Bovine mastitis test for somatic cell count on a centrifugal microfluidic disc

Abstract

Bovine mastitis – an infection in a cow’s udder – is the one of the largest problems facing the dairy industry. This thesis presents the results of a project to design, prototype and experimentally validate an automated method to detect bovine mastitis in milk rapidly, inexpensively and reliably. The system constructed is well-suited to deployment in the milking shed, as a hands-off, “point-of-cow” diagnostic instrument for detecting bovine mastitis in individual cows. The method exploits the details of the viscosity behaviour of the non-Newtonian gel that is formed upon lysis of the somatic cells present in milk from infected cattle. This physical phenomenon also underpins the most commonly used method today, the California Mastitis Test (CMT). The resulting technology from this thesis provides a significant advance on the state-of-the-art in this field, including improved compactness, automation and higher accuracy than is currently available. The platform is a spinning disc with microfluidic channels, within which the operations of mixing, timing and viscosity measurement are performed. Critical conditions for controlling the extent and temporal dynamics of the gelation process were first determined in order to optimise the sensitivity to somatic cell count, accuracy and time to result. An instrument was developed to produce the somatic cell and sodium dodecyl sulphate (SDS) gel (SCS) with low milk volumes (400 μL) and measure its kinematic viscosity. The device made use of recent developments in centrifugal microfluidics, a technology that can perform complex multistep reactions by spinning micron scale channels and millimetre scale chambers on a speed and acceleration-controlled platform. A centrifuge platform and method for making centrifugal discs was developed, making use of speed controlled servo motors, microCNC machining and laser cutting. Measurement of a complex fluid on the disc was performed as a proof of principle. By increasing the angular speed of the disc, different shearing rates of the fluid could be accessed and the shear thinning effect could be observed. Optimisation of this viscometer using finite element computational fluid dynamic simulations, vortical mixing and elution spin speeds allowed a somatic cell count to be performed on a disc, using low sample volumes, which can determine the cell count in milk and diagnose cows with mastitis at the point of milking more accurately and easily than before.