Applications involving freeze-thaw, such as cryoplasty or cryopreservation can significantly alter artery biomechanics including an increase in physiological elastic modulus. Since artery biomechanics plays a significant role in hemodynamics, it is important to understand the mechanisms underlying these changes to be able to help control the biomechanical outcome post-treatments. Understanding of these mechanisms requires investigation of the freeze-thaw effect on arterial components (collagen, smooth muscle cells or SMCs), as well as the components' contribution to the overall artery biomechanics. To do this, isolated fresh swine arteries were subjected to thermal (freeze-thaw to -20 °C for 2 min or hyperthermia to 43 °C for 2 h) and osmotic (0.1-0.2 M mannitol) treatments; these treatments preferentially altered either the collagen matrix (hydration/stability) or smooth muscle cells (SMCs), respectively. Tissue dehydration, thermal stability and SMC functional changes were assessed from bulk weight measurements, analyses of the thermal denaturation profiles using Fourier transform infrared (FTIR) spectroscopy and in vitro arterial contraction/relaxation responses to norepinephrine (NE) and acetylcholine (AC), respectively. Additionally, Second Harmonic Generation (SHG) microscopy was performed on fresh and frozen-thawed arteries to directly visualize the changes in collagen matrix following freeze-thaw. Finally, the overall artery biomechanics was studied by assessing responses to uniaxial tensile testing. Freeze-thaw of arteries caused: (a) tissue dehydration (15% weight reduction), (b) increase in thermal stability (∼6.4 °C increase in denaturation onset temperature), (c) altered matrix arrangement observed using SHG and d) complete SMC destruction. While hyperthermia treatment also caused complete SMC destruction, no tissue dehydration was observed. On the other hand, while 0.2 M mannitol treatment significantly increased the thermal stability (∼4.8 °C increase in denaturation onset), 0.1 M mannitol treatment did not result in any significant change. Both 0.1 and 0.2 M treatments caused no change in SMC function. Finally, freeze-thaw (506 ± 159 kPa), hyperthermia (268 ± 132 kPa) and 0.2 M mannitol (304 ± 125 kPa) treatments all caused significant increase in the physiological elastic modulus (Eartery) compared to control (185 ± 92 kPa) with the freeze-thaw resulting in the highest modulus. These studies suggest that changes in collagen matrix arrangement due to dehydration as well as SMC destruction occurring during freeze-thaw are important mechanisms of freeze-thaw induced biomechanical changes.
Bibliographical noteFunding Information:
The authors would like to thank Dr. Schmechel for the histological analysis and Jerry Sedgewick for SHG microscopy. The authors would also like to thank the Experimental surgical services (ESS) and the Visible Heart laboratory at the University of Minnesota for access to fresh pig arteries. The authors would also like to thank Dr. Alptekin Aksan for the numerous discussions during the course of the study as well as Sue Clemmings for proofreading the manuscript. Funding from Boston Scientific, Institute for Engineering in Medicine at the University of Minnesota, and the Doctoral Dissertation Fellowship from the University of Minnesota Graduate School is gratefully acknowledged.
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- Thermal denaturation