ith an increasing number of gel-based dermalfillers on the market, there is an increasinginterest in correlating the different properties, such asrheological and cohesive properties, with the clinicaleffects and uses. The rheological properties ofhyaluronan (HA) fillers have been explored for quitea long time, but more recently the discussions havebeen focused around the cohesive properties of HAfillers.Cohesion is described as the force between particles ofthe same substance that acts to unite them. In otherwords, it is easy to separate the particles in a lowcohesivematerial but more difficult in a material withhigh cohesion. In IUPACs Golden book,1 the definitionof cohesion is:Theworkof cohesion per unit area of a singlepure liquidor solid phase a,wac is the work done on the systemwhen a column a of unit area is split, reversibly, normalto the axis of the column to form 2 new surfaces each ofunit area in contact with the equilibrium gas phase.Currently, there are no ready-made instrumentsdesigned to measure cohesion, or methods whichcould be easily adapted to gels. Several manufacturersof HA gels have investigated their cohesive propertiesusing various different methods but as yet, unlikerheology, there is no standardized methodology that isacknowledged by the scientific community as a validway of evaluating cohesion.In a vast majority of articles where investigators stategels to be cohesive, no explanation is given on how thiswas concluded. It seems that the appearance andgeneral behavior of the gel are often considered sufficientto give an estimate of this property. Because ofthis, it is of interest to include sensory analysis intocomparison of methods for cohesivity measurement.There have been suggestions that the differentphysicochemical properties of a gel relate to specificclinical effects, e.g., the effect of the gel strength, i.e.,the elastic modulus and/or the complex modulus asmeasured by rheology, has been said to relate tolifting capacity and tissue integration. Severalauthors suggest that firm gels have a better ability toresist deformation,2–6 whereas softer gels have beensaid to better integrate into the tissue as they deformmore easily.7The suggested effects from cohesion are more diverse.Falcone and Berg8 state that cohesion is not an advantagefor fillers, whereas others believe that it is importantfor the lifting capacity.6,9 Two studies haveinvestigated the integration into tissue and describe thehigh-cohesive products to have a larger effect.10,11Otherinvestigators speculate that low cohesive fillers spreador migrate in the tissue dependent on injection depth.9To study and compare the possible clinical effects fromthe cohesive properties of a gel, it is necessary tostandardize the definition and measurement of cohesion.The purpose of this study was to develop andevaluate methods for determining cohesion. Two differentfamilies of HA fillers (Emervel and Restylane[Galderma Aesthetics]) were used to evaluate the differentmethods.Materials and MethodsMaterialsSeveral batches ofRestylane (RES),Restylane Perlane inUS Restylane Lyft (PER), Restylane Vital in US RestylaneSilk (RESV), Emervel Touch (EMET), EmervelClassic (EMEC), Emervel Lips (EMEL), Emervel Volume(EMEV), and Emervel Deep (EMED) were used.To replicate earlier published results from the compressionforcemethod Juvederm Ultra 2 (JUVU2) (batchX24L703701), Juvederm Ultra 3 (JUVU3) (batchX30L901494), and Juvederm Ultra 4 (JUVU4) (batchs30L804422) were used.