Med-Tech Innovation Materials


3D networks of cross-linked, polymeric chains, capable of absorbing multiple times its own weight in water. As a component of smart bandages, the porosity of the material permits air circulation at the wound site, accelerating the healing process. Hydrogels possess a hydro-molecular composition very similar to that of human tissue, and they are flexible enough to be moulded into almost any form. In tissue scaffold applications they have been found to promote healing and the growth of cell populations. The material can be engineered to degrade gradually, providing artificial support for the regeneration of tissue when inserted before slowly decomposing over time. Their unique properties have attracted the interest of


researchers in the Convergent Technologies Research Group (CTRG) at WIT, where experimental studies with hydrogels include: • Investigating the suitability of hydrogels to model venous tissue with accurate viscoelasticity and mechanical properties using a freeze/thaw processing technique


• The development of a series of formulations to achieve comparable, mechanical properties of blood vessels, suitable for rapid prototyping


• Optimisation of ultraviolet curing methods for soft contact lens manufacture.


Mock circulatory systems In the last article1


by CTRG, a mock circulatory system


for pre-clinical evaluation of implantable medical devices was described. The need for such a system is driven by the advancement of endovascular procedures over traditional therapeutic interventions in the treatment of cardiovascular disease. Skill and precision is demanded in performing these procedures, and a realistic test bed for testing implantable devices or for training is a necessity. Biomodelling offers a tactile evaluation and training environment to existing alternatives such as virtual reality simulation or animal testing.


Hydrogels for vascular modeling With the mock circulatory loop explored, it remains to be explained how artificial veins and arteries can be synthesised in a laboratory setting to enhance the operability of cardiovascular simulators. The choice of material for this application will be influenced by a number of factors. The material ideally needs to have similar characteristics to human blood vessels in a number of key areas: mechanical properties, viscoelasticity and surface friction resistance. For the purpose of visual


Table I: Concentration of samples PVOH


Sample


1 2 3 4


www.med-techinnovation.com


Mass (g)


0.8 0.8 0.8 0.8


Molecular weight (kMW)


100


146–186 100


146–186


Mass (g)


0.4 0.4 0.4 0.4


feedback, a material with good transparency is preferable. Flexibility is also desirable so that the physical form and tortuosity of vessels can be accurately reconstructed. Real blood vessels exhibit viscoelastic responses. Periodic events such as respiration and heartbeats subject the vessel walls to stresses and changes, necessitating the use of dynamic viscoelasticity measurements to reproduce the conditions of pulsatile flow.


Existing mock cardiovascular systems often use silicon or glass for modelling blood vessels. Although glass offers good transparency, tactile or physical similarities are absent. Silicon is a more realistic approximation, but remains sub-optimum in terms of surface friction resistance and mechanical properties. Hydrogels offer an alternative to silicon and glass in blood vessel biomodelling. Hydrogel compositions exhibit good transparency for accurate visual feedback, and have lower surface friction resistance than silicon and superior mechanical characteristics. In a recent study,2


researchers at CTRG aimed to


produce laboratory-engineered hydrogels with similar properties to human blood vessels, replicating storage modulus, viscoelasticity, mechanical properties and physical shape.


The study Hydrogel compositions were cast in a core to create structures with similar physical dimensions and tortuosity to sections of human blood vessels, with the aim of approximating the physical and mechanical properties of vascular tissue. The research sought to manipulate the viscoelastic


properties of hydrogels to closely mimic those of human blood vessels. Published viscoelasticity measurements for human blood vessels were used as a reference point for comparison. Prior research has demonstrated the stability of hydrogels within the range of physiological temperature in biomodelling applications.3


Preparation of hydrogels Samples were fabricated using poly(vinyl alcohol) (PVOH) and poly(acrylic acid) (PAA). These polymers were chosen for their ability to produce hydrogels with properties closely matching those of human blood vessels; this selection was informed by prior study.4 Researchers used a freeze/thaw processing technique


to promote the development of crystalline regions in the gels. This, in turn, promotes physical crosslinking, rendering a material with superior mechanical strength to


PAA


Molecular weight (kMW)


3,000 3,000 3,000 3,000


DMSO


Volume (mL)


0 0 8 8


Distilled H2O Volume (mL)


40 40 32 32


September/October 2013 ¦ 13


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