From Woburn Startup Extrudes Diesel Filters Like Pasta; The Way to Cleaner Cars?:

GEO2’s exhaust filters contain a network of interlocking tubes. Exhaust gas enters the open holes that are open to the upstream end of the exhaust system. It’s forced through the walls, and exits through the channels that open to the other end of the cylinder. The dark and light filters are made of slightly different materials.