TissueDB/Materials/Hydrogels
License: CC-BY-SA-3.0 by Danielle dk
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.
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 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.[1]
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.[2] 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.
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.[3] 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.[4] No simulator in this database is built with either polyvinyl alcohol gel form, so neither appears in the table below.
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.
Tissues
| Tissue | Visual | Tactile | Simulator | Notes |
|---|---|---|---|---|
| Adipose Tissue (subcutaneous) | Z-Plasty Simulator, UGHE Low-Cost Soft-Tissue Simulation Module | 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.[5] | ||
| Esophagus | Neonatal ETT Ultrasound Simulator (Qaim Ali) | 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.[6] | ||
| Kidney | Ewald Percutaneous Renal Access Trainer | 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.[7] | ||
| Blood Vessel | Pulsatile REBOA Simulator (Keller), Patent Ductus Arteriosus Ligation Simulator | 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.[8] 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.[2] | ||
| Pleura (parietal) | Patent Ductus Arteriosus Ligation Simulator | 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.[2] |
Troubleshooting
- Refrigeration is the constraint, not the recipe. A cast gelatin gel has to be refrigerated both to set it and to slow spoilage.[1] 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.[5]
- 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.[1]
- A damaged cast is remeltable. A block whose needle tracks have accumulated can be melted down and poured again rather than thrown away.
- 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.[7]
- 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.
Alternatives
| Alternative | Best For | Trade-offs |
|---|---|---|
| Agar | Where there is no refrigeration, and where more needle passes are needed | 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.[1] |
| Castable silicone | Where the part must last, or must recoil elastically | 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. |
References
- ↑ 1.0 1.1 1.2 1.3 1.4 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: 10.1016/j.afjem.2015.09.003. PMID: 30456059. PMC: PMC6233231.
- ↑ 2.0 2.1 2.2 SELF-Training team / Global Surgical Training Challenge. Phase 2: Simulator Setup, Cardiac Surgical Skills Training Module, Appropedia, 2021. Not indexed in PubMed; primary source linked directly.
- ↑ 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: 10.1088/0031-9155/55/19/022. PMID: 20858913.
- ↑ 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: 17375819.
- ↑ 5.0 5.1 5.2 5.3 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: 10.1186/s12909-025-07910-4. PMID 41034833. PMC PMC12487576. Licensed CC BY-NC-ND 4.0.
- ↑ 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: 10.1186/s12887-019-1717-y. PMID: 31722685. PMC: PMC6852924.
- ↑ 7.0 7.1 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: 10.5152/tud.2018.43569. PMID: 30668307.
- ↑ 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: 10.1097/TA.0000000000001153. PMID: 27270855.