Device Design

Initial Cardboard Models and Sketches

The process of finalizing our final prototype began with initial designs created in the form of sketches. Several of these sketches were taken to the cardboard prototyping stage to gather feedback within our team and from our mentors. Our final design concept is illustrated in the sketch to the right.

Initial Sketches and Cardboard Prototypes. Top Left. An initial design concept utilizing four rack and pinion mechanisms connected to a larger gear train. Bottom left. A cardboard model of the initial sketch. Right. A sketch of the final design concept.

Final CAD Design

The final device design CAD assembly was created in SolidWorks, as shown to the right. Every component of the device was created using 3D printing techniques except for the timing belt and rubber bands. The final prototype accomplishes three major functions: 1) submerges tissue samples in a formalin solution to undergo a fixation process, 2) stretches each side of the samples to physiological conditions, and 3) allows stretch to occur simultaneously on all four sides of the sample, or in other words, biaxially stretches the samples. Each component of the device will be described in further detail below.

Current CAD assembly of Planar Biaxial Device. Isometric View Isometric View of planar biaxial device showing loading arms, rack and pinion, driving gears, solution bath, locking mechanisms, and timing belt.

Bath for the Device, CAD Part. The bath will be used to hold the formalin solution. The front bottom-right corner of the bath’s interior includes a small pour spout to assist in removing the formalin.

Component 1:

Solution Bath

The bath is located at the center of the device. Here, a 10% formalin solution is poured into the bath and the tissue samples will be suspended in the center. Each tissue sample will stay here for 48 hours to become fixed in their stretched configuration.

Component 2:

Load Arm

To suspend the samples in the formalin solution, a total of four loading arms will be placed in the bath with one at each edge respectively. The middle section of the arm allows only the front portion to be submerged in the bath. The two vertical posts will attach to each side of the sample using sutures and hooks, keeping the samples in position. The back portion will be attached to other components of the device through contact with the ridged side of the arm shown above.

Loading Arm for the Device, CAD Part. The loading arm consists of posts for the sutures to wrap around, an arched body that reaches over the sides of the bath, and a T beam that fits the base of the device.

Component 3:

Rack and Pinion

The rack and pinion mechanism is composed of the loading arm and the gears, acting as the rack and pinions respectively. The user will turn the gears, causing the loading arm to move backwards in increments of 0.33mm. This fulfills the device's need to stretch the tissue at increments matching physiological conditions, or stretching 10% of each sample's initial dimensions.

Rack and Pinion Mechanism, Close-Up from CAD Assembly. The rack and pinion mechanism is composed of two gears and the loading arm, where the section of the arm in contact with the gear acts as the rack.

Component 4:

Ratchet and Pawl

The ratchet and pawl mechanism is one of two locking mechanisms that was incorporated into the final prototype. The mechanism is composed of two major components: a ratchet wheel and a pawl. The ratchet wheel is a gear with the teeth oriented at an angle rather than facing out radially. The tip of the pawl comes into contact with the spaces in between each tooth. Due to the design of the wheel's teeth, the mechanism is only able to rotate counter clockwise, which corresponds with the loading arms moving away from the center of the device. Any movements associated with clockwise movement will be resisted by the pawl, thus keeping the samples from relaxing during the fixation process. A rubber band is included to keep the two pieces in contact.

Close-Up of Ratchet and Pawl Mechanism (Purple). (A) Isometric view. (B) Top View. The ratchet wheel is transparent to show how it is connected to the rack and pinion. The ratchet wheel and the small driving gear are attached and rotate together.

Timing Belt Mechanism, Close-Up from Device Assembly. The timing belt mechanism is composed of four gears and a timing belt that wraps around them. Pictured is a close-up of one part of the mechanism.

Component 5:

Timing Belt

To stretch all four sides of each sample simultaneously, a timing belt mechanism was incorporated into the device. The mechanism is composed of four gears and a timing belt. Each timing belt gear is stacked on top of the large rack and pinion gear. Once the user turns the rack and pinion, the timing belt gears will also turn as a result of this connection. The timing belt is wrapped around all four timing belt gears, and transmits the movement of one gear to the other three. Because of this, the device is able to stretch all four sides at once, mimicking the biaxial stretch that the myocardium experiences in the body during the cardiac cycle.

Component 6:

Locking Pin

The locking pin acts as a secondary locking mechanism if the primary ratchet and pawl locking mechanism fails over repeated use. A triangular space is included in the base of the device, where the user is meant to place the pin once the sample is stretched to the necessary amount. The lower half of the pin includes an extended gear design that interfaces with the rack and pinion mechanism and resists undesired movement when put in place. The height of the pin and the circular shape at the top of the pin help the user place and remove the pin when necessary.

Locking Pin, Close-Up from CAD Assembly. (A) shows the device and a triangular space where the locking pin would go, circled in magenta. (B) shows an isometric view of the device with the locking pin placed in between the loading arm and the adjacent gear.

Component 7:

Base

The base of the device includes all of the necessary spaces, shafts, and brackets needed to keep each component and mechanism in place. The space in the middle includes a space to place the bath. All four sides include rails and L-shaped brackets meant to keep the loading arms aligned and in place. Finally, the shafts correspond with the various movement transmission and locking mechanisms.

Base of the Device, CAD part. The base of the device includes features (posts, shafts, and rails etc.) that interface with all components of the device.

Created By: Arianna Badilla, Delaney K. Donnelly

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