Wireless communication enables an ingestible device to send sensor information and support external on-demand operation while in the gastrointestinal (GI) tract. However, it is challenging to maintain stable wireless communication with an ingestible device that travels inside the dynamic GI environment as this environment easily detunes the antenna and decreases the antenna gain. In this paper, we propose an air-gap based antenna solution to stabilize the antenna gain inside this dynamic environment. By surrounding a chip antenna with 1 2 mms of air, the antenna is isolated from the environment, recovering its antenna gain and the received signal strength by 12 dB or more according to our in vitro and in vivo evaluation in swine. The air gap makes margin for the high path loss, enabling stable wireless communication at 2.4 GHz that allows users to easily access their ingestible devices by using mobile devices with Bluetooth Low Energy (BLE). On the other hand, the data sent or received over the wireless medium is vulnerable to being eavesdropped on by nearby devices other than authorized users. Therefore, we also propose a lightweight security protocol. The proposed protocol is implemented in low energy without compromising the security level thanks to the base protocol of symmetric challenge-response and Speck, the cipher that is optimized for software implementation.