There are several topics in which you can carry out your research. Process Control and Optimization has a lot of interesting topics. Embedded Systems for Instrumentation is another field. Image processing has its important specially in biomedical applications.

In the field of biomedical instrumentation and control, it is necessary the use of electrical signals to describe the phenotypic characteristics of biological samples, such that it is possible to prescind from the use of a microscope. For instance, the use of impedance measurements to describe physical characteristics of cells populations for its use in the classification of cellular subpopulations. Other useful example is the use of impedance measurements to detect the presence of pollutant in water or the presence of a specific bacterium/virus in a blood sample. This must be achieved with high throughput and high sensitivity.

I think you can work on full automated surgerical robot system. The interpretation of biomedical quantities detection and analysis will be on the part of instrumentation as well as the electronics control of surgical tools, then the surgerical operation as the automated control part.