Thirty detachable HydroCoils were used in the present study. All were tested in an open-fluid thermostatic system consisting of the following parts: a polyurethane immersion chamber, a thermostatic system with a maximal temperature variation range of 0.5 °C (VoltkraftGmbH, Hirschau, Germany), and a digital thermometer with a measurement range of − 199.9 °C to + 850.0 °C (GMH 3750, Greisinger ElectronicGmbH, Regenstauf, Germany) calibrated according to the Swiss certification (Cert. 112,954; Fig. 1).
The immersion chamber was filled with 500 ml of sterile 0.9% NaCl solution with 2000 units of heparin (Liquemin, DrossapharmAG, Basel, CH), simulating the exact conditions of the fluids used in conventional angiographic procedures.
Water temperature was regulated and maintained using an external thermostat (VoltkraftGmbH, Hirschau, Germany), thus ensuring qualitative and reproducible conditions. All measurements were recorded for subsequent analysis using a high-definition camera (Sony-HDV, Sony, Japan) and recorded on a DN-300a 250GB data video unit (Datavideo®, Utrecht, Holland). The timing of configuration changes was measured using a digital chronometer (T3-precision, Suunto™, Vantaa, Finland).
Methodology
The experimental design consisted of two parts: a one-step temperature immersion test and a two-step temperature immersion test.
One-step temperature immersion test
The time measurements were chronometrically registered from the time at which the hydrogel-coated coils were manually immersed into the fluid chamber (at the selected temperature range) until the time at which one of the configurations was achieved and no further changes were observed. In accordance with the study design, we performed the experiments under five different temperature ranges: 22.6 °C (room temperature), 37 °C (body temperature), 40–50 °C, 50–60 °C, and 60–70 °C.
Twenty-five (n = 25) HydroCoils (0.035″ wire diameter, 8-mm loop, 60-mm length) were analyzed with five in each temperature group. Each configuration change was classified as one of three morphologies (Fig. 1):
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Configuration I: Minimal curvature of the hydro-coated coil without reaching a first loop.
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Configuration II: First complete loop without complete curling of the hydro-coated coil.
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Configuration III: Complete/final hydro-coated coil curling.
Time measurements were stopped under one of the following conditions: 1) when the investigators (two observers: RLB and TR) confirmed that a complete and stable configuration of the coil (Configuration III) had been achieved or 2) at 30 min in cases of incomplete coil configuration. Values were measured and registered in minutes, seconds, and milliseconds.
Two-step temperature immersion test
In the second part of the study, five (n = 5) detachable hydrogel-coated coils were analyzed to assess the configuration memory of the material. The detachable hydro-coated coils were first immersed in a 70 °C water bath. Once a completely coiled shape was reached (Configuration III), the coil was retracted into its plastic sheath and pulled out of the immersion chamber. After 10 seconds at room temperature (approximately 26 °C), the hydrogel-coated coil was immersed in a 37 °C bath and released from its sheath. The time period until the HydroCoil reached complete coiling was measured. The purpose of this test was to assess whether a HydroCoil that was completely configured once would be able to instantly regain that configuration after being released from its sheath at physiologic temperatures, simulating a clinical environment (i.e., the preparation of the HydroCoil outside of the patient and subsequent placement in the target vessel).
Statistical analysis
All data were analyzed and evaluated using Stata software version 12.1 (Stata Corp. LP, College Station, TX, USA). The data values are expressed as means ± standard deviations.