Cryoprotectant agent toxicity in porcine articular chondrocytes

Cryopreserving the intact intervertebral disc without compromising viability

Background

Tissue cryopreservation requires saturation of the structure with cryoprotectants (CPAs) that are also toxic to cells within a short timeframe unless frozen. The race between CPA delivery and cell death is the main barrier to realizing transplantation banks that can indefinitely preserve tissues and organs. Unrealistic cost and urgency leaves less life-threatening ailments unable to capitalize on traditional organ transplantation systems that immediately match and transport unfrozen organs. For instance, human intervertebral discs (IVD) could be transplanted to treat back pain or used as ex vivo models for studying regenerative therapies, but both face logistical hurdles in organ acquisition and transport. Here we aimed to overcome those challenges by cryopreserving intact IVDs using compressive loading and swelling to accelerate CPA delivery.

Methods

CPAs were tested on bovine nucleus pulposus cells to determine the least cytotoxic solution. Capitalizing on our CPAs Computed Tomography (CT) contrast enhancement, we imaged and quantified saturation time in intact bovine IVDs under different conditions in a bioreactor. Finally, the entire protocol was tested, including 1 week of frozen storage, to confirm tissue viability in multiple IVD regions after thawing.

Results

Results showed cryopreserving medium containing dimethyl sulfoxide and ethylene glycol gave over 7.5 h before cytotoxicity. While non-loaded IVDs required over 3 days to fully saturate, a dynamic loading protocol followed by CPA addition and free-swelling decreased saturation time to <5 h. After cryopreserving IVDs for 1 week with the optimized CPA and permeation method, all IVD regions had 85% cell viability, not significantly different from fresh unfrozen controls.

Conclusions

This study created a novel solution to a roadblock in IVD research and development. Using post-compression swelling CPA can be delivered to an intact IVD over 20× more quickly than previous methods, enabling cryopreservation of the IVD with no detectable loss in cell viability.

Chondrocyte Viability of Particulated Porcine Articular Cartilage Is Maintained in Tissue Storage After Cryoprotectant Exposure, Vitrification, and Tissue Warming

OBJECTIVE

Vitrification of articular cartilage (AC) is a promising technique which may enable long-term tissue banking of AC allografts. We previously developed a 2-step, dual-temperature, multi-cryoprotectant agent (CPA) loading protocol to cryopreserve particulated AC (1 mm3 cubes). Furthermore, we also determined that the inclusion of ascorbic acid (AA) effectively mitigates CPA toxicity in cryopreserved AC. Prior to clinical translation, chondrocytes must remain viable after tissue re-warming and before transplantation. However, the effects of short-term hypothermic storage of particulated AC after vitrification and re-warming are not documented. This study evaluated the chondrocyte viability of post-vitrified particulated AC during a 7-day tissue storage period at 4 °C. We hypothesized that porcine particulated AC could be stored for up to 7 days after successful vitrification without significant loss of cell viability, and these results would be enhanced when cartilage is incubated in storage medium supplemented with clinical grade AA.

DESIGN

Three experimental groups were examined at 5 time points: a fresh control (only incubated in medium), a vitrified - AA group, and a vitrified + AA group (N = 7).

RESULTS

There was a mild decline in cell viability but both treatment groups maintained a viability of greater than 80% viable cells which is acceptable for clinical translation.

CONCLUSION

We determined that particulated AC can be stored for up to 7 days after successful vitrification without a clinically significant decline in chondrocyte viability. This information can be used to guide tissue banks regarding the implementation of AC vitrification to increase cartilage allograft availability.

Optimization of Annealing and Metal Films Radiofrequency Heating Procedures for Vitrified Umbilical Arteries

Vitrification is well known for its application in the cryopreservation of blood vessels, which will address the supply-demand imbalance in vascular grafts for the treatment of cardiovascular disease. Thermal stress damage and devitrification injury in umbilical arteries (UAs) require attention and resolution during the vitrification and rewarming process. In this study, we validated several cooling annealing protocols with temperatures (-130 to -100 °C) and annealing duration durations (10-20 s). Among these, the umbilical artery subjected to annealing at -110 °C for 10 s exhibited the most favorable glass transition and retained 93% of its elastic modulus (0.625 ± 0.030 MPa) compared to the fresh group. Extended annealing temperatures and durations can effectively reduce thermal stress damage, leading to improved mechanical properties by minimizing temperature gradients during cooling. Furthermore, three metal radiofrequency methods were utilized for rewarming, including the use of additional metal films and different magnetic field strengths (20, 25 kA/m). Metal radiofrequency (adding an extra metal film for cryoprotectants rewarming, 20 kA/m) achieved faster and more uniform rewarming, preserving the extracellular matrix (ECM), collagen fibers, and elastic fibers without significant differences compared to the fresh group (P < 0.05). Moreover, its preservation of the biomechanical properties of blood vessels was better than that of water bath heating. Theoretical analysis supports these findings, indicating that radiofrequency heating (RFH) with metal films reduces temperature gradients and thermal stresses during arterial rewarming. RFH contributes to the cryopreservation and clinical application of large-lumen biomaterials, overcoming challenges associated with vascular vitrification and rewarming.

Model-Guided Design and Optimization of CPA Perfusion Protocols for Whole Organ Cryopreservation

Vitrification could enable long-term organ preservation, but only after loading high-concentration, potentially toxic cryoprotective agents (CPAs) by perfusion. In this paper, we combine a two-compartment Krogh cylinder model with a toxicity cost function to theoretically optimize the loading of CPA (VMP) in rat kidneys as a model system. First, based on kidney perfusion experiments, we systematically derived the parameters for a CPA transport loading model, including the following: Vb = 86.0% (ra = 3.86 μm), Lp = 1.5 × 10-14 m3/(N·s), ω = 7.0 × 10-13 mol/(N·s), σ = 0.10. Next, we measured the toxicity cost function model parameters as α = 3.12 and β = 9.39 × 10-6. Combining these models, we developed an improved kidney-loading protocol predicted to achieve vitrification while minimizing toxicity. The optimized protocol resulted in shorter exposure (25 min or 18.5% less) than the gold standard kidney-loading protocol for VMP, which had been developed based on decades of empirical practice. After testing both protocols on rat kidneys, we found comparable physical and biological outcomes. While we did not dramatically reduce toxicity, we did reduce the time. As our approach is now validated, it can be used on other organs lacking defined toxicity data to reduce CPA exposure time and provide a rapid path toward developing CPA perfusion protocols for other organs and CPAs.

The effect of sucrose supplementation on chondrocyte viability in porcine articular cartilage following vitrification

Vitrification can extend the banking life of articular cartilage (AC) and improve osteochondral transplantation success. Current vitrification protocols require optimization to enable them to be implemented in clinical practice. Sucrose as a non-permeating cryoprotective agent (CPA) and clinical grade chondroitin sulfate (CS) and ascorbic acid (AA) as antioxidants were investigated for their ability to improve a current vitrification protocol for AC. The aim of this study was to assess the impact of sucrose and CS/AA supplementation on post-warming chondrocyte viability in vitrified AC. Porcine osteochondral dowels were randomly vitrified and warmed with one established protocol (Protocol 1) and seven modified protocols (Protocols 2-8) followed by chondrocyte viability assessment. Sucrose supplementation in both vitrification and warming media (Protocol 4) resulted in significantly higher (p = 0.018) post-warming chondrocyte viability compared to the protocol without sucrose (Protocol 1). There was no significant difference (p = 0.298) in terms of post-warming chondrocyte viability between sucrose-supplemented DMEM + CS solution (Protocol 4) and Unisol-CV (UCV) + CS (Protocol 6) solution. Clinical grade CS and AA contributed to similar post-warming chondrocyte viability to previous studies using research grade CS and AA, indicating their suitability for clinical use. The addition of an initial step (step 0) to reduce the initial concentration of CPAs to minimize osmotic effects did not enhance chondrocyte viability in the superficial layer of AC. In conclusion, sucrose-supplemented DMEM + clinical grade CS (Protocol 4) could be an ideal protocol to be investigated for future use in clinical applications involving vitrified AC.

Vitrified Particulated Articular Cartilage for Joint Resurfacing: A Swine Model

BACKGROUND

The use of particulated articular cartilage for repairing cartilage defects has been well established, but its use is currently limited by the availability and short shelf life of donor cartilage. Vitrification is an ice-free cryopreservation technology at ultralow temperatures for tissue banking. An optimized vitrification protocol has been developed for particulated articular cartilage; however, the equivalency of the long-term clinical efficacy of vitrified particulated articular cartilage compared with fresh articular cartilage has not yet been determined.

HYPOTHESIS

The repair effect of vitrified particulated cartilage from pigs would be equivalent to or better than that of fresh particulated cartilage stored at 4°C for 21 days.

STUDY DESIGN

Controlled laboratory study.

METHODS

A total of 19 pigs were randomly divided into 3 experimental groups: fresh particulated cartilage group (n = 8), vitrified particulated cartilage group (n = 8), and negative control group (no particulated cartilage in the defect; n = 3). An additional pig was used as the initial cartilage donor for the first set of surgical procedures. Pigs were euthanized after 6 months to obtain femoral condyles, and the contralateral condyle was used as the positive (no defect) control. Samples were evaluated for gross morphology using the Outerbridge and Osteoarthritis Research Society International (OARSI) scoring systems, histology (safranin O, collagen type I/II, DAPI), and chondrocyte viability using live-dead membrane integrity staining.

RESULTS

There were no infections after surgery, and all 19 pigs were followed for the duration of the study. The OARSI grades for the fresh and vitrified particulated cartilage groups were 2.44 ± 1.35 and 2.00 ± 0.80, respectively, while the negative control group was graded significantly higher at 4.83 ± 0.29. Analysis of histological and fluorescent staining demonstrated that the fresh and vitrified particulated cartilage groups had equivalent regeneration within cartilage defects, with similar cell viability and densities and expression of proteoglycans and collagen type I/II.

CONCLUSION

The implantation of fresh or vitrified particulated cartilage resulted in the equivalent repair of focal cartilage defects when evaluated at 6 months after surgery.

CLINICAL RELEVANCE

The vitrification of particulated cartilage is a viable option for long-term storage for cartilage tissue banking and could greatly increase the availability of donor tissue for transplantation.

Multiple cryoprotectant toxicity model for vitrification solution optimization

Vitrification is a promising cryopreservation technique for complex specimens such as tissues and organs. However, it is challenging to identify mixtures of cryoprotectants (CPAs) that prevent ice formation without exerting excessive toxicity. In this work, we developed a multi-CPA toxicity model that predicts the toxicity kinetics of mixtures containing five of the most common CPAs used in the field (glycerol, dimethyl sulfoxide (DMSO), propylene glycol, ethylene glycol, and formamide). The model accounts for specific toxicity, non-specific toxicity, and interactions between CPAs. The proposed model shows reasonable agreement with training data for single and binary CPA solutions, as well as ternary CPA solution validation data. Sloppy model analysis was used to examine the model parameters that were most important for predictions, providing clues about mechanisms of toxicity. This analysis revealed that the model terms for non-specific toxicity were particularly important, especially the non-specific toxicity of propylene glycol, as well as model terms for specific toxicity of formamide and interactions between formamide and glycerol. To demonstrate the potential for model-based design of vitrification methods, we paired the multi-CPA toxicity model with a published vitrification/devitrification model to identify vitrifiable CPA mixtures that are predicted to have minimal toxicity. The resulting optimized vitrification solution composition was a mixture of 7.4 molal glycerol, 1.4 molal DMSO, and 2.4 molal formamide. This demonstrates the potential for mathematical optimization of vitrification solution composition and sets the stage for future studies to optimize the complete vitrification process, including CPA mixture composition and CPA addition and removal methods.

A Cryoprotectant-Gel Composite Designed to Preserve Articular Cartilage during Frozen Osteoarticular Autograft Reconstruction for Malignant Bone Tumors: An Animal-Based Study

OBJECTIVE

We designed a highly adhesive cryoprotectant-gel composite (CGC), based on regular liquid-form cryoprotectant base (CB), aiming to protect cartilage tissue during frozen osteoarticular autograft reconstruction for high-grade sarcoma around the joint. This study aimed to evaluate its effectiveness in rat and porcine distal femur models.

DESIGN

Fresh articular cartilage samples harvested from distal rat and porcine femurs were divided into 4 test groups: untreated control group, liquid nitrogen (LN) freezing group, LN freezing group pretreated with CB (CB group), and LN freezing group pretreated with CGC (CGC group). Microscopic and macroscopic evaluation of cartilage condition, TUNEL (terminal deoxynucleotidyl transferase dUTP nick end labeling) assay, and apoptotic protein analysis of chondrocytes were performed to confirm our results.

RESULTS

In the rat model, CGC could prevent articular cartilage from roughness and preserve more proteoglycans when compared with the LN freezing and CB groups. Western blot analysis showed CGC could prevent cartilage from LN-induced apoptosis supported by caspase-3/8 apoptotic signaling cascade. Macroscopically, we observed CGC could reduce both articular clefting and loss of articular luminance after freezing in the porcine model. In both models, CGC could reduce articular chondrocytes from degeneration. Fewer TUNEL-positive apoptotic and more viable chondrocytes in cartilage tissue were observed in the CGC group in our animal models.

CONCLUSION

Our study proved that CGC could effectively prevent cartilage surface and chondrocytes from cryoinjury after LN freezing. Freezing articular cartilage surrounded with high concentration of CGC can be a better alternative to preserve articular cartilage during limb salvage surgery for malignant bone tumor.

Effectiveness of Clinical-Grade Chondroitin Sulfate and Ascorbic Acid in Mitigating Cryoprotectant Toxicity in Porcine Articular Cartilage

High concentrations of cryoprotective agents (CPAs) are required to achieve successful vitrification of articular cartilage; however, CPA cytotoxicity causes chondrocyte death. To reduce CPA toxicity, supplementation with research-grade additives, in particular chondroitin sulfate (CS) and ascorbic acid (AA), have previously been shown to improve chondrocyte recovery and metabolic function after exposure to CPAs at hypothermic conditions. However, it is necessary to evaluate the pharmaceutical equivalent clinical grade of these additives to facilitate the supplementation of additives into future vitrification protocols, which will be designed for vitrifying human articular cartilage in tissue banks. We sought to investigate the effectiveness of clinical-grade CS, AA, and N-acetylcysteine (NAC) in mitigating toxicity to chondrocytes during CPA exposure and removal, and determine whether a combination of two additives would further improve chondrocyte viability. We hypothesized that clinical-grade additives would exert chondroprotective effects comparable to those of research-grade additives, and that this protective effect would be enhanced if two additives were combined when compared with a single additive. The results indicated that both clinical-grade and research-grade additives significantly improved cell viability (p < 0.10) compared with the negative control (CPA with no additives). CS, AA, and NAC+AA increased cell viability significantly (p < 0.10) compared with the negative control. However, NAC, NAC+CS, and CS+AA did not improve cell viability when compared with the negative control (p > 0.10). We demonstrated that supplementation with clinical-grade CS or AA significantly improved chondrocyte viability in porcine cartilage subjected to high CPA concentrations, whereas supplementation with clinical-grade NAC did not benefit chondrocyte viability. Supplementation with clinical-grade additives in CPA solutions can mitigate CPA toxicity, which will be important in translating previously developed effective protocols for the vitrification of articular cartilage to human tissue banks.

Evaluation of the permeation kinetics of formamide in porcine articular cartilage

Cryopreservation of articular cartilage will increase tissue availability for osteochondral allografting and improve clinical outcomes. However, successful cryopreservation of articular cartilage requires the precise determination of cryoprotectant permeation kinetics to develop effective vitrification protocols. To date, permeation kinetics of the cryoprotectant formamide in articular cartilage have not been sufficiently explored. The objective of this study was to determine the permeation kinetics of formamide into porcine articular cartilage for application in vitrification. The permeation of dimethyl sulfoxide was first measured to validate existing methods from our previously published literature. Osteochondral dowels from dissected porcine femoral condyles were incubated in 6.5 M dimethyl sulfoxide for a designated treatment time (1 s, 1 min, 2 min, 5 min, 10 min, 15 min, 30 min, 60 min, 120 min, 180 min, 24 h) at 22 °C (N = 3). Methods were then repeated with 6.5 M formamide at one of three temperatures: 4 °C, 22 °C, 37 °C (N = 3). Following incubation, cryoprotectant efflux into a wash solution occurred, and osmolality was measured from each equilibrated wash solution. Concentrations of effluxed cryoprotectant were calculated and diffusion coefficients were determined using an analytical solution to Fick's law for axial and radial diffusion in combination with a least squares approach. The activation energy of formamide was determined from the Arrhenius equation. The diffusion coefficient (2.7-3.3 × 10^-10 m²/s depending on temperature) and activation energy (0.9±0.6 kcal/mol) for formamide permeation in porcine articular cartilage were established. The determined permeation kinetics of formamide will facilitate its precise use in future articular cartilage vitrification protocols.

The Natural Cryoprotectant Honey for Fertility Cryopreservation

Honey is a mixture of 25 sugars with other bioactive substances (i.e., organic acids, enzymes, antioxidants, and vitamins) and has been known as a highly nutritious functional food. Traditionally, it has been widely used in medicinal applications to cure various diseases. The effectiveness of honey in different applications has been used for its antimicrobial activity, absorption of hydrops, cleansing, removing odor, assisting granulation, recovery of nutrition, and formation of tissue and epithelium, which proved that honey has dehydrating and preserving properties to make it ideal for the cryopreservation of cells and tissues. Cryopreservation is an advanced preservation technique for tissue, cells, organelles, or other biological specimen storage, performed by cooling the sample at a very low temperature. It is the most common approach to improved preserving fertility (sperm, embryos, and oocytes) in different species that may undergo various life-threatening illnesses and allows for the genetic screening of these cells to test the sample for diseases before use. However, with toxic cryoprotectant (CPA), cryopreservation of fertility has been challenging because of their particular structure and sensitivity to chilling. Honey’s unique composition, as well as its dehydrating and preserving properties, qualify it to be used as a natural cryoprotectant. The aim of this study is to emphasize the ability of honey as a natural cryoprotectant in cryopreservation. The articles for this review were searched from Google Scholar, PubMed, Science Direct, Web of Science, and Scopus, using the keywords, honey, cryopreservation, natural cryoprotectant/CPAs, extenders, and fertility. Honey, as a natural cryoprotectant in fertility cryopreservation, yielded satisfactory results, with respect to improved post-thaw quality and viability. It is now proved as a non-toxic and highly efficient natural cryoprotectant in fertility preservation because its increasing viscosity at low temperature can provide a protective barrier to cells by reducing ice formation. Furthermore, its antioxidant property plays a vital role in protecting the cells from thermal damage by reducing the reactive oxygen species (ROS). This review provides a road map for future studies to investigate the potential of honey in the cryopreservation of other cells and tissue and contribute to the scientific research, regarding this remarkable natural product.

General tissue mass transfer model for cryopreservation applications

Successful cryopreservation of complex specimens, such as tissues and organs, would greatly benefit both the medical and scientific research fields. Vitrification is one of the most promising techniques for complex specimen cryopreservation, but toxicity remains a major challenge because of the high concentration of cryoprotectants (CPAs) needed to vitrify. Our group has approached this problem using mathematical optimization to design less toxic CPA equilibration methods for cells. To extend this approach to tissues, an appropriate mass transfer model is required. Fick's law is commonly used, but this simple modeling framework does not account for the complexity of mass transfer in tissues, such as the effects of fixed charges, tissue size changes, and the interplay between cell membrane transport and transport through the extracellular fluid. Here, we propose a general model for mass transfer in tissues that accounts for all of these phenomena. To create this model, we augmented a previously published acellular model of mass transfer in articular cartilage to account for the effects of cells. We show that the model can accurately predict changes in CPA concentration and tissue size for both articular cartilage and pancreatic islets, tissue types with vastly different properties.

Toxicity of cryoprotective agents to semen from two closely related snake species: The endangered Louisiana pinesnake (Pituophis ruthveni) and bullsnake (Pituophis cantenifer)

Cryopreservation of sperm is an important tool for the conservation of threatened species. Many species of reptile are under considerable threat of extinction and there has been limited investigation of sperm cryopreservation in this taxonomic group. We performed a comparative test of toxicity to sperm of six commonly used cryoprotective agents (CPAs) at three concentrations (5%, 10%, 20%) from the Louisiana pinesnake, Pituophis ruthveni (n = 11), and the closely related bullsnake, Pituophis cantenifer (n = 8). Our objective was to determine the general toxicity of CPAs for cryopreservation in snakes and the cryoprotective ability of CPAs for sperm from the endangered Louisiana pinesnake. We conducted three experiments to: 1) evaluate the short-term in vitro toxicity of common CPAs in two closely related snake species, 2) determine the effectiveness of cryoprotectants for freezing and thawing semen in the Louisiana pinesnake, and 3) test the possible reduction in toxic effects of individual CPAs on semen of the Louisiana pinesnake by combining two of them. We used measures of motility including total motility, forward motility, and forward progressive motility index to characterize toxic effects and cryoprotective ability of each CPA. The results of our three experiments provide several important findings: 1) sperm of the bullsnake and Louisiana pinesnake responded differently to CPAs, 2) few CPAs provided any cryoprotection, as measured by percent recovered motility, in Louisiana pinesnakes, and 3) using mixtures of CPAs did not reduce toxicity relative to the best performing CPA on its own. Motility was best maintained at a concentration of 5% for CPAs tested; however, cryoprotection was best achieved with glycerol at 20% followed by DMA and DMF at 10%. These results provide further insight into the challenges faced by researchers attempting to cryopreserve sperm from snakes. Further comparative studies are required to determine the generality of cryopreservation methods in reptiles and suggest caution should be taken when developing cryopreservation protocols across species, particularly in snakes. All CPAs tested in this study were permeating CPAs and showed a significant acute toxic effect on motility at concentrations that provided cryoprotection. Future work in snakes might consider additional avenues of cryoprotection and combinations of multiple approaches.

A Roadmap to Cardiac Tissue-Engineered Construct Preservation: Insights from Cells, Tissues, and Organs

Worldwide, over 26 million patients suffer from heart failure (HF). One strategy aspiring to prevent or even to reverse HF is based on the transplantation of cardiac tissue-engineered (cTE) constructs. These patient-specific constructs aim to closely resemble the native myocardium and, upon implantation on the diseased tissue, support and restore cardiac function, thereby preventing the development of HF. However, cTE constructs off-the-shelf availability in the clinical arena critically depends on the development of efficient preservation methodologies. Short- and long-term preservation of cTE constructs would enable transportation and direct availability. Herein, currently available methods, from normothermic- to hypothermic- to cryopreservation, for the preservation of cardiomyocytes, whole-heart, and regenerative materials are reviewed. A theoretical foundation and recommendations for future research on developing cTE construct specific preservation methods are provided. Current research suggests that vitrification can be a promising procedure to ensure long-term cryopreservation of cTE constructs, despite the need of high doses of cytotoxic cryoprotective agents. Instead, short-term cTE construct preservation can be achieved at normothermic or hypothermic temperatures by administration of protective additives. With further tuning of these promising methods, it is anticipated that cTE construct therapy can be brought one step closer to the patient.

Vitrification of particulated articular cartilage via calculated protocols

Preserving viable articular cartilage is a promising approach to address the shortage of graft tissue and enable the clinical repair of articular cartilage defects in articulating joints, such as the knee, ankle, and hip. In this study, we developed two 2-step, dual-temperature, multicryoprotectant loading protocols to cryopreserve particulated articular cartilage (cubes ~1 mm3 in size) using a mathematical approach, and we experimentally measured chondrocyte viability, metabolic activity, cell migration, and matrix productivity after implementing the designed loading protocols, vitrification, and warming. We demonstrated that porcine and human articular cartilage cubes can be successfully vitrified and rewarmed, maintaining high cell viability and excellent cellular function. The vitrified particulated articular cartilage was stored for a period of 6 months with no significant deterioration in chondrocyte viability and functionality. Our approach enables high-quality long-term storage of viable articular cartilage that can alleviate the shortage of grafts for use in clinically repairing articular cartilage defects.

Rapid quantification of multi-cryoprotectant toxicity using an automated liquid handling method

Cryopreservation in a vitrified state has vast potential for long-term storage of tissues and organs that may be damaged by ice formation. However, the toxicity imparted by the high concentration of cryoprotectants (CPAs) required to vitrify these specimens remains a hurdle. To address this challenge, we previously developed a mathematical approach to design less toxic CPA equilibration methods based on the minimization of a toxicity cost function. This approach was used to design improved methods for equilibration of bovine pulmonary artery endothelial cells (BPAEC) with glycerol. To fully capitalize on the toxicity cost function approach, it is critical to describe the toxicity kinetics of additional CPAs, including multi-CPA mixtures that are commonly used for vitrification. In this work, we used automated liquid handling to characterize the toxicity kinetics of five of the most common CPAs (glycerol, dimethyl sulfoxide (DMSO), propylene glycol, ethylene glycol, and formamide), along with their binary and ternary mixtures for BPAEC. In doing so, we developed experimental methods that can be used to determine toxicity kinetics more quickly and accurately. Our results highlight some common CPA toxicity trends, including the relatively low toxicity of ethylene glycol and a general increase in toxicity as the CPA concentration increases. Our results also suggest potential new approaches to reduce toxicity, including a surprising toxicity neutralization effect of glycerol on formamide. In the future, this dataset will serve as the basis to expand our CPA toxicity model, enabling application of the toxicity cost function approach to vitrification solutions containing multiple CPAs.

Antioxidant additives reduce reactive oxygen species production in articular cartilage during exposure to cryoprotective agents

High concentrations of cryoprotective agents (CPA) are required during articular cartilage cryopreservation but these CPAs can be toxic to chondrocytes. Reactive oxygen species have been linked to cell death due to oxidative stress. Addition of antioxidants has shown beneficial effects on chondrocyte survival and functions after cryopreservation. The objectives of this study were to investigate (1) oxidative stress experienced by chondrocytes and (2) the effect of antioxidants on cellular reactive oxygen species production during articular cartilage exposure to high concentrations of CPAs. Porcine cartilage dowels were exposed to a multi-CPA solution supplemented with either 0.1 mg/mL chondroitin sulfate or 2000 μM ascorbic acid, at 4 °C for 180 min (N = 7). Reactive oxygen species production was measured with 5 μM dihydroethidium, a fluorescent probe that targets reactive oxygen species. The cell viability was quantified with a dual cell membrane integrity stain containing 6.25 μM Syto 13 + 9 μM propidium iodide using confocal microscopy. Supplementation of CPA solutions with chondroitin sulfate or ascorbic acid resulted in significantly lower dihydroethidium counts (p < 0.01), and a lower decrease in the percentage of viable cells (p < 0.01) compared to the CPA-treated group without additives. These results indicated that reactive oxygen species production is induced when articular cartilage is exposed to high CPA concentrations, and correlated with the amount of dead cells. Both chondroitin sulfate and ascorbic acid treatments significantly reduced reactive oxygen species production and improved chondrocyte viability when articular cartilage was exposed to high concentrations of CPAs.

Review of non-permeating cryoprotectants as supplements for vitrification of mammalian tissues

Vitrification of mammalian tissues is important in the areas of human assisted reproduction, animal reproduction, and regenerative medicine. Non-permeating cryoprotectants (CPAs), particularly sucrose, are increasingly used in conjunction with permeating CPAs for vitrification of mammalian tissues. Combining non-permeating and permeating CPAs was found to further improve post-thaw viability and functionalities of vitrified mammalian tissues, showing the potential applications of such tissues in various clinical and veterinary settings. With the rising demand for the use of non-permeating CPAs in vitrification of mammalian tissues, there is a strong need for a timely and comprehensive review on the supplemental effects of non-permeating CPAs toward vitrification outcomes of mammalian tissues. In this review, we first discuss the roles of non-permeating CPAs including sugars and high molecular weight polymers in vitrification. We then summarize the supplemental effects of non-permeating CPAs on viability and functionalities of mammalian embryos, and ovarian, testicular, articular cartilage, tracheal, and kidney tissues following vitrification. Lastly, challenges associated with the use of non-permeating CPAs in vitrification of mammalian tissues are briefly discussed.

Using engineering models to shorten cryoprotectant loading time for the vitrification of articular cartilage

Osteochondral allograft transplantation can treat full thickness cartilage and bone lesions in the knee and other joints, but the lack of widespread articular cartilage banking limits the quantity of cartilage available for size and contour matching. To address the limited availability of cartilage, vitrification can be used to store harvested joint tissues indefinitely. Our group's reported vitrification protocol [Biomaterials 33 (2012) 6061-6068] takes 9.5 h to load cryoprotectants into intact articular cartilage on bone and achieves high cell viability, but further optimization is needed to shorten this protocol for clinical use. Herein, we use engineering models to calculate the spatial and temporal distributions of cryoprotectant concentration, solution vitrifiability, and freezing point for each step of the 9.5-h protocol. We then incorporate the following major design choices for developing a new shorter protocol: (i) all cryoprotectant loading solution concentrations are reduced, (ii) glycerol is removed as a cryoprotectant, and (iii) an equilibration step is introduced to flatten the final cryoprotectant concentration profiles. We also use a new criterion-the spatially and temporally resolved prediction of solution vitrifiability-to assess whether a protocol will be successful instead of requiring that each cryoprotectant individually reaches a certain concentration. A total cryoprotectant loading time of 7 h is targeted, and our new 7-h protocol is predicted to achieve a level of vitrifiability comparable to the proven 9.5-h protocol throughout the cartilage thickness.

Evaluation of five additives to mitigate toxicity of cryoprotective agents on porcine chondrocytes

Cryoprotective agents (CPAs) are used in cryopreservation protocols to achieve vitrification. However, the high CPA concentrations required to vitrify a tissue such as articular cartilage are a major drawback due to their cellular toxicity. Oxidation is one factor related to CPA toxicity to cells and tissues. Addition of antioxidants has proven to be beneficial to cell survival and cellular functions after cryopreservation. Investigation of additives for mitigating cellular CPA toxicity will aid in developing successful cryopreservation protocols. The current work shows that antioxidant additives can reduce the toxic effect of CPAs on porcine chondrocytes. Our findings showed that chondroitin sulphate, glucosamine, 2,3,5,6-tetramethylpyrazine and ascorbic acid improved chondrocyte cell survival after exposure to high concentrations of CPAs according to a live-dead cell viability assay. In addition, similar results were seen when additives were added during CPA removal and articular cartilage sample incubation post CPA exposure. Furthermore, we found that incubation of articular cartilage in the presence of additives for 2 days improved chondrocyte recovery compared with those incubated for 4 days. The current results indicated that the inclusion of antioxidant additives during exposure to high concentrations of CPAs is beneficial to chondrocyte survival and recovery in porcine articular cartilage and provided knowledge to improve vitrification protocols for tissue banking of articular cartilage.

The effect of additive compounds on glycerol-induced damage to human chondrocytes

High concentrations of cryoprotective agents are required for cryopreservation techniques such as vitrification. Glycerol is a common cryoprotective agent used in cryopreservation protocols but this agent is toxic at high concentrations. This work is an attempt to mitigate the toxic effects of high concentrations of glycerol on intact chondrocytes in human knee articular cartilage from total knee arthroplasty patients by simultaneous exposure to glycerol and a variety of additive compounds. The resulting cell viability in the cartilage samples as measured by membrane integrity staining showed that, in at least one concentration or in combination, all of the tested additive compounds (tetramethylpyrazine, ascorbic acid, chondroitin sulphate, glucosamine sulphate) were able to reduce the deleterious effects of glycerol exposure when examination of membrane integrity took place on a delayed time frame. The use of additive compounds to reduce cryoprotectant toxicity in articular cartilage may help improve cell recovery after cryopreservation.

Cryoprotectant kinetic analysis of a human articular cartilage vitrification protocol

We recently published a protocol to vitrify human articular cartilage and a method of cryoprotectant removal in preparation for transplantation. The current study's goal was to perform a cryoprotectant kinetic analysis and theoretically shorten the procedure used to vitrify human articular cartilage. First, the loading of the cryoprotectants was modeled using Fick's law of diffusion, and this information was used to predict the kinetics of cryoprotectant efflux after the cartilage sample had been warmed. We hypothesized that diffusion coefficients obtained from the permeation of individual cryoprotectants into porcine articular cartilage could be used to provide a reasonable prediction of the cryoprotectant loading and of the combined cryoprotectant efflux from vitrified human articular cartilage. We tested this hypothesis with experimental efflux measurements. Osteochondral dowels from three patients were vitrified, and after warming, the articular cartilage was immersed in 3 mL X-VIVO at 4 °C in two consecutive solutions, each for 24 h, with the solution osmolality recorded at various times. Measured equilibrium values agreed with theoretical values within a maximum of 15% for all three samples. The results showed that diffusion coefficients for individual cryoprotectants determined from experiments with 2-mm thick porcine cartilage can be used to approximate the rate of efflux of the combined cryoprotectants from vitrified human articular cartilage of similar thickness. Finally, Fick's law of diffusion was used in a computational optimization to shorten the protocol with the constraint of maintaining the theoretical minimum cryoprotectant concentration needed to achieve vitrification. The learning provided by this study will enable future improvements in tissue vitrification.

Cryoprotectant Toxicity: Facts, Issues, and Questions

High levels of penetrating cryoprotectants (CPAs) can eliminate ice formation during cryopreservation of cells, tissues, and organs to cryogenic temperatures. But CPAs become increasingly toxic as concentration increases. Many strategies have been attempted to overcome the problem of eliminating ice while minimizing toxicity, such as attempting to optimize cooling and warming rates, or attempting to optimize time of adding individual CPAs during cooling. Because strategies currently used are not adequate, CPA toxicity remains the greatest obstacle to cryopreservation. CPA toxicity stands in the way of cryogenic cryopreservation of human organs, a procedure that has the potential to save many lives. This review attempts to describe what is known about CPA toxicity, theories of CPA toxicity, and strategies to reduce CPA toxicity. Critical analysis and suggestions are also included.

Mesenchymal stromal cells derived from various tissues: Biological, clinical and cryopreservation aspects

Originally isolated from bone marrow, mesenchymal stromal cells (MSCs) have since been obtained from various fetal and post-natal tissues and are the focus of an increasing number of clinical trials. Because of their tremendous potential for cellular therapy, regenerative medicine and tissue engineering, it is desirable to cryopreserve and bank MSCs to increase their access and availability. A remarkable amount of research and resources have been expended towards optimizing the protocols, freezing media composition, cooling devices and storage containers, as well as developing good manufacturing practices in order to ensure that MSCs retain their therapeutic characteristics following cryopreservation and that they are safe for clinical use. Here, we first present an overview of the identification of MSCs, their tissue sources and the properties that render them suitable as a cellular therapeutic. Next, we discuss the responses of cells during freezing and focus on the traditional and novel approaches used to cryopreserve MSCs. We conclude that viable MSCs from diverse tissues can be recovered after cryopreservation using a variety of freezing protocols, cryoprotectants, storage periods and temperatures. However, alterations in certain functions of MSCs following cryopreservation warrant future investigations on the recovery of cells post-thaw followed by expansion of functional cells in order to achieve their full therapeutic potential.

Numerical simulation of the effect of superparamagnetic nanoparticles on microwave rewarming of cryopreserved tissues

In this study, the microwave rewarming process of cryopreserved samples with embedded superparamagnetic (SPM) nanoparticles was numerically simulated. The Finite Element Method (FEM) was used to calculate the coupling of the electromagnetic field and the temperature field in a microwave rewarming system composed of a cylindrical resonant cavity, an antenna source, and a frozen sample phantom with temperature-dependent properties. The heat generated by the sample and the nanoparticles inside the electromagnetic field of the microwave cavity was calculated. The dielectric properties of the biological tissues were approximated using the Debye model, which is applicable at different temperatures. The numerical results showed that, during the rewarming process of the sample phantom without nanoparticles, the rewarming rate was 29.45°C/min and the maximum temperature gradient in the sample was 3.58°C/mm. If nanoparticles were embedded in the sample, and the cavity power was unchanged, the rewarming rate was 47.76°C/min and the maximum temperature gradient in the sample was 1.64°C/mm. In the presence of SPM nanoparticles, the rewarming rate and the maximum temperature gradient were able to reach 20.73°C/min and 0.68°C/mm at the end of the rewarming under the optimized cavity power setting, respectively. The ability to change these temperature behaviors may prevent devitrification and would greatly diminish thermal stress during the rewarming process. The results indicate that the rewarming rate and the uniformity of temperature distribution are increased by nanoparticles. This could be because nanoparticles generated heat in the sample homogeneously and the time-dependent parameters of the sample improved after nanoparticles were homogeneously embedded within it. We were thus able to estimate the positive effect of SPM nanoparticles on microwave rewarming of cryopreserved samples.

Influence of cryopreservation, cultivation time and patient's age on gene expression in Hyalograft® C cartilage transplants

PURPOSE

Our aim was to evaluate the impact of cryopreservation, cultivation time and patient's age on the expression of specific chondrogenic markers in Hyalograft® C transplants.

METHODS

Gene expression of chondrocyte markers [collagen type I (COL1A1), COL2A1, aggrecan, versican, melanoma inhibitory activity (MIA) and interleukin (IL)-1β] was analysed in cartilage biopsies (n = 17) and Hyalograft® C transplant samples (non-cryopreserved = 78, cryopreserved = 13) by quantitative real-time polymerase chain reaction (PCR). Correlation analyses were performed to evaluate the influence of the above-named parameters on the level of gene expression.

RESULTS

Cryopreservation of cells was found to decrease COL2A1 and MIA significantly (4.6-fold, p < 0.01 and 2-fold, p < 0.045, respectively). The duration of cryopreservation had no further influence on the expression of these factors. No correlation was detected between cultivation time (75 ± 31 days) and the expression level of any gene. Cartilage transplants from older patients (>35 years) exhibited a significantly higher IL-1β expression (3.7-fold, p < 0.039) than transplants from younger patients (≤ 35 years).

CONCLUSIONS

Our data demonstrate that cryopreservation has a profound impact on chondrocyte metabolic activity by decreasing the expression of COL2A1 and MIA in Hyalograft® C transplants, independent of the duration of cryopreservation.

Cryopreservation of articular cartilage

Cryopreservation has numerous practical applications in medicine, biotechnology, agriculture, forestry, aquaculture and biodiversity conservation, with huge potentials for biological cell and tissue banking. A specific tissue of interest for cryopreservation is the articular cartilage of the human knee joint for two major reasons: (1) clinically, there exists an untapped potential for cryopreserved cartilage to be used in surgical repair/reconstruction/replacement of injured joints because of the limited availability of fresh donor tissue and, (2) scientifically, successful cryopreservation of cartilage, an avascular tissue with only one cell type, is considered a stepping stone for transition from biobanking cell suspensions and small tissue slices to larger and more complicated tissues. For more than 50years, a great deal of effort has been directed toward understanding and overcoming the challenges of cartilage preservation. In this article, we focus mainly on studies that led to the finding that vitrification is an appropriate approach toward successful preservation of cartilage. This is followed by a review of the studies on the main challenges of vitrification, i.e. toxicity and diffusion, and the novel approaches to overcome these challenges such as liquidus tracking, diffusion modeling, and cryoprotective agent cocktails, which have resulted in the recent advancements in the field.

Vitrification of intact human articular cartilage

Articular cartilage injuries do not heal and large defects result in osteoarthritis with major personal and socioeconomic costs. Osteochondral transplantation is an effective treatment for large joint defects but its use is limited by the inability to store cartilage for long periods of time. Cryopreservation/vitrification is one method to enable banking of this tissue but decades of research have been unable to successfully preserve the tissue while maintaining cartilage on its bone base - a requirement for transplantation. To address this limitation, human knee articular cartilage from total knee arthroplasty patients and deceased donors was exposed to specified concentrations of 4 different cryoprotective agents for mathematically determined periods of time at lowering temperatures. After complete exposure, the cartilage was immersed in liquid nitrogen for up to 3 months. Cell viability was 75.4 ± 12.1% determined by membrane integrity stains and confirmed with a mitochondrial assay and pellet culture documented production of sulfated glycosaminoglycans and collagen II similar to controls. This report documents successful vitrification of intact human articular cartilage on its bone base making it possible to bank this tissue indefinitely.

Preclinical evaluation of ice-free cryopreserved arteries: structural integrity and hemocompatibility

OBJECTIVE

Arterial allografts are routinely employed for reconstruction of infected prosthetic grafts. Usually, banked cryopreserved arteries are used; however, existing conventional freezing cryopreservation techniques applied to arteries are expensive. In contrast, a new ice-free cryopreservation technique results in processing, storage and shipping methods that are technically simpler and potentially less costly. The objective of this study was to determine whether or not ice-free cryopreservation causes tissue changes that might preclude clinical use.

METHODS

Conventionally frozen cryopreserved porcine arteries were compared with ice-free cryopreserved arteries and untreated fresh controls using morphological (light, scanning electron and laser scanning microscopy), viability (alamarBlue assay) and hemocompatibility methods (blood cell adhesion, thrombin/antithrombin-III-complex, polymorphonuclear neutrophil-elastase, β-thromboglobulin and terminal complement complex SC5b-9).

RESULTS

No statistically significant structural or hemocompatibility differences between ice-free cryopreserved and frozen tissues were detectable. There were no quantitative differences observed for either autofluorescence (elastin) or second harmonic generation (collagen) measured by laser scanning microscopy. Cell viability in ice-free cryopreserved arteries was significantly reduced compared to fresh and frozen tissues (p < 0.05).

CONCLUSIONS

The formation of ice in aortic artery preservation did not make a difference in histology, structure or thrombogenicity, but significantly increased viability compared with a preservation method that precludes ice formation. Reduced cell viability should not reduce in vivo performance. Therefore, ice-free cryopreservation is a potentially safe and cost-effective technique for the cryopreservation of blood vessel allografts.

Mathematical modeling of cryoprotectant addition and removal for the cryopreservation of engineered or natural tissues

Long-term storage of natural tissues or tissue-engineered constructs is critical to allow off-the-shelf availability. Vitrification is a method of cryopreservation that eliminates ice formation, as ice may be detrimental to the function of natural or bioartificial tissues. In order to achieve the vitreous state, high concentrations of CPAs must be added and later removed. The high concentrations may be deleterious to cells as the CPAs are cytotoxic and single-step addition or removal will result in excessive osmotic excursions and cell death. A previously described mathematical model accounting for the mass transfer of CPAs through the sample matrix and cell membrane was expanded to incorporate heat transfer and CPA cytotoxicity. Simulations were performed for two systems, an encapsulated system of insulin-secreting cells and articular cartilage, each with different transport properties, geometry and size. Cytotoxicity and mass transfer are dependent on temperature, with a higher temperature allowing more rapid mass transfer but also causing increased cytotoxicity. The effects of temperature are exacerbated for articular cartilage, which has larger dimensions and slower mass transport through the matrix. Simulations indicate that addition and removal at 4°C is preferable to 25°C, as cell death is higher at 25°C due to increased cytotoxicity in spite of the faster mass transport. Additionally, the model indicates that less cytotoxic CPAs, especially at high temperature, would significantly improve the cryopreservation outcome. Overall, the mathematical model allows the design of addition and removal protocols that insure CPA equilibration throughout the sample while still minimizing CPA exposure and maximizing cell survival.

Toxicity of cryoprotectants to honey bee semen and queens

Given the threats to the intraspecific biodiversity of Apis mellifera and the pressure on bee breeding to come up with disease-tolerant lines, techniques to cryopreserve drone semen are of great interest. Freeze-thawed drone semen of high viability and/or motility has repeatedly been obtained, but fertility of such semen, when it was measured, was always low. The cryoprotective agent (CPA) most frequently used with drone semen is dimethyl sulfoxide (DMSO), although this substance has been suspected of causing genetic damage in sperm. No form of sperm washing is currently performed. Using a membrane permeability assay, we measured the short-term toxicity of four possible replacements for DMSO, 1,3-propane diol, 2,3-butane diol, ethylene glycol, and dimethyl formamide. We also tested whether the practice of inseminating queens with CPA-containing semen affects sperm numbers in the storage organs of queens, or sperm fertility. Finally, we tested whether CPA-toxicity in vivo can be reduced by using mixtures of two CPAs, DMSO, and ethylene glycol. Our results show that, although short-term toxicity of all CPAs tested was low, the presence of single CPAs in insemination mixtures at concentrations required for slow freezing greatly reduced the number of sperm reaching the spermatheca. Contrary to earlier reports, this was also true for DMSO. Ethylene glycol was additionally shown to reduce the viability of spermatozoa reaching the storage organ. Mixtures of DMSO and EthGly performed better than either substance used singly at the same concentration. We conclude that the toxicity of CPAs, including DMSO, on honey bee semen and/or queens has been underestimated in the past. This could partly explain the discrepancy between in vitro and in vivo quality of cryopreserved drone semen, described by others. Combinations of several CPAs and techniques to partly remove CPAs after thawing could help to solve this problem.