Nondestructive evaluation of hydrogel mechanical properties using ultrasound

Tissue stiffness mapping by light sheet elastography

Tissue stiffness plays a crucial role in regulating morphogenesis. The ability to measure and monitor the dynamic progression of tissue stiffness is important for generating and testing mechanistic hypotheses. Methods to measure tissue properties in vivo have been emerging but present challenges with spatial and temporal resolution especially in 3D, by their reliance on highly specialized equipment, and/or due to their invasive nature. Here, we introduce light sheet elastography, a noninvasive method that couples low-frequency shear waves with light sheet fluorescence microscopy by adapting commercially available instruments. With this method, we achieved in toto stiffness mapping of organ-stage mouse and zebrafish embryos at cellular resolution. Versatility of the method enabled time-lapse stiffness mapping during tissue remodeling and of the beating embryonic heart. This method expands the spectrum of tools available to biologists and presents opportunities for uncovering the mechanical basis of morphogenesis.

Bioinks for bioprinting using plant-derived biomaterials

Three-dimensional (3D) bioprinting has revolutionized tissue engineering by enabling the fabrication of complex and functional human tissues and organs. An essential component of successful 3D bioprinting is the selection of an appropriate bioink capable of supporting cell proliferation and viability. Plant-derived biomaterials, because of their abundance, biocompatibility, and tunable properties, hold promise as bioink sources, thus offering advantages over animal-derived biomaterials, which carry immunogenic concerns. This comprehensive review explores and analyzes the potential of plant-derived biomaterials as bioinks for 3D bioprinting of human tissues. Modification and optimization of these materials to enhance printability and biological functionality are discussed. Furthermore, cancer research and drug testing applications of the use of plant-based biomaterials in bioprinting various human tissues such as bone, cartilage, skin, and vascular tissues are described. Challenges and limitations, including mechanical integrity, cell viability, resolution, and regulatory concerns, along with potential strategies to overcome them, are discussed. Additionally, this review provides insights into the potential use of plant-based decellularized ECM (dECM) as bioinks, future prospects, and emerging trends in the use of plant-derived biomaterials for 3D bioprinting applications. The potential of plant-derived biomaterials as bioinks for 3D bioprinting of human tissues is highlighted herein. However, further research is necessary to optimize their processing, standardize their properties, and evaluate their long-termin vivoperformance. Continued advancements in plant-derived biomaterials have the potential to revolutionize tissue engineering and facilitate the development of functional and regenerative therapies for diverse clinical applications.

Noncontact elasticity measurement of hydrogels in a culture dish using reverberant optical coherence elastography

Estimating the elasticity of hydrogel phantoms in a cell culture plane is important for understanding the cell behavior in response to various types of mechanical stimuli. Hence, a noncontact tool for measuring the elastic properties of hydrogel phantoms in such three-dimensional cell cultures is required. A well-known method to determine the mechanical properties of hydrogels is the transient wave method. However, due to the multiple reflections of waves from the boundaries, a bigger cell culture plane or multiple directional filters may be required. In this study, we utilized reverberant shear wave elastography, which is based on the autocorrelation principle, to evaluate the shear wave speed in hydrogel samples within a culture dish. Numerical simulations were performed first to confirm the validity of the reverberant elastography method. Subsequently, we used this method to measure the wave speeds in hydrogel phantoms with different concentrations. Shear rheology tests were also performed, and their results were found to be in good agreement with the measured shear wave speeds. The proposed method could be useful for measuring the elasticity of tissues in tissue engineering applications in an inexpensive and noncontact manner.

Rheological Considerations of Pharmaceutical Formulations: Focus on Viscoelasticity

Controlling rheological properties offers the opportunity to gain insight into the physical characteristics, structure, stability and drug release rate of formulations. To better understand the physical properties of hydrogels, not only rotational but also oscillatory experiments should be performed. Viscoelastic properties, including elastic and viscous properties, are measured using oscillatory rheology. The gel strength and elasticity of hydrogels are of great importance for pharmaceutical development as the application of viscoelastic preparations has considerably expanded in recent decades. Viscosupplementation, ophthalmic surgery and tissue engineering are just a few examples from the wide range of possible applications of viscoelastic hydrogels. Hyaluronic acid, alginate, gellan gum, pectin and chitosan are remarkable representatives of gelling agents that attract great attention applied in biomedical fields. This review provides a brief summary of rheological properties, highlighting the viscoelasticity of hydrogels with great potential in biomedicine.

Unique Mechanical Properties of Gel-Incorporating Protein Crystals

Crystalline materials that are grown in gel media exhibit reinforced mechanical characteristics. Studies on the mechanical properties of protein crystals are limited in numbers because of the difficulty in growing high-quality large crystals. This study shows the demonstration of the unique macroscopic mechanical properties by compression tests of large protein crystals grown in both solution and agarose gel. Particularly, the gel-incorporating protein crystals exhibit larger elastic limits and a higher fracture stress compared with the native protein crystals without gel. Conversely, the change in the Young's modulus corresponding to if the crystals incorporate the gel network is negligible. This suggests that gel networks affect only the fracture phenomenon. Thus, reinforced mechanical characteristics that cannot be obtained by the gel or the protein crystal alone can be developed. By combining the gel media and protein crystals, the gel-incorporating protein crystals show the potential to toughen without sacrificing other mechanical properties.

Microstructured soft devices for the growth and analysis of populations of homogenous multicellular tumor spheroids

Multicellular tumor spheroids are rapidly emerging as an improved in vitro model with respect to more traditional 2D culturing. Microwell culturing is a simple and accessible method for generating a large number of uniformly sized spheroids, but commercially available systems often do not enable researchers to perform complete culturing and analysis pipelines and the mechanical properties of their culture environment are not commonly matching those of the target tissue. We herein report a simple method to obtain custom-designed self-built microwell arrays made of polydimethylsiloxane or agarose for uniform 3D cell structure generation. Such materials can provide an environment of tunable mechanical flexibility. We developed protocols to culture a variety of cancer and non-cancer cell lines in such devices and to perform molecular and imaging characterizations of the spheroid growth, viability, and response to pharmacological treatments. Hundreds of tumor spheroids grow (in scaffolded or scaffold-free conditions) at homogeneous rates and can be harvested at will. Microscopy imaging can be performed in situ during or at the end of the culture. Fluorescence (confocal) microscopy can be performed after in situ staining while retaining the geographic arrangement of spheroids in the plate wells. This platform can enable statistically robust investigations on cancer biology and screening of drug treatments.

Histotripsy Bubble Dynamics in Elastic, Anisotropic Tissue-Mimicking Phantoms

Elastic, anisotropic tissue such as tendon has proven resistant to mechanical fractionation by histotripsy, a subset of focused ultrasound that uses the creation, oscillation and collapse of cavitation bubbles to fractionate tissue. Our objective was to fabricate an optically transparent hydrogel that mimics tendon for evaluation of histotripsy bubble dynamics. Ex vivo bovine deep digital flexor tendons were obtained (n = 4), and varying formulations of polyacrylamide (PA), collagen and fibrin hydrogels (n = 3 each) were fabricated. Axial sound speeds were measured at 1 MHz for calculation of anisotropy. All samples were treated with a 1.5-MHz focused ultrasound transducer with 10-ms pulses repeated at 1 Hz (p+ = 127 MPa, p- = 35 MPa); treatments were monitored with passive cavitation imaging and high-speed photography. Dehydrated fibrin gels were found to be the most similar to tendon in cavitation emission energy (fibrin = 0.69 ± 0.24, tendon = 0.64 ± 0.19 [× 1010 V2]) and anisotropy (fibrin = 3.16 ± 1.12, tendon = 19.4). Bubble cloud area in dehydrated fibrin (0.79 ± 0.14 mm2) was significantly smaller than most other tested hydrogels. Finally, anisotropy was found to have moderately strong linear relationships with cavitation energy and bubble cloud size (r = -0.65 and -0.80, respectively). Dehydrated fibrin shows potential as a repeatable, transparent, tissue-mimicking hydrogel for evaluation of histotripsy bubble dynamics in elastic, anisotropic tissues.

Assessing engineered tissues and biomaterials using ultrasound imaging: In vitro and in vivo applications

Quantitative assessment of the structural, functional, and mechanical properties of engineered tissues and biomaterials is fundamental to their development for regenerative medicine applications. Ultrasound (US) imaging is a non-invasive, non-destructive, and cost-effective technique capable of longitudinal and quantitative monitoring of tissue structure and function across centimeter to sub-micron length scales. Here we present the fundamentals of US to contextualize its application for the assessment of biomaterials and engineered tissues, both in vivo and in vitro. We review key studies that demonstrate the versatility and broad capabilities of US for clinical and pre-clinical biomaterials research. Finally, we highlight emerging techniques that further extend the applications of US, including for ultrafast imaging of biomaterials and engineered tissues in vivo and functional monitoring of stem cells, organoids, and organ-on-a-chip systems in vitro.

Monitoring cavitation dynamics evolution in tissue mimicking hydrogels for repeated exposures via acoustic cavitation emissions

A 700 kHz histotripsy array is used to generate repeated cavitation events in agarose, gelatin, and polyacrylamide hydrogels. High-speed optical imaging, a broadband hydrophone, and the narrow-band receive elements of the histotripsy array are used to capture bubble dynamics and acoustic cavitation emissions. Bubble radii, lifespan, shockwave amplitudes are noted to be measured in close agreement between the different observation methods. These features also decrease with increasing hydrogel stiffness for all of the tested materials. However, the evolutions of these properties during the repeated irradiations vary significantly across the different material subjects. Bubble maximum radius initially increases, then plateaus, and finally decreases in agarose, but remains constant across exposures in gelatin and polyacrylamide. The bubble lifespan increases monotonically in agarose and gelatin but decreases in polyacrylamide. Collapse shockwave amplitudes were measured to have different-shaped evolutions between all three of the tested materials. Bubble maximum radii, lifespans, and collapse shockwave amplitudes were observed to express evolutions that are dependent on the structure and stiffness of the nucleation medium.

The effects of elastic modulus and impurities on bubble nuclei available for acoustic cavitation in polyacrylamide hydrogels

Safety of biomedical ultrasound largely depends on controlling cavitation bubbles in vivo, yet bubble nuclei in biological tissues remain unexplored compared to water. This study evaluates the effects of elastic modulus (E) and impurities on bubble nuclei available for cavitation in tissue-mimicking polyacrylamide (PA) hydrogels. A 1.5 MHz focused ultrasound transducer with f# = 0.7 was used to induce cavitation in 17.5%, 20%, and 22.5% v/v PA hydrogels using 10-ms pulses with pressures up to peak negative pressure (p-) = 35 MPa. Cavitation was monitored at 0.075 ms through high-speed photography at 40 000 fps. At p- = 29 MPa for all hydrogels, cavitation occurred at random locations within the -6 dB focal area [9.4 × 1.2 mm (p-)]. Increasing p- to 35 MPa increased bubble location consistency and caused shock scattering in the E = 282 MPa hydrogels; as the E increased to 300 MPa, bubble location consistency decreased (p = 0.045). Adding calcium phosphate or cholesterol at 0.25% w/v or bovine serum albumin at 5% or 10% w/v in separate 17.5% PA as impurities decreased the cavitation threshold from p- = 13.2 MPa for unaltered PA to p- = 11.6 MPa, p- = 7.3 MPa, p- = 9.7 MPa, and p- = 7.5 MPa, respectively. These results suggest that both E and impurities affect the bubble nuclei available for cavitation in tissue-mimicking hydrogels.

Non-small cell lung carcinoma spheroid models in agarose microwells for drug response studies

There is a growing interest in developing personalized treatment strategies for each cancer patient, especially those with non-small cell lung carcinoma (NSCLC) which annually accounts for the majority of cancer related deaths in the US. Yet identifying the optimal NSCLC treatment strategy for each cancer patient is critical due to a multitude of mutations, some of which develop following initial therapy and can result in drug resistance. A key difficulty in developing personalized therapies in NSCLC is the lack of clinically relevant assay systems that are suitable to evaluate drug sensitivity using a minuscule amount of patient-derived material available following biopsies. Herein we leverage 3D printing to demonstrate a platform based on miniature microwells in agarose to culture cancer cell spheroids. The agarose wells were shaped by 3D printing molds with 1000 microwells with a U-shaped bottom. Three NSCLC cell lines (HCC4006, H1975 and A549) were used to demonstrate size uniformity, spheroid viability, biomarker expressions and drug response in 3D agarose microwells. Results show that our approach yielded spheroids of uniform size (coefficient of variation <22%) and high viability (>83% after 1 week-culture). Studies using epidermal growth factor receptor (EGFR) tyrosine kinase inhibitor (TKIs) drugs gefitinib and osimertinib showed clinically relevant responses. Based on the physical features, cell phenotypes, and responses to therapy of our spheroid models, we conclude that our platform is suitable for in vitro culture and drug evaluation, especially in cases when tumor sample is limited.

Nanoindentation for Monitoring the Time-Variant Mechanical Strength of Drug-Loaded Collagen Hydrogel Regulated by Hydroxyapatite Nanoparticles

Hydroxyapatite nanoparticle-complexed collagen (HAP/Col) hydrogels have been widely used in biomedical applications as a scaffold for controlled drug release (DR). The time-variant mechanical properties (Young's modulus, E) of HAP/Col hydrogels are highly relevant to the precise and efficient control of DR. However, the correlation between the DR and the E of hydrogels remains unclear because of the lack of a nondestructive and continuous measuring system. To reveal the correlations, herein, we investigate the time-variant behavior of E for HAP/Col hydrogels during 28 days using the atomic force microscopy (AFM) nanoindentation technique. The initial E of hydrogels was controlled from 200 to 9000 Pa by the addition of HAPs. Subsequently, we analyzed the relationship between the DR of the hydrogels and the changes in their mechanical properties (ΔE) during hydrogel degradation. Interestingly, the higher the initial E value of HAP/Col hydrogels is, the higher is the rate of hydrogel degradation over time. However, the DR of hydrogels with higher initial E appeared to be significantly delayed by up to 40% at a maximum. The results indicate that adding an appropriate amount of HAPs into hydrogels plays a crucial role in determining the initial E and their degradation rate, which can contribute to the properties that prolong DR. Our findings may provide insights into designing hydrogels for biomedical applications such as bone regeneration and drug-delivery systems.

Resonant acoustic rheometry for non-contact characterization of viscoelastic biomaterials

Resonant Acoustic Rheometry (RAR) is a new, non-contact technique to characterize the mechanical properties of soft and viscoelastic biomaterials, such as hydrogels, that are used to mimic the extracellular matrix in tissue engineering. RAR uses a focused ultrasound pulse to generate a microscale perturbation at the sample surface and tracks the ensuing surface wave using pulse-echo ultrasound. The frequency spectrum of the resonant surface waves is analyzed to extract viscoelastic material properties. In this study, RAR was used to characterize fibrin, gelatin, and agarose hydrogels. Single time point measurements of gelled samples with static mechanical properties showed that RAR provided consistent quantitative data and measured intrinsic material characteristics independent of ultrasound parameters. RAR was also used to longitudinally track dynamic changes in viscoelastic properties over the course of fibrin gelation, revealing distinct phase and material property transitions. Application of RAR was verified using finite element modeling and the results were validated against rotational shear rheometry. Importantly, RAR circumvents some limitations of conventional rheology methods and can be performed in a high-throughput manner using conventional labware. Overall, these studies demonstrate that RAR can be a valuable tool to noninvasively quantify the viscoelastic mechanical properties of soft hydrogel biomaterials.

Manufacturing and Characterization of Hybrid Bulk Voxelated Biomaterials Printed by Digital Anatomy 3D Printing

The advent of 3D digital printers has led to the evolution of realistic anatomical organ shaped structures that are being currently used as experimental models for rehearsing and preparing complex surgical procedures by clinicians. However, the actual material properties are still far from being ideal, which necessitates the need to develop new materials and processing techniques for the next generation of 3D printers optimized for clinical applications. Recently, the voxelated soft matter technique has been introduced to provide a much broader range of materials and a profile much more like the actual organ that can be designed and fabricated voxel by voxel with high precision. For the practical applications of 3D voxelated materials, it is crucial to develop the novel high precision material manufacturing and characterization technique to control the mechanical properties that can be difficult using the conventional methods due to the complexity and the size of the combination of materials. Here we propose the non-destructive ultrasound effective density and bulk modulus imaging to evaluate 3D voxelated materials printed by J750 Digital Anatomy 3D Printer of Stratasys. Our method provides the design map of voxelated materials and substantially broadens the applications of 3D digital printing in the clinical research area.

Novel 2D Dynamic Elasticity Maps for Inspection of Anisotropic Properties in Fused Deposition Modeling Objects

In this study, a novel ultrasonic non-destructive and non-invasive elastography method was introduced and demonstrated to evaluate the mechanical properties of fused deposition modeling 3D printed objects using two-dimensional dynamical elasticity mapping. Based on the recently investigated dynamic bulk modulus and effective density imaging technique, an angle-dependent dynamic shear modulus measurement was performed to extract the dynamic Young’s modulus distribution of the FDM structures. The elastographic image analysis demonstrated the presence of anisotropic dynamic shear modulus and dynamic Young’s modulus existing in the fused deposition modeling 3D printed objects. The non-destructive method also differentiated samples with high contrast property zones from that of low contrast property regions. The angle-dependent elasticity contrast behavior from the ultrasonic method was compared with conventional and static tensile tests characterization. A good correlation between the nondestructive technique and the tensile test measurements was observed.

Thermally Tunable Dynamic and Static Elastic Properties of Hydrogel Due to Volumetric Phase Transition

The temperature dependence of the mechanical properties of polyvinyl alcohol-based poly n-isopropyl acrylamide (PVA-PNIPAm) hydrogel was studied from the static and dynamic bulk modulus of the material. The effect of the temperature-induced volumetric phase transition on Young’s Modulus, Poisson’s ratio, and the density of PVA-PNIPAm was experimentally measured and compared with a non-thermo-responsive Alginate hydrogel as a reference. An increase in the temperature from 27.5 to 32 °C results in the conventional temperature-dependent de-swelling of the PVA-PNIPAm hydrogel volume of up to 70% at the lower critical solution temperature (LCST). However, with the increase in temperature, the PVA-PNIPAm hydrogel showed a drastic increase in Young’s Modulus and density of PVA-PNIPAm and a corresponding decrease in the Poisson’s ratio and the static bulk modulus around the LCST temperature. The dynamic bulk modulus of the PVA-PNIPAm hydrogel is highly frequency-dependent before the LCST and highly temperature-sensitive after the LCST. The dynamic elastic properties of the thermo-responsive PVA-PNIPAm hydrogel were compared and observed to be significantly different from the thermally insensitive Alginate hydrogel.

Investigation of cell mechanics using single-beam acoustic tweezers as a versatile tool for the diagnosis and treatment of highly invasive breast cancer cell lines: an in vitro study

Advancements in diagnostic systems for metastatic cancer over the last few decades have played a significant role in providing patients with effective treatment by evaluating the characteristics of cancer cells. Despite the progress made in cancer prognosis, we still rely on the visual analysis of tissues or cells from histopathologists, where the subjectivity of traditional manual interpretation persists. This paper presents the development of a dual diagnosis and treatment tool using an in vitro acoustic tweezers platform with a 50 MHz ultrasonic transducer for label-free trapping and bursting of human breast cancer cells. For cancer cell detection and classification, the mechanical properties of a single cancer cell were quantified by single-beam acoustic tweezers (SBAT), a noncontact assessment tool using a focused acoustic beam. Cell-mimicking phantoms and agarose hydrogel spheres (AHSs) served to standardize the biomechanical characteristics of the cells. Based on the analytical comparison of deformability levels between the cells and the AHSs, the mechanical properties of the cells could be indirectly measured by interpolating the Young's moduli of the AHSs. As a result, the calculated Young's moduli, i.e., 1.527 kPa for MDA-MB-231 (highly invasive breast cancer cells), 2.650 kPa for MCF-7 (weakly invasive breast cancer cells), and 2.772 kPa for SKBR-3 (weakly invasive breast cancer cells), indicate that highly invasive cancer cells exhibited a lower Young's moduli than weakly invasive cells, which indicates a higher deformability of highly invasive cancer cells, leading to a higher metastasis rate. Single-cell treatment may also be carried out by bursting a highly invasive cell with high-intensity, focused ultrasound.

Apparatus and Method for Rapid Detection of Acoustic Anisotropy in Cartilage

Purpose

Articular cartilage is known to be mechanically anisotropic. In this paper, the acoustic anisotropy of bovine articular cartilage and the effects of freeze-thaw cycling on acoustic anisotropy were investigated.

Methods

We developed apparatus and methods that use a magnetic L-shaped sample holder, which allowed minimal handling of a tissue, reduced the number of measurements compared to previous studies, and produced highly reproducible results.

Results

SOS was greater in the direction perpendicular to the articular surface compared to the direction parallel to the articular surface (N=17, P = 0.00001). Average SOS was 1,758 ± 107 m/s perpendicular to the surface, and 1,617 ± 55 m/s parallel to it. The average percentage difference in SOS between the perpendicular and parallel directions was 8.2% (95% CI: 5.4% to 11%). Freeze-thaw cycling did not have a significant effect on SOS (P>0.4).

Conclusion

Acoustic measurement of tissue properties is particularly attractive for work in our laboratory since it has the potential for nondestructive characterization of the properties of developing engineered cartilage. Our approach allowed us to observe acoustic anisotropy of articular cartilage rapidly and reproducibly. This property was not significantly affected by freeze-thawing of the tissue samples, making cryopreservation practical for these assays.

Enhanced Instantaneous Elastography in Tissues and Hard Materials Using Bulk Modulus and Density Determined Without Externally Applied Material Deformation

Ultrasound is a continually developing technology that is broadly used for fast, non-destructive mechanical property detection of hard and soft materials in applications ranging from manufacturing to biomedical. In this study, a novel monostatic longitudinal ultrasonic pulsing elastography imaging method is introduced. The existing elastography methods require an acoustic radiational or dynamic compressive externally applied force to determine the effective bulk modulus or density. This new, passive M-mode imaging technique does not require an external stress and can be effectively used for both soft and hard materials. Strain map imaging and shear wave elastography are two current categories of M-mode imaging that show both relative and absolute elasticity information. The new technique is applied to hard materials and soft material tissue phantoms for demonstrating effective bulk modulus and effective density mapping. When compared with standard techniques, the effective parameters fall within 10% of standard characterization methods for both hard and soft materials. As neither the standard A-mode imaging technique nor the presented technique require an external applied force, the techniques are applied to composite heterostructures and the findings presented for comparison. The presented passive M-mode technique is found to have enhanced resolution over standard A-mode modalities.

In Vitro Evaluation of the Influence of Substrate Mechanics on Matrix-Assisted Human Chondrocyte Transplantation

Matrix-assisted chondrocyte transplantation (MACT) is of great interest for the treatment of patients with cartilage lesions. However, the roles of the matrix properties in modulating cartilage tissue integration during MACT recovery have not been fully understood. The objective of this study was to uncover the effects of substrate mechanics on the integration of implanted chondrocyte-laden hydrogels with native cartilage tissues. To this end, agarose hydrogels with Young’s moduli ranging from 0.49 kPa (0.5%, w/v) to 23.08 kPa (10%) were prepared and incorporated into an in vitro human cartilage explant model. The hydrogel-cartilage composites were cultivated for up to 12 weeks and harvested for evaluation via scanning electron microscopy, histology, and a push-through test. Our results demonstrated that integration strength at the hydrogel-cartilage interface in the 1.0% (0.93 kPa) and 2.5% (3.30 kPa) agarose groups significantly increased over time, whereas hydrogels with higher stiffness (>8.78 kPa) led to poor integration with articular cartilage. Extensive sprouting of extracellular matrix in the interfacial regions was only observed in the 0.5% to 2.5% agarose groups. Collectively, our findings suggest that while neocartilage development and its integration with native cartilage are modulated by substrate elasticity, an optimal Young’s modulus (3.30 kPa) possessed by agarose hydrogels is identified such that superior quality of tissue integration is achieved without compromising tissue properties of implanted constructs.

In Vivo Tracking of Tissue Engineered Constructs

To date, the fields of biomaterials science and tissue engineering have shown great promise in creating bioartificial tissues and organs for use in a variety of regenerative medicine applications. With the emergence of new technologies such as additive biomanufacturing and 3D bioprinting, increasingly complex tissue constructs are being fabricated to fulfill the desired patient-specific requirements. Fundamental to the further advancement of this field is the design and development of imaging modalities that can enable visualization of the bioengineered constructs following implantation, at adequate spatial and temporal resolution and high penetration depths. These in vivo tracking techniques should introduce minimum toxicity, disruption, and destruction to treated tissues, while generating clinically relevant signal-to-noise ratios. This article reviews the imaging techniques that are currently being adopted in both research and clinical studies to track tissue engineering scaffolds in vivo, with special attention to 3D bioprinted tissue constructs.

Quantitative ultrasound imaging of cell-laden hydrogels and printed constructs

In the present work we have revisited the application of quantitative ultrasound imaging (QUI) to cellular hydrogels, by using the reference phantom method (RPM) in combination with a local attenuation compensation algorithm. The investigated biological samples consisted of cell-laden collagen hydrogels with PC12 neural cells. These cell-laden hydrogels were used to calibrate the integrated backscattering coefficient (IBC) as a function of cell density, which was then used to generate parametric images of local cell density. The image resolution used for QUI and its impact on the relative IBC error was also investigated. Another important contribution of our work was the monitoring of PC12 cell proliferation. The cell number estimates obtained via the calibrated IBC compared well with data obtained using a conventional quantitative method, the MTS assay. Evaluation of spectral changes as a function of culture time also provided additional information on the cell cluster size, which was found to be in close agreement with that observed by microscopy. Last but not least, we also applied QUI on a 3D printed cellular construct in order to illustrate its capabilities for the evaluation of bioprinted structures. STATEMENT OF SIGNIFICANCE: While there is intensive research in the areas of polymer science, biology, and 3D bio-printing, there exists a gap in available characterisation tools for the non-destructive inspection of biological constructs in the three-dimensional domain, on the macroscopic scale, and with fast data acquisition times. Quantitative ultrasound imaging is a suitable characterization technique for providing essential information on the development of tissue engineered constructs. These results provide a detailed and comprehensive guide on the capabilities and limitations of the technique.

Rapid Detection of Shear-Induced Damage in Tissue-Engineered Cartilage Using Ultrasound

Previous investigations have shown that tissue-engineered articular cartilage can be damaged under a combination of compression and sliding shear. In these cases, damage was identified in histological sections after a test was completed. This approach is limited, in that it does not identify when damage occurred. This especially limits the utility of an assay for evaluating damage when comparing modifications to a tissue-engineering protocol. In this investigation, the feasibility of using ultrasound (US) to detect damage as it occurs was investigated. US signals were acquired before, during, and after sliding shear, as were stereomicroscope images of the cartilage surface. Histology was used as the standard for showing if a sample was damaged. We showed that US reflections from the surface of the cartilage were attenuated due to roughening following sliding shear. Furthermore, it was shown that by scanning the transducer across a sample, surface roughness and erosion following sliding shear could be identified. Internal delamination could be identified by the appearance of new echoes between those from the front and back of the sample. Thus, it is feasible to detect damage in engineered cartilage using US.

A contactless approach for monitoring the mechanical properties of swollen hydrogels

Using a customized ultrasound setup we investigate the feasibility of using a contactless approach to study the bulk mechanical properties of swollen hydrogels. The study involved two different hydrogels, gelatin methacrylate (GelMa) and green algae extract methacrylate (GAEM), which were prepared to provide materials with varying modulus and different swelling properties. Two approaches have been developed. In the first case, ultrasound was compared to Young's modulus measured by indentation. It was found that can be linearly related to indentation modulus values only when the hydrogel swelling ratio is taken into account. In the second approach, an exponential dependency between swelled thickness and indentation modulus was found. This is representative for each hydrogel and purification method in addition to being independent of the conditions used within the toughness range explored. The results of this study indicate that a simple thickness measurement via the proposed approach can provide a direct relationship to Young's modulus upon calibration.

A combined three-dimensional in vitro-in silico approach to modelling bubble dynamics in decompression sickness

The growth of bubbles within the body is widely believed to be the cause of decompression sickness (DCS). Dive computer algorithms that aim to prevent DCS by mathematically modelling bubble dynamics and tissue gas kinetics are challenging to validate. This is due to lack of understanding regarding the mechanism(s) leading from bubble formation to DCS. In this work, a biomimetic in vitro tissue phantom and a three-dimensional computational model, comprising a hyperelastic strain-energy density function to model tissue elasticity, were combined to investigate key areas of bubble dynamics. A sensitivity analysis indicated that the diffusion coefficient was the most influential material parameter. Comparison of computational and experimental data revealed the bubble surface's diffusion coefficient to be 30 times smaller than that in the bulk tissue and dependent on the bubble's surface area. The initial size, size distribution and proximity of bubbles within the tissue phantom were also shown to influence their subsequent dynamics highlighting the importance of modelling bubble nucleation and bubble-bubble interactions in order to develop more accurate dive algorithms.

Tracking the Evolution of Transiently Transfected Individual Cells in a Microfluidic Platform

Transient gene expression (TGE) technology enables the rapid production of large amount of recombinant proteins, without the need of fastidious screening of the producing cells required for stable transfection (ST). However, several barriers must be overcome before reaching the production yields using ST. For optimizing the production yields from suspended cells using TGE, a better understanding of the transfection conditions at the single cell level are required. In this study, a universal droplet microfluidic platform was used to assess the heterogeneities of CHO-S population transiently transfected with cationic liposomes (CL) (lipoplexes) complexed with GFP-coding plasmid DNA (pDNA). A single cell analysis of GFP production kinetics revealed the presence of a subpopulation producing higher levels of GFP compared with the main population. The size of high producing (HP) cells, their relative abundance, and their specific productivity were dependent on the charge and the pDNA content of the different lipoplexes: HPs showed increased cell size in comparison to the average population, lipoplexes with positive charge produced more HPs, and lipoplexes carrying a larger amount of pDNA yielded a higher specific productivity of HPs. This study demonstrates the potential for time-resolved single-cell measurements to explain population dynamics from a microscopic point of view.

Toward jet injection by continuous-wave laser cavitation

This is a study motivated by the need to develop a needle-free device for eliminating major global healthcare problems caused by needles. The generation of liquid jets by means of a continuous-wave laser, focused into a light absorbing solution, was studied with the aim of developing a portable and affordable jet injector. We designed and fabricated glass microfluidic devices, which consist of a chamber where thermocavitation is created and a tapered channel. The growth of a vapor bubble displaces and expels the liquid through the channel as a fast traveling jet. Different parameters were varied with the purpose of increasing the jet velocity. The velocity increases with smaller channel diameters and taper ratios, whereas larger chambers significantly reduce the jet speed. It was found that the initial position of the liquid-air meniscus interface and its dynamics contribute to increased jet velocities. A maximum velocity of 94±3  m/s for a channel diameter of D=120  μm, taper ratio n=0.25, and chamber length E=200  μm was achieved. Finally, agarose gel-based skin phantoms were used to demonstrate the potential of our devices to penetrate the skin. The maximum penetration depth achieved was ∼1  mm, which is sufficient to penetrate the stratum corneum and for most medical applications. A meta-analysis shows that larger injection volumes will be required as a next step to medical relevance for laser-induced jet injection techniques in general.

Nondestructive evaluation of a new hydrolytically degradable and photo-clickable PEG hydrogel for cartilage tissue engineering

Photopolymerizable and hydrolytically labile poly(ethylene glycol) (PEG) hydrogels formed from photo-clickable reactions were investigated as cell delivery platforms for cartilage tissue engineering (TE). PEG hydrogels were formed from thiol-norbornene PEG macromers whereby the crosslinks contained caprolactone segments with hydrolytically labile ester linkages. Juvenile bovine chondrocytes encapsulated in the hydrogels were cultured for up to four weeks and assessed biochemically and histologically, using standard destructive assays, and for mechanical and ultrasound properties, as nondestructive assays. Bulk degradation of acellular hydrogels was confirmed by a decrease in compressive modulus and an increase in mass swelling ratio over time. Chondrocytes deposited increasing amounts of sulfated glycosaminoglycans and collagens in the hydrogels with time. Spatially, collagen type II and aggrecan were present in the neotissue with formation of a territorial matrix beginning at day 21. Nondestructive measurements revealed an 8-fold increase in compressive modulus from days 7 to 28, which correlated with total collagen content. Ultrasound measurements revealed changes in the constructs over time, which differed from the mechanical properties, and appeared to correlate with ECM structure and organization shown by immunohistochemical analysis. Overall, non-destructive and destructive measurements show that this new hydrolytically degradable PEG hydrogel is promising for cartilage TE.

STATEMENT OF SIGNIFICANCE

Designing synthetic hydrogels whose degradation matches tissue growth is critical to maintaining mechanical integrity as the hydrogel degrades and new tissue forms, but is challenging due to the nature of the hydrogel crosslinks that inhibit diffusion of tissue matrix molecules. This study details a promising, new, photo-clickable and synthetic hydrogel whose degradation supports cartilaginous tissue matrix growth leading to the formation of a territorial matrix, concomitant with an increase in mechanical properties. Nondestructive assays based on mechanical and ultrasonic properties were also investigated using a novel instrument and found to correlate with matrix deposition and evolution. In sum, this study presents a new hydrogel platform combined with nondestructive assessments, which together have potential for in vitro cartilage tissue engineering.

Finite difference time domain model of ultrasound propagation in agarose scaffold containing collagen or chondrocytes

Measurement of ultrasound backscattering is a promising diagnostic technique for arthroscopic evaluation of articular cartilage. However, contribution of collagen and chondrocytes on ultrasound backscattering and speed of sound in cartilage is not fully understood and is experimentally difficult to study. Agarose hydrogels have been used in tissue engineering applications of cartilage. Therefore, the aim of this study was to simulate the propagation of high frequency ultrasound (40 MHz) in agarose scaffolds with varying concentrations of chondrocytes (1 to 32 × 10(6) cells/ml) and collagen (1.56-200 mg/ml) using transversely isotropic two-dimensional finite difference time domain method (FDTD). Backscatter and speed of sound were evaluated from the simulated pulse-echo and through transmission measurements, respectively. Ultrasound backscatter increased with increasing collagen and chondrocyte concentrations. Furthermore, speed of sound increased with increasing collagen concentration. However, this was not observed with increasing chondrocyte concentrations. The present study suggests that the FDTD method may have some applicability in simulations of ultrasound scattering and propagation in constructs containing collagen and chondrocytes. Findings of this study indicate the significant role of collagen and chondrocytes as ultrasound scatterers and can aid in development of modeling approaches for understanding how cartilage architecture affects to the propagation of high frequency ultrasound.

Ultrasound Imaging Techniques for Spatiotemporal Characterization of Composition, Microstructure, and Mechanical Properties in Tissue Engineering

Ultrasound techniques are increasingly being used to quantitatively characterize both native and engineered tissues. This review provides an overview and selected examples of the main techniques used in these applications. Grayscale imaging has been used to characterize extracellular matrix deposition, and quantitative ultrasound imaging based on the integrated backscatter coefficient has been applied to estimating cell concentrations and matrix morphology in tissue engineering. Spectral analysis has been employed to characterize the concentration and spatial distribution of mineral particles in a construct, as well as to monitor mineral deposition by cells over time. Ultrasound techniques have also been used to measure the mechanical properties of native and engineered tissues. Conventional ultrasound elasticity imaging and acoustic radiation force imaging have been applied to detect regions of altered stiffness within tissues. Sonorheometry and monitoring of steady-state excitation and recovery have been used to characterize viscoelastic properties of tissue using a single transducer to both deform and image the sample. Dual-mode ultrasound elastography uses separate ultrasound transducers to produce a more potent deformation force to microscale characterization of viscoelasticity of hydrogel constructs. These ultrasound-based techniques have high potential to impact the field of tissue engineering as they are further developed and their range of applications expands.

Nondestructive Techniques to Evaluate the Characteristics and Development of Engineered Cartilage

In this review, methods for evaluating the properties of tissue engineered (TE) cartilage are described. Many of these have been developed for evaluating properties of native and osteoarthritic articular cartilage. However, with the increasing interest in engineering cartilage, specialized methods are needed for nondestructive evaluation of tissue while it is developing and after it is implanted. Such methods are needed, in part, due to the large inter- and intra-donor variability in the performance of the cellular component of the tissue, which remains a barrier to delivering reliable TE cartilage for implantation. Using conventional destructive tests, such variability makes it near-impossible to predict the timing and outcome of the tissue engineering process at the level of a specific piece of engineered tissue and also makes it difficult to assess the impact of changing tissue engineering regimens. While it is clear that the true test of engineered cartilage is its performance after it is implanted, correlation of pre and post implantation properties determined non-destructively in vitro and/or in vivo with performance should lead to predictive methods to improve quality-control and to minimize the chances of implanting inferior tissue.

Tendinopathy alters ultrasound transmission in the patellar tendon during squatting

Measurement of loading patterns of the patellar tendon during activity is important in understanding tendon injury. We used transmission-mode ultrasonography to investigate patellar tendon loading during squatting in adults with and without tendinopathy. It was hypothesized that axial ultrasonic velocity, a surrogate measure of the elastic modulus of tendon, would be lower in tendinopathy. Ultrasound velocity was measured in both patellar tendons of adults with unilateral patellar tendinopathy (n = 9) and in healthy controls (n = 16) during a bilateral squat maneuver. Sagittal knee movement was measured simultaneously with an electrogoniometer. Statistical comparisons between healthy and injured tendons were made using two-way mixed-design ANOVAs. Axial ultrasound velocity in both symptomatic and asymptomatic patellar tendons in tendinopathy was approximately 15% higher than in healthy tendons at the commencement (F_1,23  = 5.2, P < 0.05) and completion (F_1,23  = 4.5, P < 0.05) of the squat. While peak velocity was ≈5% higher during both flexion (F_1,23  = 5.4, P < 0.05) and extension (F_1,23  = 5.3, P < 0.05) phases, there was no significant between-group difference at the midpoint of the movement. There were no significant differences in the rate and magnitude of knee movement between groups. Although further research is required, these findings suggest enhanced baseline muscle activity in patellar tendinopathy and highlight fresh avenues for its clinical management.

Quantitative Ultrasound for Nondestructive Characterization of Engineered Tissues and Biomaterials

Non-invasive, non-destructive technologies for imaging and quantitatively monitoring the development of artificial tissues are critical for the advancement of tissue engineering. Current standard techniques for evaluating engineered tissues, including histology, biochemical assays and mechanical testing, are destructive approaches. Ultrasound is emerging as a valuable tool for imaging and quantitatively monitoring the properties of engineered tissues and biomaterials longitudinally during fabrication and post-implantation. Ultrasound techniques are rapid, non-invasive, non-destructive and can be easily integrated into sterile environments necessary for tissue engineering. Furthermore, high-frequency quantitative ultrasound techniques can enable volumetric characterization of the structural, biological, and mechanical properties of engineered tissues during fabrication and post-implantation. This review provides an overview of ultrasound imaging, quantitative ultrasound techniques, and elastography, with representative examples of applications of these ultrasound-based techniques to the field of tissue engineering.

Characterization of material properties of soft solid thin layers with acoustic radiation force and wave propagation

Evaluation of tissue engineering constructs is performed by a series of different tests. In many cases it is important to match the mechanical properties of these constructs to those of native tissues. However, many mechanical testing methods are destructive in nature which increases cost for evaluation because of the need for additional samples reserved for these assessments. A wave propagation method is proposed for characterizing the shear elasticity of thin layers bounded by a rigid substrate and fluid-loading, similar to the configuration for many tissue engineering applications. An analytic wave propagation model was derived for this configuration and compared against finite element model simulations and numerical solutions from the software package Disperse. The results from the different models found very good agreement. Experiments were performed in tissue-mimicking gelatin phantoms with thicknesses of 1 and 4 mm and found that the wave propagation method could resolve the shear modulus with very good accuracy, no more than 4.10% error. This method could be used in tissue engineering applications to monitor tissue engineering construct maturation with a nondestructive wave propagation method to evaluate the shear modulus of a material.

Ultrasound Elastography for Estimation of Regional Strain of Multilayered Hydrogels and Tissue-Engineered Cartilage

Tissue-engineered (TE) cartilage constructs tend to develop inhomogeneously, thus, to predict the mechanical performance of the tissue, conventional biomechanical testing, which yields average material properties, is of limited value. Rather, techniques for evaluating regional and depth-dependent properties of TE cartilage, preferably non-destructively, are required. The purpose of this study was to build upon our previous results and to investigate the feasibility of using ultrasound elastography to non-destructively assess the depth-dependent biomechanical characteristics of TE cartilage while in a sterile bioreactor. As a proof-of-concept, and to standardize an assessment protocol, a well-characterized three-layered hydrogel construct was used as a surrogate for TE cartilage, and was studied under controlled incremental compressions. The strain field of the construct predicted by elastography was then validated by comparison with a poroelastic finite-element analysis (FEA). On average, the differences between the strains predicted by elastography and the FEA were within 10%. Subsequently engineered cartilage tissue was evaluated in the same test fixture. Results from these examinations showed internal regions where the local strain was 1-2 orders of magnitude greater than that near the surface. These studies document the feasibility of using ultrasound to evaluate the mechanical behaviors of maturing TE constructs in a sterile environment.

A new method for evaluating the degeneration of articular cartilage using pulse-echo ultrasound

This paper presents a novel nondestructive ultrasonic technique for measuring the sound speed and acoustic impedance of articular cartilage using the pulsed Vz,t technique. Vz,t data include a series of pulsed ultrasonic echoes collected using different distances between the ultrasonic transducer and the specimen. The 2D Fourier transform is applied to the Vz,t data to reconstruct the 2D reflection spectrum Rθ,ω. To obtain the reflection coefficient of articular cartilage, the Vz,t data from a reference specimen with a well-known reflection coefficient are obtained to eliminate the dependence on the general system transfer function. The ultrasound-derived aggregate modulus (Ha) is computed based on the measured reflection coefficient and the sound speed. In the experiment, 32 cartilage-bone samples were prepared from bovine articular cartilage, and 16 samples were digested using 0.25% trypsin solution. The sound speed and Ha of these cartilage samples were evaluated before and after degeneration. The magnitude of the sound speed decreased with trypsin digestion (from 1663 ± 5.6 m/s to 1613 ± 5.3 m/s). Moreover, the Young's modulus in the corresponding degenerative state was measured and was correlated with the ultrasound-derived aggregate modulus. The ultrasound-derived aggregate modulus was determined to be highly correlated with the Young's modulus (n = 16, r>0.895, p<0.003, Pearson correlation test for each measurement). The results demonstrate the effectiveness of using the proposed method to assess the changes in sound speed and the ultrasound-derived aggregate modulus of cartilage after degeneration.

Ultrasound technologies for biomaterials fabrication and imaging

Ultrasound is emerging as a powerful tool for developing biomaterials for regenerative medicine. Ultrasound technologies are finding wide-ranging, innovative applications for controlling the fabrication of bioengineered scaffolds, as well as for imaging and quantitatively monitoring the properties of engineered constructs both during fabrication processes and post-implantation. This review provides an overview of the biomedical applications of ultrasound for imaging and therapy, a tutorial of the physical mechanisms through which ultrasound can interact with biomaterials, and examples of how ultrasound technologies are being developed and applied for biomaterials fabrication processes, non-invasive imaging, and quantitative characterization of bioengineered scaffolds in vitro and in vivo.

Pre-clinical characterization of tissue engineering constructs for bone and cartilage regeneration

Pre-clinical animal models play a crucial role in the translation of biomedical technologies from the bench top to the bedside. However, there is a need for improved techniques to evaluate implanted biomaterials within the host, including consideration of the care and ethics associated with animal studies, as well as the evaluation of host tissue repair in a clinically relevant manner. This review discusses non-invasive, quantitative, and real-time techniques for evaluating host-materials interactions, quality and rate of neotissue formation, and functional outcomes of implanted biomaterials for bone and cartilage tissue engineering. Specifically, a comparison will be presented for pre-clinical animal models, histological scoring systems, and non-invasive imaging modalities. Additionally, novel technologies to track delivered cells and growth factors will be discussed, including methods to directly correlate their release with tissue growth.

Towards the feasibility of using ultrasound to determine mechanical properties of tissues in a bioreactor

Our ultimate goal is to non-destructively evaluate mechanical properties of tissue-engineered (TE) cartilage using ultrasound (US). We used agarose gels as surrogates for TE cartilage. Previously, we showed that mechanical properties measured using conventional methods were related to those measured using US, which suggested a way to non-destructively predict mechanical properties of samples with known volume fractions. In this study, we sought to determine whether the mechanical properties of samples, with unknown volume fractions could be predicted by US. Aggregate moduli were calculated for hydrogels as a function of SOS, based on concentration and density using a poroelastic model. The data were used to train a statistical model, which we then used to predict volume fractions and mechanical properties of unknown samples. Young's and storage moduli were measured mechanically. The statistical model generally predicted the Young's moduli in compression to within <10% of their mechanically measured value. We defined positive linear correlations between the aggregate modulus predicted from US and both the storage and Young's moduli determined from mechanical tests. Mechanical properties of hydrogels with unknown volume fractions can be predicted successfully from US measurements. This method has the potential to predict mechanical properties of TE cartilage non-destructively in a bioreactor.

Noninvasive quantification of in vitro osteoblastic differentiation in 3D engineered tissue constructs using spectral ultrasound imaging

Non-destructive monitoring of engineered tissues is needed for translation of these products from the lab to the clinic. In this study, non-invasive, high resolution spectral ultrasound imaging (SUSI) was used to monitor the differentiation of MC3T3 pre-osteoblasts seeded within collagen hydrogels. SUSI was used to measure the diameter, concentration and acoustic attenuation of scatterers within such constructs cultured in either control or osteogenic medium over 21 days. Conventional biochemical assays were used on parallel samples to determine DNA content and calcium deposition. Construct volume and morphology were accurately imaged using ultrasound. Cell diameter was estimated to be approximately 12.5–15.5 µm using SUSI, which corresponded well to measurements of fluorescently stained cells. The total number of cells per construct assessed by quantitation of DNA content decreased from 5.6±2.4×10⁴ at day 1 to 0.9±0.2×10⁴ at day 21. SUSI estimation of the equivalent number of acoustic scatters showed a similar decreasing trend, except at day 21 in the osteogenic samples, which showed a marked increase in both scatterer number and acoustic impedance, suggestive of mineral deposition by the differentiating MC3T3 cells. Estimation of calcium content by SUSI was 41.7±11.4 µg/ml, which agreed well with the biochemical measurement of 38.7±16.7 µg/ml. Color coded maps of parameter values were overlaid on B-mode images to show spatiotemporal changes in cell diameter and calcium deposition. This study demonstrates the use of non-destructive ultrasound imaging to provide quantitative information on the number and differentiated state of cells embedded within 3D engineered constructs, and therefore presents a valuable tool for longitudinal monitoring of engineered tissue development.

Imaging challenges in biomaterials and tissue engineering

Biomaterials are employed in the fields of tissue engineering and regenerative medicine (TERM) in order to enhance the regeneration or replacement of tissue function and/or structure. The unique environments resulting from the presence of biomaterials, cells, and tissues result in distinct challenges in regards to monitoring and assessing the results of these interventions. Imaging technologies for three-dimensional (3D) analysis have been identified as a strategic priority in TERM research. Traditionally, histological and immunohistochemical techniques have been used to evaluate engineered tissues. However, these methods do not allow for an accurate volume assessment, are invasive, and do not provide information on functional status. Imaging techniques are needed that enable non-destructive, longitudinal, quantitative, and three-dimensional analysis of TERM strategies. This review focuses on evaluating the application of available imaging modalities for assessment of biomaterials and tissue in TERM applications. Included is a discussion of limitations of these techniques and identification of areas for further development.

Non-invasive speed of sound measurement in cartilage by use of combined magnetic resonance imaging and ultrasound: an initial study

The speed of sound (SOS) is available as an index of elasticity. Using a combination of magnetic resonance imaging (MRI) and ultrasound, one can measure the SOS. In this study, we verified the accuracy of SOS measurements by using a combination of MRI and ultrasound. The accuracy of the thickness measurements was confirmed by comparison of the results obtained with use of MRI with those of a non-contact laser, and the accuracy of the calculated SOS values was confirmed by comparison of the results of the combined method and ultrasound measurements with the transmission method ex vivo. There was no significant difference between thickness measurements by MRI and those with the non-contact laser, and there was a significant linear correlation between SOS measurement results by use of the combined method and those by use of the transmission method. We also showed that the SOS values obtained agreed with those of previously published studies.

Estimation of diffraction effect in ultrasonic attenuation by through-transmission substitution technique

Measurement of ultrasonic attenuation is important in clinical and industrial applications. The overall goal of this research was to characterize the diffraction effect in ultrasonic attenuation. We have followed a systematic approach, beginning with the theoretical analysis of the calculation method using the transfer function of the signal spectrum, moving on to numerical computations and experimental confirmation. The relation of sample thickness to pulse duration is presented and the transmission coefficient of the sample for different propagation modes is discussed. Particular attention is paid to the diffraction effect which is easy to be neglected but a potential source of artifacts. Numerical computations demonstrated that lower frequencies, shorter transducer distances and larger velocity difference can result in significant diffraction effect. Due to the complexity of determining interface loss in Single Sample Substitution Method (SSM), Two Samples Substitution Method (TSM) was proposed to avoid this drawback. Comparison experiment with SSM illustrates that the proposed diffraction correction model is sound in theory and feasible in practice.

Multimodal evaluation of tissue-engineered cartilage

Tissue engineering (TE) has promise as a biological solution and a disease modifying treatment for arthritis. Although cartilage can be generated by TE, substantial inter- and intra-donor variability makes it impossible to guarantee optimal, reproducible results. TE cartilage must be able to perform the functions of native tissue, thus mechanical and biological properties approaching those of native cartilage are likely a pre-requisite for successful implantation. A quality-control assessment of these properties should be part of the implantation release criteria for TE cartilage. Release criteria should certify that selected tissue properties have reached certain target ranges, and should be predictive of the likelihood of success of an implant in vivo. Unfortunately, it is not currently known which properties are needed to establish release criteria, nor how close one has to be to the properties of native cartilage to achieve success. Achieving properties approaching those of native cartilage requires a clear understanding of the target properties and reproducible assessment methodology. Here, we review several main aspects of quality control as it applies to TE cartilage. This includes a look at known mechanical and biological properties of native cartilage, which should be the target in engineered tissues. We also present an overview of the state of the art of tissue assessment, focusing on native articular and TE cartilage. Finally, we review the arguments for developing and validating non-destructive testing methods for assessing TE products.

Noninvasive, quantitative, spatiotemporal characterization of mineralization in three-dimensional collagen hydrogels using high-resolution spectral ultrasound imaging

As tissue engineering products move toward the clinic, nondestructive methods to monitor their development and ensure quality are needed. In this study, high-resolution spectral ultrasound imaging (SUSI) was used to noninvasively characterize mineral content in collagen hydrogels. SUSI was used to generate three-dimensional (3D) grayscale (GS) images of construct morphology with submillimeter resolution. Spectral analysis of the backscattered radio frequency (RF) ultrasound signals was used to determine the midband fit (MBF) and slope of the linearized RF spectrum. These parameters are operator and instrument independent, and were used to characterize the spatial distribution of mineral in constructs supplemented with hydroxyapatite particles. GS and MBF correlated closely with mineral content, while slope was not dependent on concentration. SUSI also was used to monitor mineralization of collagen constructs by immersion in simulated body fluid (SBF) over 21 days. The construct surface was mineralized before the interior, and there was a dose-dependent effect of SBF concentration on degree of mineralization and deposited particle size. MBF density was closely correlated with the amount of calcium deposited. These data demonstrate that SUSI has utility as a noninvasive imaging method for quantitative analysis of mineralization in 3D protein constructs. Such techniques may assist the development of engineered orthopedic tissues.

Ultrasonic wave propagation assessment of native cartilage explants and hydrogel scaffolds for tissue engineering

Non-destructive techniques characterising the mechanical properties of cells, tissues, and biomaterials provide baseline metrics for tissue engineering design. Ultrasonic wave propagation and attenuation has previously demonstrated the dynamics of extracellular matrix synthesis in chondrocyte-seeded hydrogel constructs. In this paper, we describe an ultrasonic method to analyse two of the construct elements used to engineer articular cartilage in real-time, native cartilage explants and an agarose biomaterial. Results indicated a similarity in wave propagation velocity ranges for both longitudinal (1500-1745 m/s) and transverse (350-950 m/s) waveforms. Future work will apply an acoustoelastic analysis to distinguish between the fluid and solid properties including the cell and matrix biokinetics as a validation of previous mathematical models.