[0063] Inner catheter lumen 20 is configured to transport fluid, such as aspiration fluid, blood, and particulates in the fluid, from a distal end of inner catheter 14, such as from inner catheter distal opening 22 and/or the plurality of sidewall openings 24, to a proximal end of inner catheter 14, such as to aspiration pump 6. Inner catheter 14 may be sized to be housed in outer catheter lumen 16 while still enabling aspiration fluid under a positive pressure provided by an aspiration pump to be delivered to outer catheter opening 18 (in the space between an inner surface of outer catheter 12 and an outer surface of inner catheter 14) and allowing fluid and/or particles from a site of a thrombus to pass through inner catheter lumen 20. In some examples, inner catheter 14 may have an outer diameter Dz between or equal to about 0.050 inches (about 1.3 mm) and/or about 0.074 inches (about 1.9 mm). In some examples, the outer diameter and/or outer circumference of inner catheter 14 may be relatively constant (e.g., constant or nearly constant) from a proximal end of inner catheter 14 to inner catheter opening 22. In other examples, inner catheter 14 may taper in distal direction or otherwise vary in outer diameter along its length. [0064] Inner catheter distal opening 22 is an opening of inner catheter lumen 20 at a distal end of inner catheter 14. In some examples, one or more portions of an inner surface of inner catheter 14 may be lubricious to facilitate the introduction and passage of a medical device, particulates such as pieces of a clot/thrombus, a therapeutic agent, or the like, through inner catheter lumen 20. In some examples, the material from which portions of the inner surface is formed may itself be lubricious (e.g., PTFE). For example, a lubricious inner surface that may allow relatively easy movement of a guidewire, particulates, and/or fluid through inner catheter lumen 20. In addition to, or instead of, being formed from a lubricious material, in some examples, an inner surface of may be coated with a lubricious coating such as a hydrophilic coating. The inner surface may be formed from any suitable material including, but not limited to, polytetrafluoroethylene (PTFE), expanded PTFE a fluoropolymer, perfluoroalkyoxy alkane (PFA), fluorinated ethylene propylene (FEP), polyolefin elastomer, and the like. [0065] In some examples, one or more portions of an inner surface of inner catheter lumen 20 at a distal end of inner catheter 14 may be configured to have a relatively high affinity to a clot material by, for example, using a suitable surface treatment (e.g., a coating and/or etching) on inner liner 18 to promote mechanical or chemical engagement with the clot. (Such affinity may be measured, for example, with a DMA (dynamic mechanical analyzer) equipped with a shear sandwich clamp.) For example, the inner