This paper presents a charge-balanced 4-wire interface on medical platinum wires for biomedical implants. This interface was originally designed to deliver power and full duplex data between implanted units of a retinal prosthesis. Detailed circuits on both sides of the wire interface are depicted. The proposed method ensures the total electrical charge is balanced over time within the implant to avoid the risk of harmful irreversible electrochemical reactions. Experiments show that the data links using this 4-wire interface design has minimal Bit Error Rate (BER) and very low cost in terms of power and area consumptions. The forward data recovery consumes 300 μW at 600 kbps with an area of 15×200 μm2 in 65nm CMOS. The backward data encoding circuit requires an average current of a mere 3 μA at 100 kbps while its area is 15×140 μm2.