Progress Report 10, 3/31/2010

Currently, a working cardiovascular model intended for testing devices and techniques is not available commercially. It is the intention of this design effort to develop a modular heart which may be used to test newly designed devices, including catheters, stents, and replacement valves. Further, the model will be made available to research institutions and medical device companies for testing in early stages of research. The foundation of the model has been built but it is our intention to rebuild and upgrade the functionality of the model so that it will be in a testable state.

Work Completed:

  • Met with Dr. Barnett to discuss goals of project and means of improving current heart model
  • Met with the machine shop coordinator (John Fellenstein) to discuss potential construction on the model
  • Constructed Website
  • Established approximate budget
  • Brainstormed potential cardiac functions to simulate with our model
  • add a pump (cyclical pressure cycle)
  • use acrylic tubes connected by chloroform, bent with heat
  • 2 circuits (closed) modelling arterial and venous systems
  • 1 interchangeable system, output may be relocated from arterial position to venous position
  • add model of coronary system
  • hinging heart for testing valves with pressurized flow through valve
  • make heart more viewable
  • Conducted research on current cardiovascular models and uses for our model
  • Current models were found to be significantly more complex than our intended model. Current models primarily measure flow dynamics associated with valve replacements, stent deliveries and catheter use
  • Models researched were found to be very specific to device under review
  • A more unique model used ultrasound of an agar gel, which mimics the ultrasound characteristics of biological tissues, containing an arterial or venous structure to characterize flow dynamics or flow associated with a valve within the model
  • Completed NCIIA grant proposal
  • Contacted Simulation lab and scheduled time to observe cardiovascular model
  • Establishing design of updated model
  • Designed closed loop system
  • Implemented pump and valve
  • Met with Dr. Merryman to discuss design features.
  • Boring of tubes to produce modular water tight model
  • Negative molding fabrication technique to produce a one piece water tight design- possibly using polycarbonate
  • Use of a corrosive chemical solvent to "weld" polymers together at joints to prevent leakage
  • Use of a hydrophobic adhesive at catheter insert location to prevent water loss
  • Met with Dr. Michael Barnett to explicitly discuss specific devices our model is designed to test.
  • The following purposes were discussed
  • Demonstrate catheters designed to use optical scopes in the heart
  • Proof of concept for this scope device
  • Demonstrate Swan-Ganz catheter to measure blood pressure in the heart
  • Sail on end carries it by venous flow to heart- mimicked by water flow and pressure gradient in our model
  • Perform right heart catheterization- measure pressures in the heart
  • Explored Dynamic Med Demo Model created by DynamicMed.com
  • Obtained pictures of model from company, considered their design and construction method especially for the model of the heart itself
  • Discussed with Dr. Barnett his main focuses for our model
  • Heart itself
  • Clarity
  • Portability
  • Spoke with materials representative – at Tygon. Discussed better means of bonding silicone tubing used by last year’s design team.
  • Discovered liquid silicone heated to 250 degrees is needed to create actual joint adhesion and improve the appearance of the joints on the current model. At high temperatures, the silicone is able to achieve polymer cross-linking for water proofing the joints
  • Discovered Tygon silicone tubing is not a thermoplastic and heat will not improve bending
  • Discovered 1in tubing used in previous model has a 5in bend radius
  • Met with Mr. John Fellenstein in the machine shop a second time to discuss final material selections and to place a materials order
  • Discussed casting the heart- casting materials and strategies
  • Discussed using acrylic tubing and strategies for creating a bend in the continuous flow system
  • Explored cutting acrylic tubing at angle and bonding with dichloroethylenefor creating the bend at the model’s inferior end and found it to be a feasible solution
  • Use of McMaster-Carr material supplier to identify and order materials
  • Tubing
  • Heart casing
  • Pump – metering bellows pump
  • Sealing material
  • Plexiglass base
  • Determined acrylic tubing will be tubing material used.
  • Had success in creating a seamless 180º hairpin turn by heating acrylic tubing and bending slowly. Completely clear and scratch resistant.
  • Determined joint adhesion materials (acrylic tubing with dichloroethylene adhesive) for waterproofing joints
  • Tested flexible polyurethane (80A shore hardness) as casting material for heart- using heat resistant molding material for negative of heart
  • Initial casting results: Urethane material determined to be too soft for heart mold. Also procedure pointed out the need for a reliable method to hold the imprinting mold in the urethane as it is hardening.
  • Clay used did not cleanly separate from urethane. It smudged on the mold and this decreased the clarity. Possibly due to inadequate use of lubricating spray to prevent this adhesion or that this spray is inappropriate for accomplishing this need.
  • Point to need for different approach for imprinting the interior geometry of the heart.
  • Purchased Life-size plastic heart model to use in creating negative of heart
  • Optimal for creating anatomically correct heart
  • Test two different negatives as outside of the heart into which we pressed atria and ventricles
  • Atria and ventricle presses were made out of two urethane spheres held steady using an upright stand
  • During the demold time, molds were removed but the structure lacked clarity (molding clay was smudged on the outside)
  • In a second attempt we coated a pyrex bowl with urethane release spray but the model could not be removed from the pryex bowl at the 1 hr. demold time & neither could the ventricle presses
  • Mold material sufficiently hard
  • Mold and presses may not have been removed at the proper time though. They instead may need to be removed at the 15 min mark during gelling.
  • Nonetheless the urethane mold lacks clarity. Possibly due to a misinterpretation of the processing directions as outlined by those given by the supplier
  • Experimented with bending acrylic tubes
  • Two bending methods were performed: in the presence and absence of sand (the sand trial entailed filling the tube with sand to prevent the tubes from kinking)
  • Sand: Very tight bend created (~2.5 in diameter) but sand particles remained melted into interior of tubing after heating. These could not be washed out and therefore decreased clarity of the tubing.
  • W/out sand: Bends created with 4.25 in and 4.6 in diameters. Some kinking created if bent too quickly. Kinking could be avoided by heating a large section of tubing (~ 3 in) and bending the tubing as it cooled. Clarity was maintained and improved relative to using sand.

Figure: From left to right – 2 original bends without sand (Large diameter and significantly kinked), 1 bend with sand (Tight bend w/out kinks but reduced clarity due to sand embedded in plastic), 2 bends without sand (Tighter diameter and not kinked as much)

  • Worked with John Fellenstein to finalize design of Y-connectors in a CAD program
  • O-rings used to maintain water tight seal at each connection site.
  • Both Y connectors designed, 1 with 1in input and ½ in outputs and 1 with ½ in input and ½ in outputs.
  • Ordered Metering Bellows Pump and pump connectors for connecting pump output to acrylic tubing.
  • Determined means of inserting catheter into system. Will glue on a short acrylic tube for entry at 35* angle from system tubing. This will allow a rubber stopper to seal the entry point and prevent leaking- we previously determined that boring a hole into the system tubing itself at an angle could not be sealed with a rubber stopper without any leaking.
  • Also determined catheters will enter that short acrylic tube through rubber stoppers with bored holes to prevent leaking during use. Through this method we can bore holes of various diameters in rubber stoppers to accommodate several different catheters

Current Work:

  • Produce a complex model of the heart in PRO-E. Design based on drill which can hollow out a block into any geometry as long as the geometry is symmetrical.
  • Work with Simulation lab to understand internal geometry of the heart
  • Incorporate Dr. Barnett's desired atrial and ventricular sizes into this design
  • Finalize simple sphere model of the heart in PRO-E with accurate dimensions
  • Create one more bent tube out of the acrylic tubing with a catheter input closer to the heart
  • Determine final dimensions of tubes
  • Femoral vein
  • Inferior vena cava
  • Experiment with catheter insertion points.
  • Work with John Fellenstein to create entry tubes into the femoral vein acrylic tubes
  • Drill different sized holes into various stoppers to reduce the leaking of water out of the model during use

Future Work:

  • Exploring making model modular using 2 joints and O rings
  • Determine how to integrate the pump with the rest of the cardiovascular circuit
  • Determine the correct flow and pressure of liquid throughout the model
  • Complete design of model
  • Design of the heart
  • Working closely with Dr. Barnett to ensure heart is laid out as he wishes
  • Determine final materials for construction of heart
  • Determine correct procedure for flexible polyurethane casting
  • Construct Y connector using O ring to make model disassemble