Amonophasic material can consist of 1 material, but itcan also consist of a mix of 2 or more components. Infor example saline, there is both water and salt, butbecause the salt is uniformly distributed in the water,the composition is the same at all locations, andtherefore, saline should be described as a monophasicmaterial. In fact, all materials where the compositionin different locations cannot be demonstrated to bedifferent should be regarded as monophasic.Materials and MethodsMicroscopyMicroscopy was performed by adding a small amountof gel into a 9-cm petri dish containing 10 mL of waterand 30 mL of a 1% toluidine blue solution. Approximately0.1 mL of gel typically yields a particle concentrationsuitable for imaging. The petri dish was puton a gentle shaker allowing the gel particles time todisperse and absorb water and staining color toequilibrium. The samples were imaged at 10· magnificationusing a MZ-16 A stereo microscope supplyingtransmitted light from a CLS 150 X cold light source(Leica Microsystems GmbH, Wetzlar, Germany).Water UptakeThe water uptake test was performed as described forswelling factor in Edsman and colleagues.17 Approximately0.5 g of product was weighed into a 10-mLmeasuring glass. Approximately 10 mL of saline wasadded and then mixed thoroughly until the gel particleswere fully dispersed. Softer gels required a longerdispersion time than firmer gels. The gel particles wereallowed time to settle and the volume of the gel columnwas read. The swelling factor was calculated as V/V0,where V0 is the initial volume of the gel, and V is thevolume of the fully swollen gel in 0.9%NaCl. All gelswere assumed to have a density of 1 g/cm3. Typicalprecision of the method is 2% relative standarddeviation.Extractable Hyaluronic AcidThe amount of extractable HA was analyzed asdescribed for gel content in Edsman and colleagues.17The gel content was determined by adding an excess ofsaline to a known amount of product and dispersingthe gel thoroughly to form a dilute suspension. Thedilute suspension of the gel was filtered througha 0.22-mm filter, and the concentration of HA in thefiltrate (the extractable part) was determined using thecarbazole method. In Edsman and colleagues17 the gelcontent was calculated as the fraction of HA in thefiller that could not pass through the 0.22-mm filterwhen filtering the diluted suspension of the product. Inthis study, the fraction of HA in the filler that did passthrough the 0.22-mm filter was evaluated, to demonstratethe fraction of extractable HA rather than thefraction of actual gel bound HA. Typical precision ofthe method is 2% relative standard deviation.Results and DiscussionMicroscopyAfter dispersion in water and staining with toluidineblue, the gel particles are easily discernible in productsfrom both product families (Figures 1 and 2). The most obvious difference between the product families is theparticle size distribution resulting from the differenttypes of particle sizing processes used. The Restylane(RES) products each have their own specific particlesize, whereas the Juvederm (JUV) products containa wider array of particle sizes.Nonparticulate Hyaluronic Acid FillersAlthough the particulate nature of all cross-linked HAfillers has been demonstrated and recognized by thosein the field, products are sometimes described as“homogenous gel”18–21 “continuous gel,”22 or “nonparticulate”1,13–15,23 and “smooth”1,24–26 implyingthat there is only 1 single piece of gel in the syringe. It istrue that when an HA filler is being formed, the crosslinkingprocess will create 1 single bulk of material,actually 1 immense molecule of cross-linked HA.However, because of the requirement that the materialis to be filled into a syringe and injected througha needle, the bulk material has to be fragmented intosmaller pieces. This can be performed in differentways, for example by cutting with rotating knives orby pressing the material through a mesh of a chosensize. The resulting material is a bulk ofHAgel particlesthat has flow properties suitable for injection. Becausethe HA gel particles are soft, colorless, and transparent,it is difficult to observe the separate particles.Because of the softness, the particles pack tightly andthe material seems to consist of 1 single body ofmaterial. Using the dispersion and staining technique,however, the particulate nature of any cross-linkedHA filler on the market can be demonstrated.Particle Firmness