Cardiovascular disease (CAD) is a major global public health issue, with mortality rates being significantly impacted by cold temperatures. Stable and reliable electrocardiogram (ECG) monitoring in cold environments is crucial for early detection and treatment of CAD. However, existing skin sensor struggle to balance freeze resistance, breathability, flexibility, conductivity and adhesion at cold temperatures. Here, we introduce a solvent cross-linking strategy and an in situ transfer method to prepare ultrathin bioionic gels, featuring a freezing point below -80 °C and a thickness of only 12.6 μm. The strong and abundant interactions between the ionic liquid solvent and the zwitterionic polymer effectively suppress low-temperature crystallization, forming a toughened and highly adhesive network structure. This network enables the in situ formation of an ultrathin morphology, which can be seamlessly transferred onto various substrates. Furthermore, the solvent-cross-linked network maintains a large interpolymer chain spacing, facilitating rapid ion transport pathways. Even at subzero temperatures, the gel maintains its multifunctionality, demonstrating tissue-like softness (34.6 kPa), high ionic conductivity (10.06 mS cm