Nanowarming using Au-tipped Co₃₅Fe₆₅ ferromagnetic nanowires

Ferromagnetic Co₃₅Fe₆₅, Fe, Co, and Ni nanowires have high saturation magnetizations (M_s) and magnetic anisotropies, making them ideal for magnetic heating in an alternating magnetic field (AMF). Here, Au-tipped nanowires were coated with polyethylene glycol (PEG) and specific absorption rates (SAR) were measured in glycerol. SAR increased when using metals with increasing M_s (Co₃₅Fe₆₅ > Fe > Co > Ni), reaching 1610 ± 20 W g^-1 metal at 1 mg metal per ml glycerol for Co₃₅Fe₆₅ nanowires using 190 kHz and 20 kA m^-1. Aligning these nanowires parallel to the AMF increased SAR up to 2010 W g^-1 Co₃₅Fe₆₅. Next, Co₃₅Fe₆₅ nanowires were used to nanowarm vitrified VS55, a common cryoprotective agent (CPA).Nanowarming rates up to 1000 °C min^-1 (5 mg Co₃₅Fe₆₅ per ml VS55) were achieved, which is 20× faster than the critical warming rate (50 °C min^-1) for VS55 and other common CPAs. Human dermal fibroblast cells exposed to VS55, and Co₃₅Fe₆₅ nanowire concentrations of 0, 1 and 2.5 mg Fe per ml all showed similar cell viability, indicating that the nanowires had minimal cytotoxicity. With the ability to provide rapid and uniform heating, ferromagnetic nanowires have excellent potential for nanowarming cryopreserved tissues.