The control over the release of molecules from mesoporous materials is a pivotal factor in developing efficient drug delivery devices. Conventional drug delivery systems, which have been described in Chapter 2, involve the release of adsorbed molecules following sustained release mechanisms that can be expressed in terms of diffusion of adsorbed molecules throughout the mesopore channels in the silica matrix. Consequently, release kinetics can be understood in terms of the Fickean diffusion coefficients, which depend on the characteristics of both the guest molecule and the host matrix. However, certain biomedical applications require the development of stimuli-responsive release systems (Shi et al. 2010). For instance, many site-selective delivery systems, such as those for highly toxic antitumor drugs, require zero release before reaching the targeted cell or tissue. In this sense, it is possible to use the available channels of mesoporous materials as drug reservoirs and block the pore entrances with appropriate nanogates. For example, inorganic nanoparticles, polymers, and larger supramolecular assemblies have been used as the blocking caps to control opening/closing of pore entrances of mesoporous silica. Different stimuli, pH, temperature, redox potential, light, magnetic field, ultrasounds, enzymes, antibodies, aptamer targets, or even the combination of more than one stimulus have been used as triggers for uncapping the pores and releasing the guest molecules from mesoporous silica (Figure 3.1) (Ambrogio et al. 2011; Coti et al. 2009; Liong et al. 2008, 2009; Manzano and Vallet-Regí 2010; Saha et al. 2007; Slowing et al. 2008; Trewyn et al. 2007; Vallet-Regí et al. 2011; Vallet-Regí, Balas, and Arcos 2007; Ashley et al. 2011; R. Liu, Zhang, and Feng 2009; Vallet-Regí, Colilla, and Gonzalez 2011; Vallet-Regí and Ruiz-Hernández 2011; Manzano, Colilla, and Vallet-Regí 2009; J. Liu et al. 2009).