Evaluation of single versus multiple cryogen spray cooling spurts on in vitro model human skin

Many commercially available dermatologic lasers utilize cryogen spray cooling for epidermal protection. A previous tissue culture study demonstrated that single cryogen spurts (SCS) of 80 ms or less were unlikely to cause cryo-injury in light-skinned individuals. More recently, multiple cryogen spurts (MCS) have been incorporated into commercial devices, but the effects of MCS have not been evaluated. The aim was to study an in vitro tissue culture model and the epidermal and dermal effects of SCS vs patterns of shorter MCS with the same preset total cryogen delivery time (Δt_c) and provide an explanation for noted differences. Four different spurt patterns were evaluated: SCS: one 40-ms cryogen spurt; MCS2: two 20-ms cryogen spurts; MCS4: four 10-ms cryogen spurts; MCS8: eight 5-ms cryogen spurts. Actual Δt_c and total cooling time (Δt_Total) were measured for each spurt pattern. RAFT tissue culture specimens were exposed to cryogen spurt patterns and biopsies were taken immediately and at days 3 and 7. Actual Δt_c was increased while Δt_Total remained relatively constant as the preset Δt_c of 40 ms was delivered as shorter MCS. Progressively more epidermal damage was noted with exposure to the MCS patterns. No dermal injury was noted with either SCS or MCS. For a constant preset Δt_c of 40 ms, delivering cryogen in patterns of shorter MCS increased the actual Δt_c and consequently the observed epidermal cryo-injury as compared to an SCS.