The i-gel mask is a second-generation, single-use supraglottic airway device which does not have an inflatable cuff. The cuff consists of a thermoplastic elastomer and its mechanism of sealing still remains unclear. Various theories such as temperature-dependent changes in hardness and volume expansion as well as fluid absorption have been discussed. The aim of the present study is to investigate changes in volume, weight and density caused by temperature changes. Three i-gel masks (sizes 3, 4

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... nd 5) were heated in a water bath from 10°C to 50°C in 5°C steps. The changes in weight, volume, and density were measured at each temperature step. In a second study, the identical parameters of masks (n = 5 of sizes 3, 4 and 5) used in patients were investigated. Results of volume and density between benchtop and patient measurements were examined for correlation. All masks, regardless of the size investigated, showed a significant linear temperature-dependent volume expansion (sizes 3, 4 and 5: P < 0.001), a weight increase (sizes 3: P = 0.018; 4: P = 0.027; and 5: P = 0.006) and a linear decrease in density (sizes 3, 4 and 5: P < 0.001). There was a very strong correlation between the results of benchtop and patient measurements (volume: P < 0.001; density: P < 0.001). We observed significant changes in all three properties investigated. Thus, a temperature-dependent volume expansion and weight increasement was observed, which can be considered as a further explanation for improved sealing after insertion.