Alginate is also a biodegradable polymer; the backbone can be degraded via hydrolysis of the glycosidic bonds [7] or an alginate gel can dissolve due to the elution of the cross-linking calcium cation [6]. Molecular weight has been shown to affect alginate polymer chain degradation [8] and the elution of the crosslinking calcium can also lead to dissolution of gel [9]. This paper considers both high M and high G calcium crosslinked alginate gel beads as devices for use in embolisation.Therapeutic embolisation is the intentional introduction of a (foreign) embolic material into a blood vessel in order to reduce or completely obstruct the blood flow. This procedure is commonly used to treat hypervascularised tumours and arteriovenous malformations (AVMs). The choice of embolic agent depends on the desired level of occlusion down the vascular tree and permanency. The ideal occlusion technique is one that allows for accurate guidance and delivery to the target with low risk of injury to normal structures. To achieve this, interventional radiologists (IRs) are of the opinion that the embolic agent requires a combination of a number of attributes [10]. In handling the agent, it must be either radiopaque or have the ability to be mixed into a radiopaque suspension to enable visualisation during delivery. Simplicity and reliability of delivery is preferred (glue is notoriously difficult to handle for instance), without blocking of the catheters used to deliver the agents. Adaptability to allow selective and rapid occlusion of various blood vessels sizes and types is desirable; and an ability to reach distal vascular beds is particularly important in the treatment of tumours. If possible, the agent should have amenability to trouble shooting/salvage in case of complications or device malfunction. Other important considerations include biocompatibility and cost competitiveness.
One important and growing area of embolotherapy is uterine artery embolisation (UAE), or sometimes referred to as uterine fibroid embolisation (UFE). Since its fist report in 1995 [11] it is estimated that more that 100,000 UAE procedures have been performed (mainly in the USA and Europe) [12]. Outcomes from a large US registry [13] and four separate (albeit relatively small) randomised trials over the last few years [14–17] demonstrate that UAE is safe and effective, gives similar results to hysterectomy but with faster and shorter recovery and uterine preservation. This is balanced against the need to retreat some patients following UAE. Given that some patients opt for UAE as they wish to maintain the potential for future pregnancy, there is some debate over whether the embolisation agent should be permanent or temporary in nature. Gelatin sponge (or Gelfoam®) provides the only current degradable option, which is non-calibrated, has an irreproducible degradation rate and can induce a strong inflammatory response. A calibrated microspherical embolisation agent with controlled degradability could therefore fulfil a role in the clinical practice of UAE.
This paper reports the results from some in vitro and in vivo models used to assess all the key necessary attributes of alginate beads as an embolisation device. The in vitro tests examined bead compressibility compared to other embolic beads on the market, microcatheter deliverability and bead degradation. A sheep uterine artery embolisation model, designed to mimic UAE conditions, was used to assess bead deliverability, biocompatibility and degradation over 12 weeks in vivo.