This paper presents two ultra-miniaturized and ultra-low-power Low-Dropout (LDO) regulators, implemented using Folded-Cascode (FC) and Two-Stage (TO) Operational Transconductance Amplifier (OTA) topologies. Both designs deliver a 400 mV output, operating in the weak inversion region to significantly minimize power consumption. A bulk-driven input approach is employed to enhance the input common-mode range (ICMR) for stable operation. The TO-based LDO achieves 23.4 mV/V line sensitivity, 33 μs Transient Response (TR), 21.5 nA quiescent current, and 11.0 μm × 30.8 μm area, demonstrating fast dynamic response and efficient regulation within a small footprint. In contrast, the FC-based LDO reduces the quiescent current to 7.3 nA with 31.8 mV/V line sensitivity, 1.9 ms TR, and 11.9 μm × 131.4 μm area, favoring applications where static power is prioritized over transient speed. These designs demonstrate an optimal balance between area, power, and regulation performance, making them highly suitable for biosensor and implantable device applications.