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Collagen sponges, conventional polyvinyl alcohol (PVA), and microspheres are the most commonly used particles (Loffroy et al. 2009). Collagen sponge particles are used for temporary occlusion (Abada and Golzarian 2007). Most studies suggest that recanalization occurs within 14 days (Abada and Golzarian 2007; Loffroy et al. 2009); however, one study found that 78% of cases were recanalized at 3 days (Louail et al. 2006). These sponges likely perform occlusion through physical effects and by enhancing thrombus formation (Abada and Golzarian 2007; Loffroy et al. 2009).

Conventional or nonspherical PVA particles are available in multiple sizes. These particles cause mechanical occlusion, and the subsequent blood stasis results in biological occlusion (Loffroy et al. 2009). Nonspherical PVA particles have an irregular shape and may aggregate, resulting in a more proximal occlusion; for instance, a third‐order vessel branch may be occluded when a fourth‐order branch occlusion is desired (Siskin et al. 2000; Loffroy et al. 2009). Nonspherical PVA particles are considered permanent vascular occlusion agents (Siskin et al. 2000; Patel and Soulen 2006); however, some reports have described recanalization of vessels that have been occluded with these particles (Siskin et al. 2000; Loffroy et al. 2009).

The majority of microspheres that are commercially available are made of trisacryl gelatin, PVA, or sodium acrylate/vinyl alcohol copolymer (Patel and Soulen 2006; Loffroy et al. 2009). Microspheres are available in 100–300 μm, 300–500 μm, 500–700 μm, 700–900 μm, and 900–1200 μm (Laurent 2007). Microspheres have several advantages over nonspherical PVA particles. Microspheres can be calibrated (developed with a predetermined size) and tend to reduce blood flow more quickly and reliably than nonspherical PVA (Laurent 2007; Loffroy et al. 2009). Nonspherical PVA particles can have variable behavior after discharge from a catheter, making the placement of these particles less predictable than microspheres (Laurent 2007). Another advantage of microspheres as compared to nonspherical PVA is that they do not result in catheter blockage as they do not aggregate prematurely (Loffroy et al. 2009).

Improvements in microsphere technology have allowed for even more sophisticated IO techniques. Microspheres with drug‐eluting capabilities are available and allow for delivery of high‐dose chemotherapy directly to a tumor, with minimal systemic effects (Liapi et al. 2007; Martin et al. 2009). Some of the drugs that have been incorporated into microspheres include doxorubicin, oxaliplatin, and irinotecan (Liapi et al. 2007; Kettenbach et al. 2008; Martin et al. 2009).

Further advancements in microsphere production include the use of microspheres that can be detected on MRIs and CTs and microspheres that can be resorbed in a controlled manner (Laurent 2007). Being able to visualize the location of a microsphere with CT and MRI allows the clinician to determine the final location of the microspheres and the subsequent tumor response based on that location. Resorption of the microsphere may allow these particles to be used for temporary occlusion.