MR spectroscopy measurement of the diffusion of dimethyl sulfoxide in articular cartilage and comparison to theoretical predictions

Permeation kinetics of dimethyl sulfoxide in porcine corneoscleral discs

The cornea is the transparent tissue in front of the eye that bends light to help the eye focus. More than five million people's vision can be restored by a corneal transplant (keratoplasty), but there is a scarcity of suitable donor tissue. Cryopreservation could potentially increase the on-demand availability of corneas by reducing expiration and contamination during hypothermic storage, and allow equitable distribution. Understanding the transport of water and cryoprotectants across the tissue is important in developing effective cryopreservation protocols. Here, we first measured the shrinking and swelling kinetics at 22 °C and 0 °C of porcine corneoscleral discs when exposed to phosphate-buffered saline and to a cryoprotectant vehicle solution containing 2.5% chondroitin sulfate and 1% dextran. Other valuable measurements were made including the density and osmolality of the vehicle solution at 0 °C, and the water fraction of porcine cornea and sclera. Using the knowledge gained from this first part to minimize background swelling, we then examined permeation kinetics of dimethyl sulfoxide (Me2SO) in porcine corneoscleral discs at 0 °C, the temperature at which cryoprotectant loading typically occurs. The concentration data obtained as a function of time were fitted to a Fick's law model of one-dimensional diffusion to measure an effective diffusion coefficient of Me2SO, which was found to be 5.306×10-11 m2/s. We further quantified permeation kinetics of Me2SO in sclera alone at 0 °C to support our hypothesis that our measurements for corneoscleral discs will not be affected by the presence of the sclera. The obtained effective diffusion coefficient can be used in modelling aimed at developing cryopreservation protocols that minimize the exposure time of the corneas during the cryoprotectant loading step.

Modeling the Simultaneous Transport of Multiple Cryoprotectants into Articular Cartilage Using a Triphasic Model

Cryopreserving articular cartilage by vitrification can increase the availability of tissue for osteochondral allograft transplantation to treat cartilage defects. Developing well-optimized vitrification protocols can be supported by mathematical modeling to reduce the amount of trial-and-error experimentation needed. Fick's law has been used to model cryoprotectant diffusion, but it assumes ideal, dilute solution behavior, neglects water movement, and assumes diffusion of each cryoprotectant is independent of the presence of other cryoprotectants. The modified triphasic model addresses some of these shortcomings by accounting for water movement and the nonideal, nondilute nature of cryoprotectant vitrification solutions. However, it currently only exists for solutions containing a single cryoprotectant. As such, we extend the modified triphasic model to include two permeating cryoprotectants so that simultaneous diffusion occurring in vitrification protocols can be more accurately modeled. Using previously published experimental data, we determine suitable values for the fitting parameters of the new model. We then model a successful vitrification protocol for particulated cartilage cubes by calculating concentration, freezing point, vitrifiability, and strain profiles at the end of each loading step. We observe that Fick's law consistently underestimates cryoprotectant concentration throughout the cartilage compared to the modified triphasic model, leading to an underestimation of tissue vitrifiability. We additionally observe that simultaneous diffusion of cryoprotectants increases the permeation rate of each individual cryoprotectant, which Fick's law fails to consider. This suggests that using the two-cryoprotectant modified triphasic model to develop vitrification protocols could reduce excess exposure to cryoprotectants and improve preserved tissue outcomes.

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.

Osmometric Measurements of Cryoprotective Agent Permeation into Tissues

Quantification of the amount of cryoprotective agent (CPA) in a tissue is an essential step in the design of successful cryopreservation protocols. This chapter details two inexpensive methods to measure cryoprotective agent permeation into tissues as functions of time. One of the methods to measure the CPA permeation is to permeate a series of tissue samples from a surrounding solution at a specified concentration of CPA, each sample for a different amount of time, and then to quantitate the amount of CPA that was taken up in the tissue during that time period. The quantification is performed by equilibrating the permeated tissue with a surrounding solution and then measuring the osmolality of the solution to determine the amounts of CPAs that have come out of each tissue sample corresponding to each permeation time. An alternative method to measuring the CPA permeation as a function of time, which requires fewer tissue samples, is to measure the CPA efflux as a function of time. In the efflux method, a CPA-permeated tissue sample is placed in a surrounding solution, and solution samples are taken at different time points throughout the efflux to quantitate how much CPA has left the tissue by each time point.

Principles Underlying Cryopreservation and Freeze-Drying of Cells and Tissues

Cryopreservation and freeze-drying can be used to preserve cells or tissues for prolonged periods. Vitrification, or ice-free cryopreservation, is an alternative to cryopreservation that enables cooling cells to cryogenic temperatures in the absence of ice. The processing pathways involved in (ice-free) cryopreservation and freeze-drying of cells and tissues, however, can be very damaging. In this chapter, we describe the principles underlying preservation of cells for which freezing and drying are normally lethal processes as well as for cells that are able to survive in a reversible state of suspended animation. Freezing results in solution effects injury and/or intracellular ice formation, whereas drying results in removal of (non-freezable) water normally bound to biomolecules, which is generally more damaging. Cryopreservation and freeze-drying require different types of protective agents. Different mechanistic modes of action of cryoprotective and lyoprotective agents are described including minimizing ice formation, preferential exclusion, water replacement, and vitrification. Furthermore, it is discussed how protective agents can be introduced into cells avoiding damage due to too large cell volume excursions, and how knowledge of cell-specific membrane permeability properties in various temperature regimes can be used to rationally design (ice-free) cryopreservation and freeze-drying protocols.

Transport processes in equine oocytes and ovarian tissue during loading with cryoprotective solutions

BACKGROUND

Rational design of cryopreservation strategies for oocytes and ovarian cortex tissue requires insights in the rate at which cryoprotective agents (CPA) permeate and concomitant water transport takes place. The aim of the current study was to investigate possible differences in permeation kinetics of different CPAs (i.e., glycerol/GLY, ethylene glycol/EG, dimethyl sulfoxide/DMSO, and propylene glycol/PG), in equine oocytes as well as ovarian tissue.

METHODS

Membrane permeability of oocytes to water (Lp) and to CPAs (Ps) was inferred from video microscopic imaging of oocyte volume responses during perfusion with anisotonic and CPA solutions. CPA diffusion into ovarian tissue and tissue dehydration was monitored during incubation, using osmometer and weight measurements, to derive CPA diffusion coefficients (D).

RESULTS

Membrane permeability of oocytes towards CPAs was found to increase in the order GLY < EG < DMSO<PG. Permeability towards water in anisotonic solutions was determined to be higher than in CPA solutions, indicating CPAs alter membrane permeability properties. CPA diffusion in ovarian tissue increased in the order GLY,PG < EG,DMSO. Tissue dehydration was found to increase with exposure to increasing CPA concentrations, which inversely correlated with CPA diffusivity.

CONCLUSIONS

In conclusion, it is shown here that the rate of CPA movement across membrane bilayers is determined by different physical barrier factors than those determining CPA movement in tissues.

GENERAL SIGNIFICANCE

The parameters presented in this study can be applied in models describing solute and water transport in cells and tissues, as well as in cryopreservation protocols.

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.

Spectroscopic monitoring of transport processes during loading of ovarian tissue with cryoprotective solutions

There is an increasing demand for female fertility preservation. Cryopreservation of ovarian cortex tissue by means of vitrification can be done ad-hoc and for pre-pubertal individuals. Obtaining a homogeneous distribution of protective agents in tissues is one of the major hurdles for successful preservation. Therefore, to rationally design vitrification strategies for tissues, it is needed to determine permeation kinetics of cryoprotective agents; to ensure homogeneous distribution while minimizing exposure time and toxicity effects. In this study, Fourier transform infrared spectroscopy (FTIR) was used to monitor diffusion of different components into porcine ovarian cortex tissue. Water fluxes and permeation kinetics of dimethyl sulfoxide (DMSO), glycerol (GLY), ethylene glycol (EG), and propylene glycol (PG) were investigated. Diffusion coefficients derived from FTIR data, were corroborated with differential scanning calorimetry and osmometer measurements. FTIR allowed real-time spectral fingerprinting of tissue during loading with mixtures of protective agents, while discriminating between different components and water. Exposure to vitrification solutions was found to cause drastic initial weight losses, which could be correlated with spectral features. Use of heavy water allowed distinguishing water fluxes associated with dehydration and permeation, both of which were found to precede permeation of cryoprotective agents. Overall, DMSO and EG were found to permeate faster than GLY and PG. In mixtures, however, solutes behave differently. The non-invasive spectroscopic method described here to study permeation of vitrification solution components into ovarian tissue can be applied to many other types of engineered constructs, tissues, and possibly organs.

Advances in the slow freezing cryopreservation of microencapsulated cells

Over the past few decades, the use of cell microencapsulation technology has been promoted for a wide range of applications as sustained drug delivery systems or as cells containing biosystems for regenerative medicine. However, difficulty in their preservation and storage has limited their availability to healthcare centers. Because the preservation in cryogenic temperatures poses many biological and biophysical challenges and that the technology has not been well understood, the slow cooling cryopreservation, which is the most used technique worldwide, has not given full measure of its full potential application yet. This review will discuss the different steps that should be understood and taken into account to preserve microencapsulated cells by slow freezing in a successful and simple manner. Moreover, it will review the slow freezing preservation of alginate-based microencapsulated cells and discuss some recommendations that the research community may pursue to optimize the preservation of microencapsulated cells, enabling the therapy translate from bench to the clinic.

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.

Anatomical study: comparing the human, sheep and pig knee meniscus

BACKGROUND

Animal models are commonly used in investigating new treatment options for knee joint injuries including injuries to the meniscus. The reliability and applicability of these models to replicate findings in humans depends on determining the most suitable animal proxy. Therefore, this study was designed to compare the wet weight, volume and dimensions of the human meniscus with two commonly used animal models: sheep and pig.

METHODS

Human menisci (n = 6 pairs) were obtained from the knee joints of cadaveric male donors. Sheep menisci (n = 6 pairs) and pig menisci (n = 22 pairs) were obtained from the stifle joints of adult sheep and pigs. Meniscal wet weight, volume and dimensions of the body were measured and compared among the species. Anatomical dimensions included circumference, width, peripheral height, articular height and superior articular length.

RESULTS

The circumference of human menisci (lateral: 84.0 mm, medial: 88.7 mm) was significantly longer than that of sheep (lateral: 50.0 mm, medial: 55.5 mm) and pig (lateral: 66.8 mm, medial: 64.9 mm). The majority of the remaining dimensions of the medial and all of the remaining dimensions of the lateral menisci in sheep showed no statistical difference in comparison to the human menisci. The meniscal weight in pig was significantly larger (lateral: 6.4 g, medial: 5.0 g) than the human (lateral: 4.9 g, medial: 4.4 g) and sheep (lateral: 2.5 g, medial: 2.2 g). Porcine meniscal volume (lateral: 6.5 ml, medial: 5.1 ml) was also larger than the human (lateral: 5.0 ml, medial: 4.5 ml) and sheep (lateral: 2.3 ml, medial: 2.2 ml) menisci. The dimensions measured in the pig meniscus were generally larger than human menisci with statistically significant differences in most categories.

CONCLUSION

Sheep meniscal dimensions more closely matched human meniscal dimensions than the pig meniscal dimensions. This information may help guide the choice of an animal proxy in meniscal research.

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.

Transport of Neutral Solute Across Articular Cartilage: The Role of Zonal Diffusivities

Transport of solutes through diffusion is an important metabolic mechanism for the avascular cartilage tissue. Three types of interconnected physical phenomena, namely mechanical, electrical, and chemical, are all involved in the physics of transport in cartilage. In this study, we use a carefully designed experimental-computational setup to separate the effects of mechanical and chemical factors from those of electrical charges. Axial diffusion of a neutral solute (Iodixanol) into cartilage was monitored using calibrated microcomputed tomography (micro-CT) images for up to 48 hr. A biphasic-solute computational model was fitted to the experimental data to determine the diffusion coefficients of cartilage. Cartilage was modeled either using one single diffusion coefficient (single-zone model) or using three diffusion coefficients corresponding to superficial, middle, and deep cartilage zones (multizone model). It was observed that the single-zone model cannot capture the entire concentration-time curve and under-predicts the near-equilibrium concentration values, whereas the multizone model could very well match the experimental data. The diffusion coefficient of the superficial zone was found to be at least one order of magnitude larger than that of the middle zone. Since neutral solutes were used, glycosaminoglycan (GAG) content cannot be the primary reason behind such large differences between the diffusion coefficients of the different cartilage zones. It is therefore concluded that other features of the different cartilage zones such as water content and the organization (orientation) of collagen fibers may be enough to cause large differences in diffusion coefficients through the cartilage thickness.

A model for predicting the permeation of dimethyl sulfoxide into articular cartilage, and its application to the liquidus-tracking method

Long-term storage of articular cartilage (AC) has excited great interest due to the practical surgical significance of this tissue. The liquidus-tracking (LT) method developed by Pegg et al. (2006) [29] for vitreous preservation of AC achieved reasonable survival of post-warming chondrocytes in situ, but the design of the entire procedure was more dependent on trial and error. Mathematical modeling would help to better understand the LT process, and thereby make possible improvements to attain higher cell survival. Mass transfer plays a dominant role in the LT process. In the present study, a diffusion model based on the free-volume theory and the Flory-Huggins thermodynamics theory was developed to predict the permeation of dimethyl sulfoxide (Me2SO) into AC. A comparison between the predicted mean concentration of Me2SO in the AC disc and the experimental data over wide temperature and concentration ranges [-30 to 37 °C, 10 to 64.5% (w/w)] shows that the developed model can accurately describe the permeation of Me2SO into AC [coefficient of determination (R(2)): 0.951-1.000, mean relative error (MRE): 0.8-12.8%]. With this model, the spatial and temporal distribution of Me2SO in the AC disc during a loading/unloading process can be obtained. Application of the model to Pegg et al.'s LT procedure revealed that the liquidus line is virtually not followed for the center part of the AC disc. The presently developed model will be a useful tool in the analysis and design of the LT method for vitreous preservation of AC.

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.

Subject-specific analysis of joint contact mechanics: application to the study of osteoarthritis and surgical planning

Advances in computational mechanics, constitutive modeling, and techniques for subject-specific modeling have opened the door to patient-specific simulation of the relationships between joint mechanics and osteoarthritis (OA), as well as patient-specific preoperative planning. This article reviews the application of computational biomechanics to the simulation of joint contact mechanics as relevant to the study of OA. This review begins with background regarding OA and the mechanical causes of OA in the context of simulations of joint mechanics. The broad range of technical considerations in creating validated subject-specific whole joint models is discussed. The types of computational models available for the study of joint mechanics are reviewed. The types of constitutive models that are available for articular cartilage are reviewed, with special attention to choosing an appropriate constitutive model for the application at hand. Issues related to model generation are discussed, including acquisition of model geometry from volumetric image data and specific considerations for acquisition of computed tomography and magnetic resonance imaging data. Approaches to model validation are reviewed. The areas of parametric analysis, factorial design, and probabilistic analysis are reviewed in the context of simulations of joint contact mechanics. Following the review of technical considerations, the article details insights that have been obtained from computational models of joint mechanics for normal joints; patient populations; the study of specific aspects of joint mechanics relevant to OA, such as congruency and instability; and preoperative planning. Finally, future directions for research and application are summarized.