surface of a distal section on inner catheter lumen 20 may be treated with a surface coating, etching, or other roughening mechanism, so that the distal section better engages with the clot, such that the inner surface of the distal section may be configured to promote at least one of mechanical or chemical clot engagement. A roughened or less lubricious surface of an inner surface that is brought in contact with the clot may allow for the clot to stick better to the inner surface, which may allow the clot to be pulled into inner catheter 14 more effectively. Examples of suitable coating materials to increase the affinity of the clot to an inner surface of inner catheter lumen 20 may include, for example, a thermoplastic elastomer such as ChronoPrene™(AdvanSource Biomaterials, Wilmington, Massachusetts); a polyolefin elastomer such as ethylene-octene or ethylene- butene copolymer, or the like. 

[0066] As mentioned above, inner catheter 14 is configured to be positioned within outer catheter lumen 16. As such, an outer diameter or circumference of inner catheter 14 and an inner diameter or circumference of outer catheter lumen 16 may be selected or configured relative to each other. In some examples, an inner circumference of outer catheter 12 may be between about 50 microns and about 400 microns greater than an outer circumference of inner catheter 18. 

[006 η The plurality of sidewall openings 24 may be configured to flow fluid from at least one of outer catheter lumen 16 and a volume of space (e.g., in a blood vessel) external to outer catheter distal opening 18 into inner catheter lumen 20. Various factors of the sidewall openings 24, such as a number of sidewall openings 24, one or more positions of sidewall openings 24 around and along inner catheter 14, one or more sizes of sidewall openings 24, and the like, may be selected to provide desired and/or predetermined flow characteristics of fluid from outer catheter lumen 16 through outer catheter distal opening 18, from outer catheter lumen 16 into inner catheter lumen 20, and/or from a volume distal to outer catheter distal opening 18 into inner catheter lumen 20 . 

[0068] In some examples, each of the plurality of sidewall openings 24 comprises a one-way valve configured to permit flow of fluid into inner catheter lumen 20. For example, sidewall openings 24 may be configured to allow flow of fluid from outer catheter lumen 16 into inner catheter lumen 20 through sidewall openings 24, such as in response to a positive pressure in outer catheter lumen 16 being greater than a negative pressure in inner catheter lumen 20, but not allow (i.e., no substantial amount of) flow of fluid from inner catheter lumen 20 into outer catheter lumen 16 through sidewall 

