls of the vial. The planetary gear has slot for holding the diametrically magnetized ring/disk magnet. The smaller magnetic particles inside the vial get attracted by the magnet, which produces simultaneous rotation around its axis and revolution around the vial axis. The rotation of the magnet along its own axis is responsible for the physical impact on the fibres. The revolutionary trajectory allows for the particles to cover the entire surface area of the swab, thereby allowing maximum exposure. By adjusting the height of the vial, the magnetic particles can be levitated from the base, which allows them to cover the entire length of the swab in the vertical direction. Smooth transfer of rotational energy has been ensured by inserting snap rings below the bearings to prevent its fallout form position. A small beam supporting the planetary gear is also connected with the motor shaft that revolves around the central axis of the device. The device is connected to a DC power supply, which provides the necessary voltage for the DC motor (Figure 2a).  
This device provides higher impact forces on the swab fibres, desired to improve elution efficiencies. In order to introduce multiple impingements and fluid circulation, a diametrically magnetized ring magnet is used and mechanically moved around the vial in a planetary fashion. The planetary rotation periodically changes the polarity of the magnet, which causes rotation of the magnetic particles in the fluid. The rotation of the particles is responsible for creating a scrubbing action to remove materials from the swab. The revolution of the ring magnet itself causes curvilinear translation of the magnetic particles around the swab. These particles strike the swab at different locations angles to provide the necessary force to elute the entrapped beads/cells. Simultaneous rotation and revolution of the particles is desired inside the vial to cover maximum surface area of the swab head. Repeated impingements along with turbulent motions of the fluid also allow for fresh fluid to enter the fibre matrix after each impact and dissolve the samples with the elution fluid.  
The most important parameters that affect the recovery efficiency are the rotational speed of the magnet around the vial, the time of elution, and the concentration of the particles/cells in the sample. The rotational magnetic field is dependent on the speed of the motor. Higher rotational speed provides more number of impingements per minute, thereby facilitating higher removal of materials from the swab fibres. Hence, RPM can be identified as the most important parameter. Elution time reflects the amount of time that the impact forces are imparted to the swab and hence the trend in recovery should also influence the elution time. Concentration of the particulates in the sample is another important criterion that shows the number of eluted particles and chances of entrapment of the organisms in the fibrous matrix. The effect of these parameters needs to be characterized in order to determine the optimal operating conditions for high recovery efficiency.