Design and Validation of Equiaxial Mechanical Strain Platform, EQUicycler, for 3D Tissue Engineered Constructs

Stable hydrogel adhesion to polydimethylsiloxane enables cyclic mechanical stimulation of 3D-bioprinted smooth muscle constructs

During normal urination, smooth muscle cells (SMCs) in the lower urinary tract (LUT) are exposed to mechanical signals that have a critical impact on tissue structure and function. Nevertheless, the mechanisms underlying the maintenance of the contractile phenotype of SMCs remain poorly understood. This is due, in part, to a lack of studies that have examined the effects of mechanical loading using three-dimensional (3D) models. In this study, surface modifications of polydimethylsiloxane (PDMS) membrane were evaluated to investigate the effects of cyclic mechanical stimulation on SMC maturation in 3D constructs. Commercially available cell stretching plates were modified with amino or methacrylate groups to promote adhesion of 3D constructs fabricated by bioprinting. After 6 days of stimulation, the effects of mechanical stimulation on the expression of contractile markers at the mRNA and protein levels were analyzed. Methacrylate-modified surfaces supported stable adhesion of the 3D constructs to the membrane and facilitated cyclic mechanical stimulation, which significantly increased the expression of contractile markers at the mRNA and protein levels. These effects were found to be mediated by activation of the p38 MAPK pathway, as inhibition of this pathway abolished the effects of stimulation in a dose-dependent manner. These results provide valuable insights into the role of mechanical signaling in maintaining the contractile phenotype of bladder SMCs, which has important implications for the development of future treatments for LUT diseases.

Extracellular Mechanical Stimuli Alters the Metastatic Progression of Prostate Cancer Cells within 3D Tissue Matrix

This study aimed to understand extracellular mechanical stimuli's effect on prostate cancer cells' metastatic progression within a three-dimensional (3D) bone-like microenvironment. In this study, a mechanical loading platform, EQUicycler, has been employed to create physiologically relevant static and cyclic mechanical stimuli to a prostate cancer cell (PC-3)-embedded 3D tissue matrix. Three mechanical stimuli conditions were applied: control (no loading), cyclic (1% strain at 1 Hz), and static mechanical stimuli (1% strain). The changes in prostate cancer cells' cytoskeletal reorganization, polarity (elongation index), proliferation, expression level of N-Cadherin (metastasis-associated gene), and migratory potential within the 3D collagen structures were assessed upon mechanical stimuli. The results have shown that static mechanical stimuli increased the metastasis progression factors, including cell elongation (p < 0.001), cellular F-actin accumulation (p < 0.001), actin polymerization (p < 0.001), N-Cadherin gene expression, and invasion capacity of PC-3 cells within a bone-like microenvironment compared to its cyclic and control loading counterparts. This study established a novel system for studying metastatic cancer cells within bone and enables the creation of biomimetic in vitro models for cancer research and mechanobiology.

Tumor removal limits prostate cancer cell dissemination in bone and osteoblasts induce cancer cell dormancy through focal adhesion kinase

BACKGROUND

Disseminated tumor cells (DTCs) can enter a dormant state and cause no symptoms in cancer patients. On the other hand, the dormant DTCs can reactivate and cause metastases progression and lethal relapses. In prostate cancer (PCa), relapse can happen after curative treatments such as primary tumor removal. The impact of surgical removal on PCa dissemination and dormancy remains elusive. Furthermore, as dormant DTCs are asymptomatic, dormancy-induction can be an operational cure for preventing metastases and relapse of PCa patients.

METHODS

We used a PCa subcutaneous xenograft model and species-specific PCR to survey the DTCs in various organs at different time points of tumor growth and in response to tumor removal. We developed in vitro 2D and 3D co-culture models to recapitulate the dormant DTCs in the bone microenvironment. Proliferation assays, fluorescent cell cycle reporter, qRT-PCR, and Western Blot were used to characterize the dormancy phenotype. We performed RNA sequencing to determine the dormancy signature of PCa. A drug repurposing algorithm was applied to predict dormancy-inducing drugs and a top candidate was validated for the efficacy and the mechanism of dormancy induction.

RESULTS

We found DTCs in almost all mouse organs examined, including bones, at week 2 post-tumor cell injections. Surgical removal of the primary tumor reduced the overall DTC abundance, but the DTCs were enriched only in the bones. We found that osteoblasts, but not other cells of the bones, induced PCa cell dormancy. RNA-Seq revealed the suppression of mitochondrial-related biological processes in osteoblast-induced dormant PCa cells. Importantly, the mitochondrial-related biological processes were found up-regulated in both circulating tumor cells and bone metastases from PCa patients' data. We predicted and validated the dormancy-mimicking effect of PF-562,271 (PF-271), an inhibitor of focal adhesion kinase (FAK) in vitro. Decreased FAK phosphorylation and increased nuclear translocation were found in both co-cultured and PF-271-treated C4-2B cells, suggesting that FAK plays a key role in osteoblast-induced PCa dormancy.

CONCLUSIONS

Our study provides the first insights into how primary tumor removal enriches PCa cell dissemination in the bones, defines a unique osteoblast-induced PCa dormancy signature, and identifies FAK as a PCa cell dormancy gatekeeper.

Reconstruction of Fibrocartilage with Fibrous Alignment of Type I Collagen in Scaffold-Free Manner

For functional reconstruction of fibrocartilage, it is necessary to reproduce the essential mechanical property exhibited by natural fibrocartilage. The distinctive mechanical property of fibrocartilage is originated from the specific histological features of fibrocartilage composed of highly aligned type I collagen (Col I) and an abundant cartilaginous matrix. While the application of tensile stimulation induces highly aligned Col I, our study reveals that it also exerts an antichondrogenic effect on scaffold-free tissues constructed with meniscal chondrocytes (MCs) and induces downregulation of Sox-9 expression and attenuated glycosaminoglycan production. Modulation of mechanotransduction by blocking nuclear translocation of Yes-associated protein (YAP) ameliorated the antichondrogenic effect in the presence of tensile stimulation. Since MCs subjected to mechanical doses either by surface stiffness or tensile stimulation showed reversibility of YAP status even after a long-term exposure to mechanotransduction, fibrocartilage tissue was constructed by sequentially inducing tissue alignment by tensile stimulation followed by inducing cartilaginous matrix production in a tension-released state. The minimal tensile dose to constitute durable tissue alignment was screened by investigating the alignment of cytoskeleton and Col I after culturing the scaffold-free tissue constructs with various tensile doses (10% static tension for 1, 3, 7, and 10 days) followed by maintaining in a released state for 5 days. Fluorescence-conjugated phalloidin binding and immunofluorescence of Col I indicated that the duration of static tension for more than 7 days resulted in durable tissue alignment for at least 5 days in the tension-released state. The tissues subjected to tensile stimulation for 7 days followed by 14 days in a released state in chondrogenic media resulted in abundant cartilaginous matrix as well as uniaxial anisotropic alignment. Our results show that the optimized tensile dose can facilitate the successful reconstruction of fibrocartilage by modulating the characteristics of matrix production by MCs.

Characterization and Biocompatibility Assessment of Boron Nitride Magnesium Nanocomposites for Orthopedic Applications

Magnesium (Mg) has been intensively studied as a promising alternative material to inert metallic alloys for orthopedic fixation devices due to its biodegradable nature inside the body and its favorable biocompatibility. However, the low mechanical strength and rapid corrosion of Mg in physiological environments represent the main challenges for the development of Mg-based devices for orthopedic applications. A possible solution to these limitations is the incorporation of a small content of biocompatible nanoparticles into the Mg matrix to increase strength and possibly corrosion resistance of the resulting nanocomposites. In this work, the effect of adding boron nitride (BN) nanoparticles (0.5 and 1.5 vol.%) on the mechanical properties, corrosion behavior, and biocompatibility of Mg-based nanocomposites was investigated. The properties of the nanocomposites fabricated using powder metallurgy methods were assessed using microstructure analyses, microhardness, compression tests, in vitro corrosion, contact angle, and cytotoxicity tests. A significant increase in the microhardness, strength, and corrosion rates of Mg-BN nanocomposites was detected compared with those of pure Mg (0% BN). Crystalline surface post-corrosion byproducts were detected and identified via SEM, EDX, and XRD. Biocompatibility assessments showed that the incorporation of BN nanoparticles had no significant impact on the cytotoxicity of Mg and samples were hydrophilic based on the contact angle results. These results confirm that the addition of BN nanoparticles to the Mg matrix can increase strength and corrosion resistance without influencing cytotoxicity in vitro. Further investigation into the chemical behavior of nanocomposites in physiological environments is needed to determine the potential impact of corrosive byproducts. Surface treatments and formulation methods that would increase the viability of these materials in vivo are also needed.

Mechanical stimulation devices for mechanobiology studies: a market, literature, and patents review

Significant advancements in various research and technological fields have contributed to remarkable findings on the physiological dynamics of the human body. To more closely mimic the complex physiological environment, research has moved from two-dimensional (2D) culture systems to more sophisticated three-dimensional (3D) dynamic cultures. Unlike bioreactors or microfluidic-based culture models, cells are typically seeded on polymeric substrates or incorporated into 3D constructs which are mechanically stimulated to investigate cell response to mechanical stresses, such as tensile or compressive. This review focuses on the working principles of mechanical stimulation devices currently available on the market or custom-built by research groups or protected by patents and highlights the main features still open to improvement. These are the features which could be focused on to perform, in the future, more reliable and accurate mechanobiology studies.

Graphic abstract

Injectable Cell-Laden Nanofibrous Matrix for Treating Annulus Fibrosus Defects in Porcine Model: An Organ Culture Study

Lower back pain commonly arises from intervertebral disc (IVD) failure, often caused by deteriorating annulus fibrosus (AF) and/or nucleus pulposus (NP) tissue. High socioeconomic cost, quality of life issues, and unsatisfactory surgical options motivate the rapid development of non-invasive, regenerative repair strategies for lower back pain. This study aims to evaluate the AF regenerative capacity of injectable matrix repair strategy in ex vivo porcine organ culturing using collagen type-I and polycaprolactone nanofibers (PNCOL) with encapsulated fibroblast cells. Upon 14 days organ culturing, the porcine IVDs were assessed using gross optical imaging, magnetic resonance imaging (MRI), histological analysis, and Reverse Transcriptase quantitative PCR (RT-qPCR) to determine the regenerative capabilities of the PNCOL matrix at the AF injury. PNCOL-treated AF defects demonstrated a full recovery with increased gene expressions of AF extracellular matrix markers, including Collagen-I, Aggrecan, Scleraxis, and Tenascin, along with anti-inflammatory markers such as CD206 and IL10. The PNCOL treatment effectively regenerates the AF tissue at the injury site contributing to decreased herniation risk and improved surgical outcomes, thus providing effective non-invasive strategies for treating IVD injuries.

Delivering Mechanical Stimulation to Cells: State of the Art in Materials and Devices Design

Biochemical signals, such as growth factors, cytokines, and transcription factors are known to play a crucial role in regulating a variety of cellular activities as well as maintaining the normal function of different tissues and organs. If the biochemical signals are assumed to be one side of the coin, the other side comprises biophysical cues. There is growing evidence showing that biophysical signals, and in particular mechanical cues, also play an important role in different stages of human life ranging from morphogenesis during embryonic development to maturation and maintenance of tissue and organ function throughout life. In order to investigate how mechanical signals influence cell and tissue function, tremendous efforts have been devoted to fabricating various materials and devices for delivering mechanical stimuli to cells and tissues. Here, an overview of the current state of the art in the design and development of such materials and devices is provided, with a focus on their design principles, and challenges and perspectives for future research directions are highlighted.

Electrical Stimulation-Mediated Tissue Healing in Porcine Intervertebral Disc Under Mechanically Dynamic Organ Culture Conditions

STUDY DESIGN

Porcine intervertebral discs (IVDs) were excised and then drilled to simulate degeneration before being electrically stimulated for 21 days while undergoing mechanical loading. The discs were then analyzed for gene expression and morphology to assess regeneration.

OBJECTIVE

The purpose of this study was to investigate the effectiveness of the electrical stimulation of IVD treatment as an early intervention method in halting the progression of degenerative disc disease using an ex-vivo porcine model.

SUMMARY OF BACKGROUND DATA

Treatments for degenerative disc disease are limited in their efficacy and tend to treat the symptoms of the disease rather than repairing the degenerated disc itself. There is a dire need for an early intervention treatment that not only halts the progression of the disease but contributes to reviving the degenerated disc.

METHODS

Lumbar IVDs were extracted from a mature pig within 1 hour of death and were drilled with a 1.5 mm bit to simulate degenerative disc disease. Four IVDs at a time were then cultured in a dynamic bioreactor system under mechanical loading for 21 days, two with and two without the electrical stimulation treatment. The IVDs were assessed using histological analysis, magnetic resonance imaging, and quantitative reverse transcriptase polymerase chain reaction to quantify the effectiveness of the treatment on the degenerated discs.

RESULTS

IVDs with electrical stimulation treatment exhibited extensive annular regeneration and prevented herniation of the nucleus pulposus (NP). In contrast, the untreated group of IVDs were unable to maintain tissue integrity and exhibited NP herniation through multiple layers of the annulus fibrosus. Gene expression showed an increase of extracellular matrix markers and antiinflammatory cytokine interleukin-4 (IL-4), while decreasing in pro-inflammatory markers and pain markers in electrically stimulated IVDs when compared to the untreated group.

CONCLUSION

The direct electrical stimulation application in NP of damaged IVDs can be a viable option to regenerate damaged NP and annulus fibrosus tissues.

Impact of Digestive Inflammatory Environment and Genipin Crosslinking on Immunomodulatory Capacity of Injectable Musculoskeletal Tissue Scaffold

The paracrine and autocrine processes of the host response play an integral role in the success of scaffold-based tissue regeneration. Recently, the immunomodulatory scaffolds have received huge attention for modulating inflammation around the host tissue through releasing anti-inflammatory cytokine. However, controlling the inflammation and providing a sustained release of anti-inflammatory cytokine from the scaffold in the digestive inflammatory environment are predicated upon a comprehensive understanding of three fundamental questions. (1) How does the release rate of cytokine from the scaffold change in the digestive inflammatory environment? (2) Can we prevent the premature scaffold degradation and burst release of the loaded cytokine in the digestive inflammatory environment? (3) How does the scaffold degradation prevention technique affect the immunomodulatory capacity of the scaffold? This study investigated the impacts of the digestive inflammatory environment on scaffold degradation and how pre-mature degradation can be prevented using genipin crosslinking and how genipin crosslinking affects the interleukin-4 (IL-4) release from the scaffold and differentiation of naïve macrophages (M0). Our results demonstrated that the digestive inflammatory environment (DIE) attenuates protein retention within the scaffold. Over 14 days, the encapsulated protein released 46% more in DIE than in phosphate buffer saline (PBS), which was improved through genipin crosslinking. We have identified the 0.5 (w/v) genipin concentration as an optimal concentration for improved IL-4 released from the scaffold, cell viability, mechanical strength, and scaffold porosity, and immunomodulation studies. The IL-4 released from the injectable scaffold could differentiate naïve macrophages to an anti-inflammatory (M2) lineage; however, upon genipin crosslinking, the immunomodulatory capacity of the scaffold diminished significantly, and pro-inflammatory markers were expressed dominantly.

High Throughput Screening of Cell Mechanical Response Using a Stretchable 3D Cellular Microarray Platform

Cells in vivo are constantly subjected to multiple microenvironmental mechanical stimuli that regulate cell function. Although 2D cell responses to the mechanical stimulation have been established, these methods lack relevance as physiological cell microenvironments are in 3D. Moreover, the existing platforms developed for studying the cell responses to mechanical cues in 3D either offer low-throughput, involve complex fabrication, or do not allow combinatorial analysis of multiple cues. Considering this, a stretchable high-throughput (HT) 3D cell microarray platform is presented that can apply dynamic mechanical strain to cells encapsulated in arrayed 3D microgels. The platform uses inkjet-bioprinting technique for printing cell-laden gelatin methacrylate (GelMA) microgel array on an elastic composite substrate that is periodically stretched. The developed platform is highly biocompatible and transfers the applied strain from the stretched substrate to the cells. The HT analysis is conducted to analyze cell mechano-responses throughout the printed microgel array. Also, the combinatorial analysis of distinct cell behaviors is conducted for different GelMA microenvironmental stiffnesses in addition to the dynamic stretch. Considering its throughput and flexibility, the developed platform can readily be scaled up to introduce a wide range of microenvironmental cues and to screen the cell responses in a HT way.

Bioreactor With Electrically Deformable Curved Membranes for Mechanical Stimulation of Cell Cultures

Physiologically relevant in vitro models of stretchable biological tissues, such as muscle, lung, cardiac and gastro-intestinal tissues, should mimic the mechanical cues which cells are exposed to in their dynamic microenvironment in vivo. In particular, in order to mimic the mechanical stimulation of tissues in a physiologically relevant manner, cell stretching is often desirable on surfaces with dynamically controllable curvature. Here, we present a device that can deform cell culture membranes without the current need for external pneumatic/fluidic or electrical motors, which typically make the systems bulky and difficult to operate. We describe a modular device that uses elastomeric membranes, which can intrinsically be deformed by electrical means, producing a dynamically tuneable curvature. This approach leads to compact, self-contained, lightweight and versatile bioreactors, not requiring any additional mechanical equipment. This was obtained via a special type of dielectric elastomer actuator. The structure, operation and performance of early prototypes are described, showing preliminary evidence on their ability to induce changes on the spatial arrangement of the cytoskeleton of fibroblasts dynamically stretched for 8 h.

Mechano-Immunomodulation: Mechanoresponsive Changes in Macrophage Activity and Polarization

In recent years, biomaterial- and scaffold-based immunomodulation strategies were implemented in tissue regeneration efforts for manipulating macrophage polarization (a.k.a. phenotype or lineage commitment, or differentiation). Yet, most of our understanding of macrophage phenotype commitment and phagocytic capacity is limited to how physical cues (extracellular matrix stiffness, roughness, and topography) and soluble chemical cues (cytokines and chemokines released from the scaffold) influence macrophage polarization. In the context of immune response-tissue interaction, the mechanical cues experienced by the residing cells within the tissue also play a critical role in macrophage polarization and inflammatory response. However, there is no compiled study discussing the effect of the dynamic mechanical environment around the tissues on macrophage polarization and the innate immune response. The aim of this comprehensive review paper is 2-fold; (a) to highlight the importance of mechanical cues on macrophage lineage commitment and function and (b) to summarize the important studies dedicated to understand how macrophage polarization changes with different mechanical loading modalities. For the first time, this review paper compiles and compartmentalizes the studies investigating the role of dynamic mechanical loading with various modalities, amplitude, and frequency on macrophage differentiation. A deeper understanding of macrophage phenotype in mechanically dominant tissues (i.e. musculoskeletal tissues, lung tissues, and cardiovascular tissues) provides mechanistic insights into the design of mechano-immunomodulatory tissue scaffold for tissue regeneration.

Interconnectable Dynamic Compression Bioreactors for Combinatorial Screening of Cell Mechanobiology in Three Dimensions

Biophysical cues can potently direct a cell's or tissue's behavior. Cells interpret their biophysical surroundings, such as matrix stiffness or dynamic mechanical stimulation, through mechanotransduction. However, our understanding of the various aspects of mechanotransduction has been limited by the lack of proper analysis platforms capable of screening three-dimensional (3D) cellular behaviors in response to biophysical cues. Here, we developed a dynamic compression bioreactor to study the combinational effects of biomaterial composition and dynamic mechanical compression on cellular behavior in 3D hydrogels. The bioreactor contained multiple actuating posts that could apply cyclic compressive strains ranging from 0 to 42% to arrays of cell-encapsulated hydrogels. The bioreactor could be interconnected with other compressive bioreactors, which enabled the combinatorial screenings of 3D cellular behaviors simultaneously. As an application of the screening platform, cell spreading, and osteogenic differentiation of human mesenchymal stem cells (hMSCs) were characterized in 3D gelatin methacryloyl (GelMA) hydrogels. Increasing hydrogel concentration from 5 to 10% restricted the cell spreading, however, dynamic compressive strain increased cell spreading. Osteogenic differentiation of hMSCs was also affected by dynamic compressive strains. hMSCs in 5% GelMA hydrogel were more sensitive to strains, and the 42% strain group showed a significant increase in osteogenic differentiation compared to other groups. The interconnectable dynamic compression bioreactor provides an efficient way to study the interactions of cells and their physical microenvironments in three dimensions.

Equiaxial Strain Modulates Adipose-derived Stem Cell Differentiation within 3D Biphasic Scaffolds towards Annulus Fibrosus

Recurrence of intervertebral disc (IVD) herniation is the most important factor leading to chronic low back pain and subsequent disability after discectomy. Efficacious annulus fibrosus (AF) repair strategy that delivers cells and biologics to IVD injury site is needed to limit the progression of disc degeneration and promote disc self-regeneration capacities after discectomy procedures. In this study, a biphasic mechanically-conditioned scaffold encapsulated with human adipose-derived stem cells (ASCs) is studied as a potential treatment strategy for AF defects. Equiaxial strains and frequencies were applied to ASCs-encapsulated scaffolds to identify the optimal loading modality to induce AF differentiation. Equiaxial loading resulted in 2–4 folds increase in secretion of extracellular matrix proteins and the reorganization of the matrix fibers and elongations of the cells along the load direction. Further, the equiaxial load induced region-specific differentiation of ASCs within the inner and outer regions of the biphasic scaffolds. Gene expression of AF markers was upregulated with 5–30 folds within the equiaxially loaded biphasic scaffolds compared to unstrained samples. The results suggest that there is a specific value of equiaxial strain favorable to differentiate ASCs towards AF lineage and that ASCs-embedded biphasic scaffold can potentially be utilized to repair the AF defects.

Creating homogenous strain distribution within 3D cell-encapsulated constructs using a simple and cost-effective uniaxial tensile bioreactor: Design and validation study

Mechanical loading bioreactors capable of applying uniaxial tensile strains are emerging to be a valuable tool to investigate physiologically relevant cellular signaling pathways and biochemical expression. In this study, we have introduced a simple and cost-effective uniaxial tensile strain bioreactor for the application of precise and homogenous uniaxial strains to 3D cell-encapsulated collagen constructs at physiological loading strains (0-12%) and frequencies (0.01-1 Hz). The bioreactor employs silicone-based loading chambers specifically designed to stretch constructs without direct gripping to minimize stress concentration at the ends of the construct and preserve its integrity. The loading chambers are driven by a versatile stepper motor ball-screw actuation system to produce stretching of the constructs. Mechanical characterization of the bioreactor performed through Finite Element Analysis demonstrated that the constructs experienced predominantly uniaxial tensile strain in the longitudinal direction. The strains produced were found to be homogenous over a 15 × 4 × 2 mm region of the construct equivalent to around 60% of the effective region of characterization. The strain values were also shown to be consistent and reproducible during cyclic loading regimes. Biological characterization confirmed the ability of the bioreactor to promote cell viability, proliferation, and matrix organization of cell-encapsulated collagen constructs. This easy-to-use uniaxial tensile strain bioreactor can be employed for studying morphological, structural, and functional responses of cell-embedded matrix systems in response to physiological loading of musculoskeletal tissues. It also holds promise for tissue-engineered strategies that involve delivery of mechanically stimulated cells at the site of injury through a biological carrier to develop a clinically useful therapy for tissue healing. Biotechnol. Bioeng. 2017;114: 1878-1887. © 2017 Wiley Periodicals, Inc.

Predicting cell viability within tissue scaffolds under equiaxial strain: multi-scale finite element model of collagen-cardiomyocytes constructs

Successful tissue engineering and regenerative therapy necessitate having extensive knowledge about mechanical milieu in engineered tissues and the resident cells. In this study, we have merged two powerful analysis tools, namely finite element analysis and stochastic analysis, to understand the mechanical strain within the tissue scaffold and residing cells and to predict the cell viability upon applying mechanical strains. A continuum-based multi-length scale finite element model (FEM) was created to simulate the physiologically relevant equiaxial strain exposure on cell-embedded tissue scaffold and to calculate strain transferred to the tissue scaffold (macro-scale) and residing cells (micro-scale) upon various equiaxial strains. The data from FEM were used to predict cell viability under various equiaxial strain magnitudes using stochastic damage criterion analysis. The model validation was conducted through mechanically straining the cardiomyocyte-encapsulated collagen constructs using a custom-built mechanical loading platform (EQUicycler). FEM quantified the strain gradients over the radial and longitudinal direction of the scaffolds and the cells residing in different areas of interest. With the use of the experimental viability data, stochastic damage criterion, and the average cellular strains obtained from multi-length scale models, cellular viability was predicted and successfully validated. This methodology can provide a great tool to characterize the mechanical stimulation of bioreactors used in tissue engineering applications in providing quantification of mechanical strain and predicting cellular viability variations due to applied mechanical strain.