Characterizing morphology and nonlinear elastic properties of normal and thermally stressed engineered oral mucosal tissues using scanning acoustic microscopy

Jingle Cell Rock: Steering Cellular Activity With Low-Intensity Pulsed Ultrasound (LIPUS) to Engineer Functional Tissues in Regenerative Medicine

Acoustic manipulation or perturbation of biological soft matter has emerged as a promising clinical treatment for a number of applications within regenerative medicine, ranging from bone fracture repair to neuromodulation. The potential of ultrasound (US) endures in imparting mechanical stimuli that are able to trigger a cascade of molecular signals within unscathed cells. Particularly, low-intensity pulsed ultrasound (LIPUS) has been associated with bio-effects such as activation of specific cellular pathways and alteration of cell morphology and gene expression, the extent of which can be modulated by fine tuning of LIPUS parameters including intensity, frequency and exposure time. Although the molecular mechanisms underlying LIPUS are not yet fully elucidated, a number of studies clearly define the modulation of specific ultrasonic parameters as a means to guide the differentiation of a specific set of stem cells towards adult and fully differentiated cell types. Herein, we outline the applications of LIPUS in regenerative medicine and the in vivo and in vitro studies that have confirmed the unbounded clinical potential of this platform. We highlight the latest developments aimed at investigating the physical and biological mechanisms of action of LIPUS, outlining the most recent efforts in using this technology to aid tissue engineering strategies for repairing tissue or modelling specific diseases. Ultimately, we detail tissue-specific applications harnessing LIPUS stimuli, offering insights over the engineering of new constructs and therapeutic modalities. Overall, we aim to lay the foundation for a deeper understanding of the mechanisms governing LIPUS-based therapy, to inform the development of safer and more effective tissue regeneration strategies in the field of regenerative medicine.

Noninvasive high-frequency acoustic microscopy for 3D visualization of microstructure and estimation of elastic properties during hydrolytic degradation of lactide and ε-caprolactone polymers

The monitoring of degradation processes' kinetics in polymers is one of the attractive possibilities of ultrasound technique applications that provide non-destructive imaging of polymers' internal microstructure and measurements of elastic properties. In this work, biodegradable polymers and copolymers based on L,L-lactide, D,L-lactide and ε-caprolactone have been studied at different stages of hydrolysis at 37 °C by high-frequency (100 and 200 MHz) ultrasound. The acoustic microscopy technique has been developed to reveal changes in the internal microstructure and bulk sound speed in polymer samples over a hydrolysis period of 25 weeks. Ultrasound imaging provides visualization of amorphous and crystalline phases, internal imperfections, variation in packing density, and other microstructural features. Acoustic images demonstrate nucleation, growth, and the changes in internal inhomogeneities in polymers during degradation accompanied by a decrease in the polymers' molecular weight. We associate the changes in the elastic properties (the speed of a longitudinal wave) with crystallinity variations in polymers during hydrothermal aging. The results of the ultrasound investigations are supplemented by gel permeation chromatography, differential scanning calorimetry, and wide-angle X-ray spectroscopy. STATEMENT OF SIGNIFICANCE: Monitoring the kinetics of degradation processes in polymers is one of the attractive possibilities of applying ultrasound techniques that provide non-destructive imaging of the polymers' internal microstructure and measurements of elastic properties. In this work, visualization of nucleation, growth, and evolution of internal inhomogeneities in the volume of polymers and variation of values of speed of longitudinal and transverse sound waves during hydrolysis are compared with measurements of molecular weight, density, data of DSC curves, and X-ray scattering analysis. We discuss several common phenomena that occur in the volume of poly(L-lactide) and poly(D,L-lactide) over the degradation process as well as improvement of elastic properties of the poly(ε -caprolactone) and poly(L-lactide-co-caprolactone) during hydrothermal aging.

Photoacoustic Imaging in Tissue Engineering and Regenerative Medicine

Several imaging modalities are available for investigation of the morphological, functional, and molecular features of engineered tissues in small animal models. While research in tissue engineering and regenerative medicine (TERM) would benefit from a comprehensive longitudinal analysis of new strategies, researchers have not always applied the most advanced methods. Photoacoustic imaging (PAI) is a rapidly emerging modality that has received significant attention due to its ability to exploit the strong endogenous contrast of optical methods with the high spatial resolution of ultrasound methods. Exogenous contrast agents can also be used in PAI for targeted imaging. Applications of PAI relevant to TERM include stem cell tracking, longitudinal monitoring of scaffolds in vivo, and evaluation of vascularization. In addition, the emerging capabilities of PAI applied to the detection and monitoring of cancer and other inflammatory diseases could be exploited by tissue engineers. This article provides an overview of the operating principles of PAI and its broad potential for application in TERM. Impact statement Photoacoustic imaging, a new hybrid imaging technique, has demonstrated high potential in the clinical diagnostic applications. The optical and acoustic aspect of the photoacoustic imaging system works in harmony to provide better resolution at greater tissue depth. Label-free imaging of vasculature with this imaging can be used to track and monitor disease, as well as the therapeutic progression of treatment. Photoacoustic imaging has been utilized in tissue engineering to some extent; however, the full benefit of this technique is yet to be explored. The increasing availability of commercial photoacoustic systems will make application as an imaging tool for tissue engineering application more feasible. This review first provides a brief description of photoacoustic imaging and summarizes its current and potential application in tissue engineering.

Computational modeling of airway instability and collapse in tracheomalacia

Background

Tracheomalacia (TM) is a condition of excessive tracheal collapse during exhalation. Both acquired and congenital forms of TM are believed to result from morphological changes in cartilaginous, fibrous and/or smooth muscle tissues reducing airway mechanical properties to a degree that precipitates collapse. However, neither the specific amount of mechanical property reduction nor the malacic segment lengths leading to life threatening airway collapse in TM are known. Furthermore, the specific mechanism of collapse is still debated.

Methods

Computational nonlinear finite element models were developed to determine the effect of malacic segment length, tracheal diameter, and reduction in tissue nonlinear elastic properties on the risk for and mechanism of airway collapse. Cartilage, fibrous tissue, and smooth muscle nonlinear elastic properties were fit to experimental data from preterm lambs from the literature. These elastic properties were systematically reduced in the model to simulate TM.

Results

An intriguing finding was that sudden mechanical instability leading to complete airway collapse occurred in airways when even a 1 cm segment of cartilage and fibrous tissue properties had a critical reduction in material properties. In general, increased tracheal diameter, increased malacic segment length coupled with decreased nonlinear anterior cartilage/fibrous tissue nonlinear mechanical properties increased the risk of sudden airway collapse from snap through instability.

Conclusion

Modeling results support snap through instability as the mechanism for life threatening tracheomalacia specifically when cartilage ring nonlinear properties are reduced to a range between fibrous tissue nonlinear elastic properties (for larger diameter airways > 10 mm) to mucosa properties (for smaller diameter airways < 6 mm). Although reducing posterior tracheal smooth muscle properties to mucosa properties decreased exhalation area, no sudden instability leading to collapse was seen in these models.

Biomechanics of oral mucosa

The prevalence of prosthodontic treatment has been well recognized, and the need is continuously increasing with the ageing population. While the oral mucosa plays a critical role in the treatment outcome, the associated biomechanics is not yet fully understood. Using the literature available, this paper provides a critical review on four aspects of mucosal biomechanics, including static, dynamic, volumetric and interactive responses, which are interpreted by its elasticity, viscosity/permeability, apparent Poisson's ratio and friction coefficient, respectively. Both empirical studies and numerical models are analysed and compared to gain anatomical and physiological insights. Furthermore, the clinical applications of such biomechanical knowledge on the mucosa are explored to address some critical concerns, including stimuli for tissue remodelling (interstitial hydrostatic pressure), pressure–pain thresholds, tissue displaceability and residual bone resorption. Through this review, the state of the art in mucosal biomechanics and their clinical implications are discussed for future research interests, including clinical applications, computational modelling, design optimization and prosthetic fabrication.

Non-invasive and Non-destructive Characterization of Tissue Engineered Constructs Using Ultrasound Imaging Technologies: A Review

With the rapid expansion of biomaterial development and coupled efforts to translate such advances toward the clinic, non-invasive and non-destructive imaging tools to evaluate implants in situ in a timely manner are critically needed. The required multi-level information is comprehensive, including structural, mechanical, and biological changes such as scaffold degradation, mechanical strength, cell infiltration, extracellular matrix formation and vascularization to name a few. With its inherent advantages of non-invasiveness and non-destructiveness, ultrasound imaging can be an ideal tool for both preclinical and clinical uses. In this review, currently available ultrasound imaging technologies that have been applied in vitro and in vivo for tissue engineering and regenerative medicine are discussed and some new emerging ultrasound technologies and multi-modality approaches utilizing ultrasound are introduced.

Imaging strategies for tissue engineering applications

Tissue engineering has evolved with multifaceted research being conducted using advanced technologies, and it is progressing toward clinical applications. As tissue engineering technology significantly advances, it proceeds toward increasing sophistication, including nanoscale strategies for material construction and synergetic methods for combining with cells, growth factors, or other macromolecules. Therefore, to assess advanced tissue-engineered constructs, tissue engineers need versatile imaging methods capable of monitoring not only morphological but also functional and molecular information. However, there is no single imaging modality that is suitable for all tissue-engineered constructs. Each imaging method has its own range of applications and provides information based on the specific properties of the imaging technique. Therefore, according to the requirements of the tissue engineering studies, the most appropriate tool should be selected among a variety of imaging modalities. The goal of this review article is to describe available biomedical imaging methods to assess tissue engineering applications and to provide tissue engineers with criteria and insights for determining the best imaging strategies. Commonly used biomedical imaging modalities, including X-ray and computed tomography, positron emission tomography and single photon emission computed tomography, magnetic resonance imaging, ultrasound imaging, optical imaging, and emerging techniques and multimodal imaging, will be discussed, focusing on the latest trends of their applications in recent tissue engineering studies.

High-frequency ultrasonic imaging of growth and development in manufactured engineered oral mucosal tissue surfaces

This study uses high-resolution ultrasound to examine the growth and development of engineered oral mucosal tissues manufactured under aseptic conditions. The specimens are a commercially available natural tissue scaffold, AlloDerm, and oral keratinocytes seeded onto AlloDerm to form an ex vivo-produced oral mucosal equivalent (EVPOME) suitable for intra-oral grafting. The seeded cells produce a keratinized protective upper layer that smooths out any remaining surface irregularities on the underlying AlloDerm. Two-dimensional acoustic imaging of unseeded AlloDerm and developing EVPOMEs was performed on each day of their growth and development, each tissue specimen being imaged under aseptic conditions (total time from seeding to maturation: 11 d). Ultrasonic monitoring offers us the ability to determine the constituents of the EVPOME that are responsible for changes in its mechanical behavior during the manufacturing process. Ultrasonic monitoring affords us an opportunity to non-invasively assess, in real time, tissue-engineered constructs before release for use in patient care.