"Dynamic microtubule bundles are involved in several motility processes in cells, for instance by interacting with mitotic chromosomes. The study of microtubule bundle dynamics and force generation can help understanding the mechanisms underlying these complex processes. Here we discuss experimental results on the force-generating capabilities of bundles of non-crosslinked growing microtubules obtained using an optical tweezers technique in vitro. We discuss the possible effects on force generation by these microtubule bundles of specific (with microtubule-associated proteins) and aspecific (with ions or crowding agents) ways of crosslinking microtubules. We present preliminary results showing that force generation by microtubule bundles is enhanced in the presence of polyethylene glycol used as a crowding agent, and discuss possible explanations for this observation.