Development of a mechanical cell stimulation system
Stevenson, Mathew Paul
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An electro-mechanical device was developed to provide mechanical stimulation to cell populations for the purpose of studying how mechanical signals affect cell activity. The system can dynamically deform cubes of hydrogel seeded with cells by applying combinations of normal and shear forces to the faces of the hydrogel cube using plastic pads attached to the cube. The compact device was fabricated using rapid prototyping methods with ABS plastic and uses shape memory alloy actuator wires to generate the necessary forces. The actuator wires can be independently activated in sequence to create stimulation routines involving compression, tension and shear forces. All of the components can be sterilized and are corrosion resistant so they are not affected by the high humidity environment of a tissue incubator where cell stimulation studies are performed. The system fits inside a standard plastic lab container measuring 7 cm tall by 4 cm in diameter to maintain sterile conditions and hold the liquid culture medium required by the cells. During operation the hydrogel cube and the contact pads are submerged in the culture medium. The shape memory alloy actuators have been modeled in a two step process: 1) the electrothermal model, relating input electrical current to the wire temperature due to resistive heating and 2) the thermomechanical model relating the wire temperature to the wire strain and actuator stroke due to the shape memory effect. Testing was completed to validate the models and calibrate the shape memory alloy actuators. There was good agreement between the model predictions and the experimental results. For experiments with a hydrogel cube with sides measuring 1 cm, the system was capable of compressing the hydrogel cube up to 8 %, and generating shear strains of up to 7%. Tensile strains were much smaller at 0.9%. The dynamic deformations were applied at a frequency of 0.5 Hz.