In recent years, biomedical wearable devices have achieved much progress in miniaturization, flexibility, and sensitivity. However, current power delivery systems remain too bulky, primarily due to inefficient packaging and integration schemes [1][2]. This bulkiness is partly because commercially available lithium-ion batteries, the industry standard for rigid portable devices, suffer electrochemical degradation under long-term mechanical deformation, such as bending, folding, twisting, and other strain modes. The space allocated for power storage and management is also ever decreasing to make more room for higher functionality. More importantly, the form/shape factor (coin, cylindrical, prismatic, and pouch shapes) of the commercial batteries severely impact the wearable device safety, reliability, flexibility, and miniaturization. Since onventional batteries are not intrinsically flexible, in the past few decades, researchers have developed various types of flexible batteries, including pouch cell batteries [3], rubber-like batteries [4], [5], [6], 1D fibrous batteries [7], [8], [9], etc.Pouch cell batteries are a relatively mature technology in the battery market. While they exhibit some degree of flexibility, they are limited to a bending radius of ~20 mm. In addition, pouch cell batteries typically contain flammable organic electrolyte, presenting serious safety concerns for wearable devices. Beyond flammability concerns, other issues, including swelling, leakage, and lack of biocompatibility, limit the application of pouch cells in wearable devices [10]. Other formats such as rubber-like batteries, utilize crosslinked rubber-based polymers to fabricate cathode, anode, and electrolyte, and offer a route for flexible batteries. While this approach provides intrinsic flexibility and stretchability, the ionic conductivity of the electrolyte remains low, resulting in low output power and high internal resistance [3], [11].

To overcome these issues in current designs of flexible batteries, we adopt two new designs 1. Battlet design 2.Interdigitated battlet design to fabricate flexible Li-ion batteries and evaluate if these designs help us meet the current flexible battery requirements in terms of mechanical and electrochemical performance. In addition, an ionic liquid electrolyte is used to address the safety concerns of the Li-ion battery for wearables. Ionic liquids are room temperature molten salts that exhibit wide electrochemical stability windows (4.5V - 5V), excellent thermal stability, and nonflammability owing to their low vapor pressure [14], [15], [16]. Thus, ionic liquid electrolytes present several advantages over traditional organic electrolytes for powering wearable devices.