Controlled-porosity alginate hydrogels are synthesized via a novel organic solvent-free method, "reactive emulsion templating", which utilizes dense-phase carbon dioxide as a reagent and as a templating agent to produce well-defined hydrogels with a narrow macropore-size distribution and high degrees of porosity and interconnectivity (see Figure). These hydrogels may find use in the biomedical field as drug delivery devices, wound dressings, or as supports for tissue engineering applications.