Electrostatic interactions within a semi-interpenetrating network (semi-IPN) gel can control the postsynthesis loading, long-term retention, and subsequent release of small-molecule cationic antibiotics. Here, electrostatic charge is introduced into an otherwise neutral gel [poly(ethylene glycol) (PEG)] by physically entrapping high-molecular-weight poly(acrylic acid) (PAA). The network structure is characterized by small-angle neutron scattering. PEG/PAA semi-IPN gels absorb over 40 times more antibiotic than PAA-free PEG gels. Subsequent soaking in physiological buffer (pH 7.4; 0.15 M NaCl) releases the loaded antibiotics for periods as long as 30 days. The loaded gels elute antibiotics with diffusivities of 4.46 x 10(-8) cm(2)/s (amikacin) and 2.08 x 10(-8) cm(2)/s (colistin), which are two orders of magnitude less than those in pure PEG gels where diffusion is controlled purely by gel tortuosity. The release and hindered diffusion can be understood based on the partial shielding of the charged groups within the loaded gel, and they have a significant effect on the antimicrobial properties of these gels. (C) 2015 Wiley Periodicals, Inc.