In pursuit of research to create a synthetic tissue scaffold by a micropunching process, material properties of polycaprolactone (PCL) in liquid nitrogen were determined experimentally and used for finite element modeling of cryogenic micropunching process. Specimens were prepared using injection molding and tested under compression to determine the stress–strain relationship of PCL below its glass transition temperature. Cryogenic conditions were maintained by keeping the PCL specimens submerged in liquid nitrogen throughout the loading cycle. Specimens of two different aspect ratios were used for testing. Yield strength, strength coefficient, and strain hardening exponent were determined for different specimen aspect ratios and extrapolated for the case with zero diameter to length ratio. Material properties were also determined at room temperature and compared against results available in the literature. Results demonstrate that PCL behaves in a brittle manner at cryogenic temperatures with more than ten times increase in Young's modulus from its value at room temperature. The results were used to predict punching forces for the design of microscale hole punching dies and for validation of a microscale hole punching model that was created with a commercially available finite element software package, deform 3D. The three parameters, yield strength, strength coefficient, and strain hardening exponent, used in Ludwik's equation to model flow stress of PCL in deform 3D were determined to be 94.8 MPa, 210 MPa, and 0.54, respectively. The predicted peak punching force from finite element simulations matched with experimentally determined punching force results.

Articular cartilage degeneration is a central pathological feature of osteoarthritis. Cartilage in the adult does not regenerate in vivo and, as a result, cartilage damage in osteoarthritis is irreversible. With our ever-aging population, osteoarthritis has become a leading cause of disability and unfortunately, no optimal treatments for osteoarthritis are currently available. To address this problem, a research community is focused on the development of both natural and synthetic biodegradable tissue scaffolds. The scaffolds must contain depressions or holes for the purpose of chondrocyte seeding and growth in order to create an implantable construct. In addition to chondrocytes, cartilage tissue consists of the extracellular matrix (ECM). Studies of many tissue types have established that ECM plays an important role in regulating cell behavior and controlling processes such as tissue differentiation and tumor progression. Unlike most natural tissues, adult cartilage ECM is exceptionally dense and lacking in vascularity, which makes it difficult for chondrocytes to be transplanted directly into the matrix. Current methods of creating cell home sites through chemical decellularization of the ECM degrade the mechanical integrity of the cartilage tissue. The research conducted here used a mechanical, rather than chemical, method to create cell home sites. A novel micropunching machine was developed to fabricate 200 μm diameter holes in cartilage, thereby creating a porous natural scaffold while maintaining a healthy ECM. Equine articular cartilage slices were harvested from the cadaver’s back knee joint and cryo-sectioned into 100 μm thick slices. Using die clearances of 3.7%, 6.8%, and 8.9%, the results indicate that micro-scale holes can be mechanically punched in cartilage tissue. The maximum punching force showed a slight trend of decreasing as die clearance increased, but there was no statistical significance. Punching force, as well as hole size, was highly dependent on sample hydration. Upon inspection, the resulting hole sizes were approximately 50 μm to 150 um, indicating 25% to 75% shrinkage in reference to the male punch diameter. Finally, the resulting hole shape was observed to be slightly non-circular and the edges of the hole exhibited a raggedness, which was indicative of the cartilage tearing during hole punching.

Microscale holes were punched at cryogenic conditions in Polycaprolactone (PCL) membranes to create synthetic 3D tissue scaffolds through multilayer stacking of 2D porous membranes. Punching forces were experimentally measured, and finite element modeling of the punching process was validated by comparing punching force results. Holes of nominal diameter of 200 µm were punched in PCL films of two different thicknesses: 40 µm and 70 µm. Die clearances used for holes in 40 µm thick films were 15.0 %, and 30.0 %, and 45.0 %. Die clearances used for holes in 70 µm films were 8.6 %, 17.1 %, and 25.7 %. All holes were punched while the PCL film was in thermal equilibrium with a bath of boiling liquid nitrogen. Punching forces were analyzed to study the effect of die clearance and film thickness. A 3D finite element simulation of the punching process was done using DEFORM 3D software. Cryogenic material properties of PCL used in the simulation were determined experimentally. It was concluded that finite element simulation for the cryogenic micropunching process can be used to predict peak punching forces with reasonable accuracy which is a key factor to be considered while designing the punching dies. The finite element simulations did not predict an optimal die clearance to minimize peak punching force. However, the measured peak punching forces for 70 µm thick film seem to favor the smallest die clearance to minimize peak punching force.

In pursuit of research to create a synthetic tissue scaffold by a micropunching process, material properties of Polycaprolactone (PCL) in liquid nitrogen were determined experimentally. Specimens were prepared using injection molding and tested under compression to determine the stress-strain relationship of PCL below its glass transition temperature. Cryogenic conditions were maintained by keeping the PCL specimens submerged in liquid nitrogen throughout the loading cycle. Specimens of two different aspect ratios were used for testing. Yield Strength, Strength Coefficient, and Strain Hardening Exponent were determined for different specimen aspect ratios and extrapolated for the case with zero diameter to length ratio. Material properties were also determined at room temperature and compared against results available in the literature. Results demonstrate that PCL behaves in a brittle manner at cryogenic temperatures with more than ten times increase in Young’s modulus from its value at room temperature. The results were used to predict punching forces for the design of microscale hole punching dies and for validation of a microscale hole punching model that was created with a commercially available finite element software package, DEFORM 3D. The three parameters Yield Strength, Strength Coefficient, and Strain Hardening Exponent used in Ludwik’s equation to model flow stress of PCL in DEFORM 3D were determined to be 94.8 MPa, 210 MPa, and 0.54, respectively.