read the US information sheet
http://www.extheramedical.com/coronavirus-resources
http://www.extheramedical.com/coronavirus-resources
My understanding is that viruses, toxins, bacteria, etc, bind to the heparin microbeads as blood is passed through the filtration column. IIRC, the Seraph is being used more for treating complications resulting from a COVID-19 infection than the virus itself (though COVID-19 does appear to have an affinity to Heparin). They're reporting improvement in lung function and greater odds of coming off the ventilator alive.
I should add that I'm not aware of any published data regarding the use of the product. My info comes from my friend who works at Exthera, so take it for what it is.
Is it possible to customise surface of these microbeads to bind with specific protein sequences?