The automated SPR (Surface Plasmon Resonance) imaging system, the SPRi-Plex, enables researchers to conduct label-free bio-assays in a multiplexed array system to aid the characterisation of biological interactions. Label-free interaction analysis is becoming increasingly important during the drug discovery process because labelling proteins with either fluorescent or radio-labels can modify their behaviour. This increased importance is mirrored by the number of different manufacturers looking to put their own spin on SPR analysis. These approaches include GE Healthcare's Biacore system , Bio-Rad's Proteon XPR36 and AlphaSniffer's common path interferometry (CPI) SPR biosensor . The analyser has been developed by Genoptics and is being distributed by Horiba's scientific instrument business, Horiba Jobin Yvon. According to Dr Chiraz Frydman, product manager for Horiba Jobin Yvon's Life Science, Molecular and Microanalysis Division, the system is very easy to use and can analyse up to 400 interactions on the surface of a functionalised biochip in just 10 minutes. "The SPRi-Plex system will be of use to biologists studying interactions between peptides, proteins, DNA, antibodies and antigens - and these can even be in complex solutions like serum" said Dr Frydman. The system can even be used to detect molecules from raw serum to enable researchers to follow the effects that drug candidates have on biomarkers for disease during preclinical animal experiments. The company has published a range of application notes on its website detailing the use of the system for imaging oligosaccharide-protein interactions, specific antibody-antigen interactions, peptide-antibody interactions and clinically related protein-peptide interactions. SPR is a phenomenon that occurs when light is reflected off the thin metal films, usually gold, and interacts with delocalised electrons (plasmons) reducing the amount of light reflected. When molecules bind to probe molecules or ligands attached to the metal surface the refractive index changes, which leads to a change in the SPR angle which can be monitored in real-time. The system generates kinetic curves of the interactions that provide insight into association and dissociation rates, affinity constants and surface coverage. In addition, a video capture function enables researchers to watch a visual image of how the light reflected from the array changes over time so that they can check the reactions are working. Dr Frydman continued by explaining that the major limitation of the technique is that the probe molecules need to be smaller than the target molecule as well as being amenable to chemical fixation to the surface. Probes can either be fixed to the biochip surface using either chemical absorption immobilisation techniques or by electropolymerisation. This flexible approach, allowing researchers to choose which immobilisation route is best for a particular assay is somewhat different to that of other instruments where the immobilisation chemistry is usually dictated by the biochip supplier. The system can detect samples at concentrations as low as 150ng/ml for large molecules (100-1000 kDa) or 5µg/ml for smaller molecules (4-20 kDa) from sample volumes of around 200µl. According to the company, the system can also be integrated with microfluidic systems for dedicated screening purposes.