Scaffolds and implants: Networks also have a high surface area-to-volume ratio but are localized to specific sites and generally are more persistent than hydrogels. They are useful as substrates for cell growth and thus are useful for wound and bone healing. For wound healing, networks of chitosan, 365 and sodium alginate with poly(vinyl alcohol) 366 can both facilitate healing and inhibit infection. Bone matrixes require yet more persistence to allow the growth of new bone and greater rigidity because of the stiffness of bone. Antibacterial agents are important because bone infections are likely less accessible to antimicrobial agents and thus are more difficult to treat; preventing them would be more efficient than treating them. One example is a gentamicincontaining porous implant for bone healing; 367 a quaternized chitosan/polyester/hydroxyapatite scaffold was also implanted in rats and rabbits and had antibacterial and bone-healing activities. Implants using cationic polymers 369 or copper nanoparticles were effective at preventing infections, with the copper/polyetheretherketone implant being effective against MRSA. An alternative antibacterial method is the use of nitric oxide-releasing agents in concert with bone matrixes to kill microbes. 371