{"id":142584,"key":"TissueDB/Materials/Hydrogels","title":"TissueDB/Materials/Hydrogels","latest":{"id":1261317,"timestamp":"2026-08-18T01:02:27Z"},"content_model":"wikitext","license":{"url":"https://www.appropedia.org/Appropedia:Copyrights","title":"CC-BY-SA-4.0"},"source":"{{TissueDB Material Layout\n| hero_image = Gelatine.png\n| hero_image_size = 350px\n| hero_image_caption = Gelatin powder mixed with water for tissue phantom preparation.\n| lead_paragraph = '''Hydrogels''' are the TissueDB class for cast, water-based gels: a gelling polymer dissolved or swollen in water and then set into a solid that holds its own shape. They are used as stand-ins for soft, wet tissue — they cut, they tear, they hold a needle track, and they can be left transparent or coloured opaque. Where a builder wants a material bought in a finished shape, this is the wrong class; a hydrogel is mixed and cast on site.\n\n'''Gelatin''' is the most common form. It is a protein obtained by hydrolysis of collagen from animal skin, bone and connective tissue, usually bovine or porcine, and is sold as dry powder, as sheets, or as granules from grocery shops and specialty suppliers. It dissolves in warm water and sets into a gel on cooling, and the firmness of the finished gel is set by how much powder goes into how much water. One published build uses 80 g of unflavoured gelatine in 800 mL of water — about 91% water by mass — poured into a mould and refrigerated for about four hours. Gelatine's limits have been measured against [[TissueDB/Materials/Agar|agar]]: it \"requires refrigeration, both to set the model when poured and to minimise spoilage\", and in side-by-side testing gelatine models \"were much more fragile and compressible than tissue and were prone to rapid degeneration from needle sticks and heat exposure\", with a 10% model more durable and denser than a 5% one. Gelatine is the cheaper of the two — about US$2.50 for enough for one 5% model bought through Amazon.com, or about US$1.50 through online suppliers in Cape Town, against about US$5.00 and US$3.60 for agar.<ref name=\"earle2016\">Earle M, Portu GD, DeVos E. Agar ultrasound phantoms for low-cost training without refrigeration. ''Afr J Emerg Med'' 2016;6(1):18-23. DOI: [https://doi.org/10.1016/j.afjem.2015.09.003 10.1016/j.afjem.2015.09.003]. PMID: [https://pubmed.ncbi.nlm.nih.gov/30456059/ 30456059]. PMC: [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6233231/ PMC6233231].</ref>\n\n'''Gelatin-glycerol gel''' is the same protein base with glycerol added, mixed and cast in a mould. The published recipe is 15 g gelatin, 40 g glycerol and 0.5 g powdered red pigment made up to 100 mL with cold water — so the liquid the gel is swollen with is roughly 45 g water alongside 40 g glycerol, not water alone. It is left to stand 20 minutes, then stirred over low heat for about ten minutes without boiling. The glycerol makes the set gel slower to dry out.<ref name=\"pda-module\">SELF-Training team / Global Surgical Training Challenge. ''Phase 2: Simulator Setup'', [https://www.appropedia.org/Cardiac_Surgical_Skills_Training_Module Cardiac Surgical Skills Training Module], Appropedia, 2021. Not indexed in PubMed; primary source linked directly.</ref> The same gelatin-and-glycerine family is mixed to four different ratios in one simulator to give four different tissues, listed in the table below.\n\n'''Polyvinyl alcohol cryogel''' and '''polyvinyl alcohol hydrogel tubing''' are the synthetic forms of this class. Polyvinyl alcohol is a water-soluble synthetic polymer; a cryogel is made by freezing and thawing a dilute solution of it repeatedly, so the network forms by freeze–thaw rather than by cooling or by a chemical setting agent. Published phantoms are 5–6% polymer by weight — that is 94–95% water — with a measured stiffness of 1.6 to 16.1 kPa and an acoustic velocity of 1540 to 1570 m/s, close to liver.<ref name=\"cournane2010\">Cournane S, Cannon L, Browne JE, Fagan AJ. Assessment of the accuracy of an ultrasound elastography liver scanning system using a PVA-cryogel phantom with optimal acoustic and mechanical properties. ''Phys Med Biol'' 2010;55(19):5965-83. DOI: [https://doi.org/10.1088/0031-9155/55/19/022 10.1088/0031-9155/55/19/022]. PMID: [https://pubmed.ncbi.nlm.nih.gov/20858913/ 20858913].</ref> Stiffness rises sharply with the number of freeze–thaw cycles: about 20 kPa of Young's modulus after one cycle, up to 600 kPa after ten.<ref name=\"fromageau2007\">Fromageau J, Gennisson JL, Schmitt C, Maurice RL, Mongrain R, Cloutier G. Estimation of polyvinyl alcohol cryogel mechanical properties with four ultrasound elastography methods and comparison with gold standard testings. ''IEEE Trans Ultrason Ferroelectr Freq Control'' 2007;54(3):498-509. PMID: [https://pubmed.ncbi.nlm.nih.gov/17375819/ 17375819].</ref> No simulator in this database is built with either polyvinyl alcohol gel form, so neither appears in the table below.\n\n'''Terms used in the table.''' A ''phantom'' is a block of material imaged or punctured in place of a patient. ''Radio-opaque'' means the material shows up under X-ray or fluoroscopy, which plain gelatin does not — a contrast agent has to be added. ''Denier'' is the weight in grams of 9,000 metres of a fibre, so a 15-denier stocking is a very fine one. A ''substrate'' is a fibre such as wool or stocking mesh mixed into or laid inside the gel to give it something to tear against.\n| overview = A hydrogel is bought as dry ingredients and made up on site. Gelatin powder, sheets and granules are sold in grocery shops and by specialty suppliers; polyvinyl alcohol is sold as cryogel or hydrogel stock by laboratory suppliers. Costs are low and are recorded per build rather than per class: enough gelatine for one 5% ultrasound model costs about US$2.50 through Amazon.com or about US$1.50 through online suppliers in Cape Town,<ref name=\"earle2016\" /> and the two cast gelatin models in the Rwandan soft-tissue set cost US$0.40 and US$0.34 each.<ref name=\"ughe2025\">Wittenberg RE, Bryce-Alberti M, Shimelash NZ, Masimbi O, Kratky LE, Thompson AA, et al. Low-cost simulation models for soft-tissue procedures for medical student education in Rwanda. ''BMC Medical Education'' 2025;25(1):1271. DOI: [https://doi.org/10.1186/s12909-025-07910-4 10.1186/s12909-025-07910-4]. PMID [https://pubmed.ncbi.nlm.nih.gov/41034833/ 41034833]. PMC [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC12487576/ PMC12487576]. Licensed CC BY-NC-ND 4.0.</ref> The specialised ingredients — ballistic-grade gelatin, iohexol contrast, psyllium fibre, powdered pigment — are named per build, not stocked class-wide. Dry powder keeps for a long time if it is kept dry; a cast gelatin gel keeps for days and must be refrigerated, which the Rwandan authors record as a real constraint where mains power is unreliable.<ref name=\"ughe2025\" /> A failed or damaged cast can be remelted and poured again. Setting times run from about one hour for a thin gelatin-glycerine layer to overnight for a full cast block.\n| tissues_rows = <tr>\n<td>[[TissueDB/Tissues/Adipose Tissue|Adipose Tissue]] (subcutaneous)</td>\n<td></td>\n<td></td>\n<td>[[TissueDB/Simulators/Z-Plasty Simulator|Z-Plasty Simulator]], [[TissueDB/Simulators/Soft Tissue Lesion Simulator|UGHE Low-Cost Soft-Tissue Simulation Module]]</td>\n<td>In the '''Z-Plasty Simulator''' the ratio is 2 parts gelatin to 6 glycerine to 12 water, with yellow food colouring and teased-out wool; it sets in about an hour, and several sheets are layered for thickness and glued together with the connective-tissue paste. In the '''UGHE module''' an approximately 2 cm layer of pigmented gelatin is poured over the lesion and left to set overnight, between a sponge base and a skin made of felt covered with clear packing tape. The source gives no gelatin concentration for this build — only the layer thickness, the pigmentation and the cost, at US$0.40 for the abscess model and US$0.34 for the nodule model in Rwanda.<ref name=\"ughe2025\" /></td>\n</tr>\n<tr>\n<td>[[TissueDB/Tissues/Esophagus|Esophagus]]</td>\n<td></td>\n<td></td>\n<td>[[TissueDB/Simulators/Neonatal ETT Ultrasound Simulator (Qaim Ali)|Neonatal ETT Ultrasound Simulator (Qaim Ali)]]</td>\n<td>Gelatin blended with psyllium fibre and cored with a hollow lumen, to make a neck phantom on which a trainee identifies by ultrasound whether a newborn's breathing tube has gone into the trachea or the oesophagus.<ref name=\"merali2019\">Merali HS, Tessaro MO, Ali KQ, Morris SK, Soofi SB, Ariff S (2019). \"A novel training simulator for portable ultrasound identification of incorrect newborn endotracheal tube placement – observational diagnostic accuracy study protocol.\" ''BMC Pediatrics'' 19:434. DOI: [https://doi.org/10.1186/s12887-019-1717-y 10.1186/s12887-019-1717-y]. PMID: [https://pubmed.ncbi.nlm.nih.gov/31722685/ 31722685]. PMC: [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6852924/ PMC6852924].</ref></td>\n</tr>\n<tr>\n<td>[[TissueDB/Tissues/Kidney|Kidney]]</td>\n<td></td>\n<td></td>\n<td>[[TissueDB/Simulators/Percutaneous Renal Access Simulator (Ewald)|Ewald Percutaneous Renal Access Trainer]]</td>\n<td>10% ballistic gelatin, 30% iohexol contrast medium (Omnipaque 300) and 60% water, by weight. The contrast is what makes the set gel radio-opaque, so the trainee can target the needle under fluoroscopy; plain gelatin would be invisible.<ref name=\"ewald2019\">Ewald JM, Cheng JW, Engelhart SM, Wilkinson MC, Hajiha M, Wagner H, Baldwin DD. A realistic, durable, and low-cost training model for percutaneous renal access using ballistic gelatin. ''Turk J Urol'' 2019;45(1):31-6. DOI: [https://doi.org/10.5152/tud.2018.43569 10.5152/tud.2018.43569]. PMID: [https://pubmed.ncbi.nlm.nih.gov/30668307/ 30668307].</ref></td>\n</tr>\n<tr>\n<td>[[TissueDB/Tissues/Blood Vessel|Blood Vessel]]</td>\n<td></td>\n<td></td>\n<td>[[TissueDB/Simulators/Resuscitative Endovascular Balloon Occlusion of the Aorta Simulator (Keller)|Pulsatile REBOA Simulator (Keller)]], [[TissueDB/Simulators/Patent Ductus Arteriosus Ligation Simulator|Patent Ductus Arteriosus Ligation Simulator]]</td>\n<td>In the '''Keller REBOA simulator''' the femoral access site is a gelatin mould that carries a pulse a trainee can feel and a probe can see, so the whole percutaneous sequence can be practised on it — needle puncture, conversion over a wire, sheath upsize and balloon inflation. The source states it as \"an ultrasound compatible gelatin mold capable of transducing discernible pulsations enables REBOA deployment starting with percutaneous vascular access and ending with balloon inflation\", and the mould takes about four deployments before it must be replaced.<ref name=\"keller2016\">Keller BA, Salcedo ES, Williams TK, Neff LP, Carden AJ, Li Y, Gotlib O, Tran NK, Galante JM. Design of a cost-effective, hemodynamically adjustable model for resuscitative endovascular balloon occlusion of the aorta (REBOA) simulation. ''J Trauma Acute Care Surg.'' 2016 Sep;81(3):606-611. DOI: [https://doi.org/10.1097/TA.0000000000001153 10.1097/TA.0000000000001153]. PMID: [https://pubmed.ncbi.nlm.nih.gov/27270855/ 27270855].</ref> In the '''Patent Ductus Arteriosus Ligation Simulator''' the gelatin-glycerol gel described above is cast in a mould holding plastic inserts, which leave behind the lumen of the ductus arteriosus and of the aorta for dissection and double-ligation practice.<ref name=\"pda-module\" /></td>\n</tr>\n<tr>\n<td>[[TissueDB/Tissues/Peritoneum and Serosa|Pleura (parietal)]]</td>\n<td></td>\n<td></td>\n<td>[[TissueDB/Simulators/Patent Ductus Arteriosus Ligation Simulator|Patent Ductus Arteriosus Ligation Simulator]]</td>\n<td>Cast from the same gelatin-glycerol mixture as the vessel in the same mould; the trainee dissects the parietal pleura off the aorta to expose the ductus.<ref name=\"pda-module\" /></td>\n</tr>\n| troubleshooting =\n* '''Refrigeration is the constraint, not the recipe.''' A cast gelatin gel has to be refrigerated both to set it and to slow spoilage.<ref name=\"earle2016\" /> The Rwandan authors record this as a real limit on deploying models where mains power is unreliable, and the two cast models in their set have to be made the day before.<ref name=\"ughe2025\" />\n* '''Gelatine is fragile under repeated needling.''' Measured against agar, gelatine models \"were much more fragile and compressible than tissue and were prone to rapid degeneration from needle sticks and heat exposure\"; a 10% gel is more durable and denser than a 5% one, and agar models of similar density withstood more punctures for venepuncture practice.<ref name=\"earle2016\" />\n* '''A damaged cast is remeltable.''' A block whose needle tracks have accumulated can be melted down and poured again rather than thrown away.\n* '''Plain gelatin is invisible to X-ray.''' A build that needs fluoroscopic targeting has to carry a contrast agent in the mix; the Ewald renal trainer uses 30% iohexol by weight for this.<ref name=\"ewald2019\" />\n* '''Match the mix to the layer, not to the class.''' In the Z-Plasty simulator four different tissues come out of one gelatin-and-glycerine family purely by changing the ratio and the fibre substrate. Copying one ratio across every layer gives four layers that behave alike.\n| alternatives_rows = <tr>\n<td>[[TissueDB/Materials/Agar|Agar]]</td>\n<td>Where there is no refrigeration, and where more needle passes are needed</td>\n<td>Sets and keeps at room temperature; a 5% agar model took about 20 needle passes before failing, against about 3 for gelatine. It costs more — about US$5.00 or US$3.60 per model against US$2.50 or US$1.50 — and it is brittle rather than compressible.<ref name=\"earle2016\" /></td>\n</tr>\n<tr>\n<td>[[TissueDB/Materials/Silicone, Castable|Castable silicone]]</td>\n<td>Where the part must last, or must recoil elastically</td>\n<td>No cold chain and no spoilage, and it survives handling that destroys a gel. It is far more expensive, it is not remeltable, and it will not pass for wet tissue under ultrasound without loading.</td>\n</tr>\n| related_materials =\n* [[TissueDB/Materials/Gelatin|Gelatin]] — the protein base of every cast form in this class.\n* [[TissueDB/Materials/Gelatin-Glycerol Gel|Gelatin-Glycerol Gel]] — the gelatin base with glycerol added, cast in a mould.\n* [[TissueDB/Materials/Glycerine|Glycerine]] — an ingredient of the gelatin-glycerol recipe and of the Z-Plasty mixes. It is added to a gel; it is not itself a gel and is not a member of this class.\n* [[TissueDB/Materials/Polyvinyl Alcohol|Polyvinyl Alcohol]] — the polymer behind the cryogel and the hydrogel tubing, and separately a dissolvable 3D-printing filament, which is not a gel.\n* [[TissueDB/Materials/Psyllium|Psyllium]] — fibre blended into gelatin for the neck ultrasound phantom.\n* [[TissueDB/Materials/Castable Materials|Castable Materials]] — the wider group of materials a builder forms on site rather than buying in a finished shape.\n| synonyms_text = '''Member forms in this class:''' Gelatin, Gelatin-Glycerol Gel, PVA Cryogel, PVA Hydrogel Tubing.\n\n'''Common names:''' Hydrogel, hydrogels, gelatin, gelatine, unflavoured gelatin, Knox gelatin, food-grade gelatin, pharmaceutical gelatin, ballistic gelatin, gelatin-glycerol gel, gelatin glycerol mixture, gelatin-water-glycerol gel, GWG gel, polyvinyl alcohol cryogel, poly(vinyl alcohol) cryogel, PVA-C, polyvinyl alcohol hydrogel tubing, PVA hydrogel tubing, PVA-H.\n\n'''Forms:''' Gelatin powder, gelatin sheets, gelatin granules, gelatin blocks, gelatin mix, set gelatin, cast gelatin-glycerol gel, freeze–thaw cryogel, cast hydrogel tubing.\n\n'''Medical:''' Pharmaceutical-grade gelatin, absorbable gelatin sponge (Gelfoam).\n\n'''Regional terms:''' Gélatine (French), Gelatina (Italian, Spanish, Portuguese), Gelatine (German, Dutch).\n| shelf_life_rows = <tr>\n<td>Dry powder or granules at room temperature; cast gel refrigerated</td>\n<td>Keep the powder dry, below about 60% humidity</td>\n<td>Remeltable — a damaged cast can be poured again</td>\n<td>Powder: long-term if kept dry. Cast gel: days, refrigerated</td>\n<td>Mould growth and bacterial contamination if a cast gel is left unrefrigerated</td>\n</tr>\n| references = <references />\n}}\n\n{{Page data\n| keywords = hydrogel, hydrogels, gelatin, gelatine, gelatin-glycerol gel, GWG gel, PVA cryogel, PVA hydrogel tubing, polyvinyl alcohol cryogel, ballistic gelatin, tissue phantom, ultrasound phantom, cast gel, castable material, surgical simulation, TissueDB\n| description = Hydrogels — the TissueDB class for cast, water-based gels used as soft wet-tissue stand-ins in surgical simulation. Covers gelatin, cast gelatin-glycerol gel and the polyvinyl alcohol gel forms, with the published recipes and the simulators that use them.\n| hide = yes\n}}"}