You are presented with multiple paper abstracts, alongside the associated literature review and chapter titles. Given the provided information, you are going to do a literature review. Please generate a coherent literature review that synthesizes the content, draws connections between different works, highlights major findings, and identifies any gaps in the existing research.

Literature Review Title: Body-Centric Wireless Communications: A Survey of Hearing-Instrument Antennas 
Chapter Title:  III. CREEPING WAVES 
1. Abstract of Cited Paper (BIB001):  A compact 2.45 GHz slot-loop antenna is implemented for the use in the outer shell of an in-the-ear (ITE) hearing instrument (HI). The antenna is optimized for high ear-to-ear path gain (|S 21 |). The antenna simulation results are presented for two identical antennas, one placed in the center of each ear. A simulated path gain of −75.0 dB is obtained. 
2. Abstract of Cited Paper (BIB002):  A novel concept for an electrically small on-body antenna targeted for 2.4 GHz ISM band custom in-the-ear (ITE) hearing instrument (HI) applications is introduced. The antenna is based upon a cavity-backed design in order to take advantage of the maximum volume available ITE while providing isolation from the user’s body, and it occupies only $40\%$ of the volume of the sphere with radius $a=12 \,\text{mm}$ . The antenna is implemented on a realistic three-dimensional (3-D)-printed lossy substrate and exhibits high efficiency of 70% and 22%, and a 6-dB impedance bandwidth of 108 and 149 MHz, when the antenna is measured in free space (FS) and ITE, respectively. A measurement campaign conducted in FS and on a specific anthropomorphic mannequin (SAM) head with ears shows that the radiation pattern is optimal for HI applications. Furthermore, the antenna is primarily polarized normal to the surface of the head to ensure the best on-body path gain (PG). This is substantiated by the study of the ear-to-ear (E2E) PG, which is measured and compared to analytic and numerical results.

Literature Review:
To improve hearing radio signals are transmitted from the antenna in the hearing instrument on one ear to the antenna in the hearing instrument on the other ear. Transmission of the signal through the head is not an option due to the very high losses at the ISM-band at 2.4 GHz. This is overcome by the BIB001 ,  , a meandered slot antenna (e) BIB002 , a custom shell antenna (f) BIB003 , and an electric coupled antenna  . use of creeping waves that propagates on the surface of the head. To do this the challenge is to construct antennas that effectively launches creeping waves, e.g., the electric field is perpendicular to the surface. Optimal antenna are developed for each type of hearing instrument. Typical measured propagation losses are in the range 50-80 dB depending on the type of hearing instrument. Simulations of the creeping waves at the surface of the head on a specific anthropomorphic mannequin (SAM) head of the electric field vectors and Poynting vector are shown in Fig. 2 .