Effects of Geometry on the Mechanics and Alignment of Three-Dimensional Engineered Microtissues

Design optimization of geometrically confined cardiac organoids enabled by machine learning techniques

Stem cell organoids are powerful models for studying organ development, disease modeling, drug screening, and regenerative medicine applications. The convergence of organoid technology, tissue engineering, and artificial intelligence (AI) could potentially enhance our understanding of the design principles for organoid engineering. In this study, we utilized micropatterning techniques to create a designer library of 230 cardiac organoids with 7 geometric designs. We employed manifold learning techniques to analyze single organoid heterogeneity based on 10 physiological parameters. We clustered and refined the cardiac organoids based on their functional similarity using unsupervised machine learning approaches, thus elucidating unique functionalities associated with geometric designs. We also highlighted the critical role of calcium transient rising time in distinguishing organoids based on geometric patterns and clustering results. This integration of organoid engineering and machine learning enhances our understanding of structure-function relationships in cardiac organoids, paving the way for more controlled and optimized organoid design.

Steering cell orientation through light-based spatiotemporal modulation of the mechanical environment

The anisotropic organization of cells and the extracellular matrix (ECM) is essential for the physiological function of numerous biological tissues, including the myocardium. This organization changes gradually in space and time, during disease progression such as myocardial infarction. The role of mechanical stimuli has been demonstrated to be essential in obtaining, maintaining and de-railing this organization, but the underlying mechanisms are scarcely known. To enable the study of the mechanobiological mechanisms involved,in vitrotechniques able to spatiotemporally control the multiscale tissue mechanical environment are thus necessary. Here, by using light-sensitive materials combined with light-illumination techniques, we fabricated 2D and 3Din vitromodel systems exposing cells to multiscale, spatiotemporally resolved stiffness anisotropies. Specifically, spatial stiffness anisotropies spanning from micron-sized (cellular) to millimeter-sized (tissue) were achieved. Moreover, the light-sensitive materials allowed to introduce the stiffness anisotropies at defined timepoints (hours) after cell seeding, facilitating the study of their temporal effects on cell and tissue orientation. The systems were tested using cardiac fibroblasts (cFBs), which are known to be crucial for the remodeling of anisotropic cardiac tissue. We observed that 2D stiffness micropatterns induced cFBs anisotropic alignment, independent of the stimulus timing, but dependent on the micropattern spacing. cFBs exhibited organized alignment also in response to 3D stiffness macropatterns, dependent on the stimulus timing and temporally followed by (slower) ECM co-alignment. In conclusion, the developed model systems allow improved fundamental understanding of the underlying mechanobiological factors that steer cell and ECM orientation, such as stiffness guidance and boundary constraints.

Structural maturation of myofilaments in engineered 3D cardiac microtissues characterized using small angle x-ray scattering

Understanding the structural and functional development of human-induced pluripotent stem-cell-derived cardiomyocytes (hiPSC-CMs) is essential to engineering cardiac tissue that enables pharmaceutical testing, modeling diseases, and designing therapies. Here we use a method not commonly applied to biological materials, small angle x-ray scattering, to characterize the structural development of hiPSC-CMs within three-dimensional engineered tissues during their preliminary stages of maturation. An x-ray scattering experimental method enables the reliable characterization of the cardiomyocyte myofilament spacing with maturation time. The myofilament lattice spacing monotonically decreases as the tissue matures from its initial post-seeding state over the span of 10 days. Visualization of the spacing at a grid of positions in the tissue provides an approach to characterizing the maturation and organization of cardiomyocyte myofilaments and has the potential to help elucidate mechanisms of pathophysiology, and disease progression, thereby stimulating new biological hypotheses in stem cell engineering.

Geometry and length control of 3D engineered heart tissues using direct laser writing

Geometry and mechanical characteristics of the environment surrounding the Engineered Heart Tissues (EHT) affect their structure and function. Here, we employed a 3D tissue culture platform fabricated using two-photon direct laser writing with a high degree of accuracy to control parameters that are relevant to EHT maturation. Using this platform, we first explore the effects of geometry based on two distinct shapes: a rectangular seeding well with two attachment sites, and a stadium-like seeding well with six attachment sites that are placed symmetrically along hemicylindrical membranes. The former geometry promotes uniaxial contraction of the tissues; the latter additionally induces diagonal fiber alignment. We systematically increase the length of the seeding wells for both configurations and observe a positive correlation between fiber alignment at the center of the EHTs and tissue length. With increasing length, an undesirable thinning and "necking" also emerge, leading to the failure of longer tissues over time. In the second step, we optimize the stiffness of the seeding wells and modify some of the attachment sites of the platform and the seeding parameters to achieve tissue stability for each length and geometry. Furthermore, we use the platform for electrical pacing and calcium imaging to evaluate the functional dynamics of EHTs as a function of frequency.

Hydrogel-Assisted Double Molding Enables Rapid Replication of Stereolithographic 3D Prints for Engineered Tissue Design

Tissue-engineered in vitro models are an essential tool in biomedical research. Tissue geometry is a key determinant of function, but controlling the geometry of microscale tissues remains challenging. Additive manufacturing approaches have emerged as a promising means for rapid and iterative changes in the geometry of microdevices. However, it has been shown that poly(dimethylsiloxane) (PDMS) cross-linking is often inhibited at the interface of materials printed with stereolithography. While approaches to replica mold stereolithographic three-dimensional (3D) prints have been described, these methods are inconsistent and often lead to print destruction when unsuccessful. Additionally, 3D-printed materials often leach toxic chemicals into directly molded PDMS. Here, we developed a double molding approach that allows precise replication of high-resolution stereolithographic prints into poly(dimethylsiloxane) (PDMS) elastomer, facilitating rapid design iterations and highly parallelized sample production. Inspired by lost wax casting, we used hydrogels as intermediary molds to transfer high-resolution features from high-resolution 3D prints into PDMS, while previously published work focused on enabling direct molding of PDMS onto 3D prints through the use of coatings and post-cross-linking treatments of the 3D print itself. Hydrogel mechanical properties, including cross-link density, predict replication fidelity. We demonstrate the ability of this approach to replicate a variety of shapes that would be impossible to create using photolithography techniques traditionally used to create engineered tissue designs. This method also enabled the replication of 3D-printed features into PDMS that would not be possible with direct molding as the stiffness of these materials leads to material fracture when unmolding, while the increased toughness in the hydrogels can elastically deform around complex features and maintain replication fidelity. Finally, we highlight the ability of this method to minimize the potential for toxic materials to transfer from the original 3D print into the PDMS replica, enhancing its use for biological applications. This minimization of the transfer of toxic materials has not been reported in other previously reported methods describing replication of 3D prints into PDMS, and we demonstrate its use through the creation of stem cell-derived microheart muscles. This method can also be used in future studies to understand the effects of geometry on engineered tissues and their constitutive cells.

Membrane Remodeling of Human-Engineered Cardiac Tissue by Chronic Electric Stimulation

Human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) show immature features, but these are improved by integration into 3D cardiac constructs. In addition, it has been demonstrated that physical manipulations such as electrical stimulation (ES) are highly effective in improving the maturation of human-engineered cardiac tissue (hECT) derived from hiPSC-CMs. Here, we continuously applied an ES in capacitive coupling configuration, which is below the pacing threshold, to millimeter-sized hECTs for 1-2 weeks. Meanwhile, the structural and functional developments of the hECTs were monitored and measured using an array of assays. Of particular note, a nanoscale imaging technique, scanning ion conductance microscopy (SICM), has been used to directly image membrane remodeling of CMs at different locations on the tissue surface. Periodic crest/valley patterns with a distance close to the sarcomere length appeared on the membrane of CMs near the edge of the tissue after ES, suggesting the enhanced transverse tubulation network. The SICM observation is also supported by the fluorescence images of the transverse tubulation network and α-actinin. Correspondingly, essential cardiac functions such as calcium handling and contraction force generation were improved. Our study provides evidence that chronic subthreshold ES can still improve the structural and functional developments of hECTs.

Fibroblast clearance of damaged tissue following laser ablation in engineered microtissues

Although the mechanisms underlying wound healing are largely preserved across wound types, the method of injury can affect the healing process. For example, burn wounds are more likely to undergo hypertrophic scarring than are lacerations, perhaps due to the increased underlying damage that needs to be cleared. This tissue clearance is thought to be mainly managed by immune cells, but it is unclear if fibroblasts contribute to this process. Herein, we utilize a 3D in vitro model of stromal wound healing to investigate the differences between two modes of injury: laceration and laser ablation. We demonstrate that laser ablation creates a ring of damaged tissue around the wound that is cleared by fibroblasts prior to wound closure. This process is dependent on ROCK and dynamin activity, suggesting a phagocytic or endocytic process. Transmission electron microscopy of fibroblasts that have entered the wound area reveals large intracellular vacuoles containing fibrillar extracellular matrix. These results demonstrate a new model to study matrix clearance by fibroblasts in a 3D soft tissue. Because aberrant wound healing is thought to be caused by an imbalance between matrix degradation and production, this model, which captures both aspects, will be a valuable addition to the study of wound healing.

Light-driven biological actuators to probe the rheology of 3D microtissues

The mechanical properties of biological tissues are key to their physical integrity and function. Although external loading or biochemical treatments allow the estimation of these properties globally, it remains difficult to assess how such external stimuli compare with cell-generated contractions. Here we engineer microtissues composed of optogenetically-modified fibroblasts encapsulated within collagen. Using light to control the activity of RhoA, a major regulator of cellular contractility, we induce local contractions within microtissues, while monitoring microtissue stress and strain. We investigate the regulation of these local contractions and their spatio-temporal distribution. We demonstrate the potential of our technique for quantifying tissue elasticity and strain propagation, before examining the possibility of using light to create and map local anisotropies in mechanically heterogeneous microtissues. Altogether, our results open an avenue to guide the formation of tissues while non-destructively charting their rheology in real time, using their own constituting cells as internal actuators.

Integrating mechanical sensor readouts into organ-on-a-chip platforms

Organs-on-a-chip have emerged as next-generation tissue engineered models to accurately capture realistic human tissue behaviour, thereby addressing many of the challenges associated with using animal models in research. Mechanical features of the culture environment have emerged as being critically important in designing organs-on-a-chip, as they play important roles in both stimulating realistic tissue formation and function, as well as capturing integrative elements of homeostasis, tissue function, and tissue degeneration in response to external insult and injury. Despite the demonstrated impact of incorporating mechanical cues in these models, strategies to measure these mechanical tissue features in microfluidically-compatible formats directly on-chip are relatively limited. In this review, we first describe general microfluidically-compatible Organs-on-a-chip sensing strategies, and categorize these advances based on the specific advantages of incorporating them on-chip. We then consider foundational and recent advances in mechanical analysis techniques spanning cellular to tissue length scales; and discuss their integration into Organs-on-a-chips for more effective drug screening, disease modeling, and characterization of biological dynamics.

Remodeling of Architected Mesenchymal Microtissues Generated on Mechanical Metamaterials

Mechanical metamaterials constitute a nascent category of architected structures comprising arranged periodic components with tailored geometrical features. These materials are now being employed as advanced medical implants due to their extraordinary mechanical properties over traditional devices. Nevertheless, to achieve desired tissue integration and regeneration, it is critical to study how the microarchitecture affects interactions between metamaterial scaffolds and living biological tissues. Based on human induced pluripotent stem cell technology and multiphoton lithography, we report the establishment of an in vitro microtissue model to study the integration and remodeling of human mesenchymal tissues on metamaterial scaffolds with different unit geometries. Microtissues showed distinct tissue morphologies and cellular behaviors between architected octet-truss and bowtie structures. Under the active force generated from mesenchymal tissues, the octet-truss and bowtie metamaterial scaffolds demonstrated unique instability phenomena, significantly different from uniform loading using conventional mechanical testing.

Programming Cellular Alignment in Engineered Cardiac Tissue via Bioprinting Anisotropic Organ Building Blocks

The ability to replicate the 3D myocardial architecture found in human hearts is a grand challenge. Here, the fabrication of aligned cardiac tissues via bioprinting anisotropic organ building blocks (aOBBs) composed of human induced pluripotent stem cell derived cardiomyocytes (hiPSC-CMs) is reported. A bioink composed of contractile cardiac aOBBs is first generated and aligned cardiac tissue sheets with linear, spiral, and chevron features are printed. Next, aligned cardiac macrofilaments are printed, whose contractile force and conduction velocity increase over time and exceed the performance of spheroid-based cardiac tissues. Finally, the ability to spatially control the magnitude and direction of contractile force by printing cardiac sheets with different aOBB alignment is highlighted. This research opens new avenues to generating functional cardiac tissue with high cell density and complex cellular alignment.

Hipster microcarriers: exploring geometrical and topographical cues of non-spherical microcarriers in biomedical applications

Structure and organisation are key aspects of the native tissue environment, which ultimately condition cell fate via a myriad of processes, including the activation of mechanotransduction pathways. By modulating the formation of integrin-mediated adhesions and consequently impacting cell contractility, engineered geometrical and topographical cues may be introduced to activate downstream signalling and ultimately control cell morphology, proliferation, and differentiation. Microcarriers appear as attractive vehicles for cell-based tissue engineering strategies aiming to modulate this 3D environment, but also as vehicles for cell-free applications, given the ease in tuning their chemical and physical properties. In this review, geometry and topography are highlighted as two preponderant features in actively regulating interactions between cells and the extracellular matrix. While most studies focus on the 2D environment, we focus on how the incorporation of these strategies in 3D systems could be beneficial. The techniques applied to design 3D microcarriers with unique geometries and surface topographical cues are covered, as well as specific tissue engineering approaches employing these microcarriers. In fact, successfully achieving a functional histoarchitecture may depend on a combination of fine-tuned geometrically shaped microcarriers presenting intricately tailored topographical cues. Lastly, we pinpoint microcarrier geometry as a key player in cell-free biomaterial-based strategies, and its impact on drug release kinetics, the production of steerable microcarriers to target tumour cells, and as protein or antibody biosensors.

Controlling Mesenchyme Tissue Remodeling via Spatial Arrangement of Mechanical Constraints

Tissue morphogenetic remodeling plays an important role in tissue repair and homeostasis and is often governed by mechanical stresses. In this study, we integrated an in vitro mesenchymal tissue experimental model with a volumetric contraction-based computational model to investigate how geometrical designs of tissue mechanical constraints affect the tissue remodeling processes. Both experimental data and simulation results verified that the standing posts resisted the bulk contraction of the tissues, leading to tissue thinning around the posts as gap extension and inward remodeling at the edges as tissue compaction. We changed the geometrical designs for the engineered mesenchymal tissues with different shapes of posts arrangements (triangle vs. square), different side lengths (6 mm vs. 8 mm), and insertion of a center post. Both experimental data and simulation results showed similar trends of tissue morphological changes of significant increase of gap extension and deflection compaction with larger tissues. Additionally, insertion of center post changed the mechanical stress distribution within the tissues and stabilized the tissue remodeling. This experimental-computational integrated model can be considered as a promising initiative for future mechanistic understanding of the relationship between mechanical design and tissue remodeling, which could possibly provide design rationale for tissue stability and manufacturing.

Scaffold geometry modulation of mechanotransduction and its influence on epigenetics

The dynamics of cell mechanics and epigenetic signatures direct cell behaviour and fate, thus influencing regenerative outcomes. In recent years, the utilisation of 2D geometric (i.e. square, circle, hexagon, triangle or round-shaped) substrates for investigating cell mechanics in response to the extracellular microenvironment have gained increasing interest in regenerative medicine due to their tunable physicochemical properties. In contrast, there is relatively limited knowledge of cell mechanobiology and epigenetics in the context of 3D biomaterial matrices, i.e., hydrogels and scaffolds. Scaffold geometry provides biophysical signals that trigger a nucleus response (regulation of gene expression) and modulates cell behaviour and function. In this review, we explore the potential of additive manufacturing to incorporate multi length-scale geometry features on a scaffold. Then, we discuss how scaffold geometry direct cell and nuclear mechanosensing. We further discuss how cell epigenetics, particularly DNA/histone methylation and histone acetylation, are modulated by scaffold features that lead to specific gene expression and ultimately influence the outcome of tissue regeneration. Overall, we highlight that geometry of different magnitude scales can facilitate the assembly of cells and multicellular tissues into desired functional architectures through the mechanotransduction pathway. Moving forward, the challenge confronting biomedical engineers is the distillation of the vast knowledge to incorporate multiscaled geometrical features that would collectively elicit a favourable tissue regeneration response by harnessing the design flexibility of additive manufacturing. STATEMENT OF SIGNIFICANCE: It is well-established that cells sense and respond to their 2D geometric microenvironment by transmitting extracellular physiochemical forces through the cytoskeleton and biochemical signalling to the nucleus, facilitating epigenetic changes such as DNA methylation, histone acetylation, and microRNA expression. In this context, the current review presents a unique perspective and highlights the importance of 3D architectures (dimensionality and geometries) on cell and nuclear mechanics and epigenetics. Insight into current challenges around the study of mechanobiology and epigenetics utilising additively manufactured 3D scaffold geometries will progress biomaterials research in this space.

Revisiting tissue tensegrity: Biomaterial-based approaches to measure forces across length scales

Cell-generated forces play a foundational role in tissue dynamics and homeostasis and are critically important in several biological processes, including cell migration, wound healing, morphogenesis, and cancer metastasis. Quantifying such forces in vivo is technically challenging and requires novel strategies that capture mechanical information across molecular, cellular, and tissue length scales, while allowing these studies to be performed in physiologically realistic biological models. Advanced biomaterials can be designed to non-destructively measure these stresses in vitro, and here, we review mechanical characterizations and force-sensing biomaterial-based technologies to provide insight into the mechanical nature of tissue processes. We specifically and uniquely focus on the use of these techniques to identify characteristics of cell and tissue “tensegrity:” the hierarchical and modular interplay between tension and compression that provide biological tissues with remarkable mechanical properties and behaviors. Based on these observed patterns, we highlight and discuss the emerging role of tensegrity at multiple length scales in tissue dynamics from homeostasis, to morphogenesis, to pathological dysfunction.

Bio-mimicked guided tissue regeneration/guided bone regeneration membranes with hierarchical structured surfaces replicated from teak leaf exhibits enhanced bioactivity

Bio-mimicked GTR/GBR membranes with hierarchical structured surfaces were developed by direct and indirect replication of teak leaf surface. The membranes were fabricated using solvent casting method with customized templates. The surfaces obtained were those with micro-trichomes (MTS) and micro-depression (MDS) that resembled a whorling pattern. Structural details of the fabricated membrane surfaces were studied under stereomicroscope and scanning electron microscopy. Surface roughness, water wetting angle, water uptake, and degradation properties of the membranes were examined. The effects of the micro-patterned hierarchical structure on in vitro bioactivities of human osteoblast-like cells (MG63) and human gingival fibroblast cells HGF1-RT1 were studied. In vivo study carried out on rat skulls to assess the response of surrounding tissues for 4 weeks showed that the bio-mimicked MTS and MDS membrane surfaces enhanced the cell proliferation. The proliferation significantly increased with increasing surface roughness and decreasing contact angle. There was also an evidence of rapid new bone maturation with membranes with MTS. It is thus suggested that the teak leaf mimicked whorling patterned hierarchical structured surface is an important design for enhancing bioactivity.

Extracellular Matrix Alignment Directs Provisional Matrix Assembly and Three Dimensional Fibrous Tissue Closure

Gap closure is a dynamic process in wound healing, in which a wound contracts and a provisional matrix is laid down, to restore structural integrity to injured tissues. The efficiency of wound closure has been found to depend on the shape of a wound, and this shape dependence has been echoed in various in vitro studies. While wound shape itself appears to contribute to this effect, it remains unclear whether the alignment of the surrounding extracellular matrix (ECM) may also contribute. In this study, we investigate the role both wound curvature and ECM alignment have on gap closure in a 3D culture model of fibrous tissue. Using microfabricated flexible micropillars positioned in rectangular and octagonal arrangements, seeded 3T3 fibroblasts embedded in a collagen matrix formed microtissues with different ECM alignments. Wounding these microtissues with a microsurgical knife resulted in wounds with different shapes and curvatures that closed at different rates. Observing different regions around the wounds, we noted local wound curvature did not impact the rate of production of provisional fibronectin matrix assembled by the fibroblasts. Instead, the rate of provisional matrix assembly was lowest emerging from regions of high fibronectin alignment and highest in the areas of low matrix alignment. Our data suggest that the underlying ECM structure affects the shape of the wound as well as the ability of fibroblasts to build provisional matrix, an important step in the process of tissue closure and restoration of tissue architecture. The study highlights an important interplay between ECM alignment, wound shape, and tissue healing that has not been previously recognized and may inform approaches to engineer tissues.

Direct laser writing for cardiac tissue engineering: a microfluidic heart on a chip with integrated transducers

We have developed a microfluidic platform for engineering cardiac microtissues in highly-controlled microenvironments. The platform is fabricated using direct laser writing (DLW) lithography and soft lithography, and contains four separate devices. Each individual device houses a cardiac microtissue and is equipped with an integrated strain actuator and a force sensor. Application of external pressure waves to the platform results in controllable time-dependent forces on the microtissues. Conversely, oscillatory forces generated by the microtissues are transduced into measurable electrical outputs. We demonstrate the capabilities of this platform by studying the response of cardiac microtissues derived from human induced pluripotent stem cells (hiPSC) under prescribed mechanical loading and pacing. This platform will be used for fundamental studies and drug screening on cardiac microtissues.

Biomaterial Properties Modulating Bone Regeneration

Biomaterial scaffolds have been gaining momentum in the past several decades for their potential applications in the area of tissue engineering. They function as three-dimensional porous constructs to temporarily support the attachment of cells, subsequently influencing cell behaviors such as proliferation and differentiation to repair or regenerate defective tissues. In addition, scaffolds can also serve as delivery vehicles to achieve sustained release of encapsulated growth factors or therapeutic agents to further modulate the regeneration process. Given the limitations of current bone grafts used clinically in bone repair, alternatives such as biomaterial scaffolds have emerged as potential bone graft substitutes. This review summarizes how physicochemical properties of biomaterial scaffolds can influence cell behavior and its downstream effect, particularly in its application to bone regeneration.

Engineering Biomaterials and Approaches for Mechanical Stretching of Cells in Three Dimensions

Mechanical stretch is widely experienced by cells of different tissues in the human body and plays critical roles in regulating their behaviors. Numerous studies have been devoted to investigating the responses of cells to mechanical stretch, providing us with fruitful findings. However, these findings have been mostly observed from two-dimensional studies and increasing evidence suggests that cells in three dimensions may behave more closely to their in vivo behaviors. While significant efforts and progresses have been made in the engineering of biomaterials and approaches for mechanical stretching of cells in three dimensions, much work remains to be done. Here, we briefly review the state-of-the-art researches in this area, with focus on discussing biomaterial considerations and stretching approaches. We envision that with the development of advanced biomaterials, actuators and microengineering technologies, more versatile and predictive three-dimensional cell stretching models would be available soon for extensive applications in such fields as mechanobiology, tissue engineering, and drug screening.

Guiding Cell Network Assembly using Shape-Morphing Hydrogels

Forces and relative movement between cells and extracellular matrix (ECM) are crucial to the self-organization of tissues during development. However, the spatial range over which these dynamics can be controlled in engineering approaches is limited, impeding progress toward the construction of large, structurally mature tissues. Herein, shape-morphing materials called "kinomorphs" that rationally control the shape and size of multicellular networks are described. Kinomorphs are sheets of ECM that change their shape, size, and density depending on patterns of cell contractility within them. It is shown that these changes can manipulate structure-forming behaviors of epithelial cells in many spatial locations at once. Kinomorphs are built using a new photolithographic technology to pattern single cells into ECM sheets that are >10× larger than previously described. These patterns are designed to partially mimic the branch geometry of the embryonic kidney epithelial network. Origami-inspired simulations are then used to predict changes in kinomorph shapes. Last, kinomorph dynamics are shown to provide a centimeter-scale program that sets specific spatial locations in which ≈50 µm-diameter epithelial tubules form by cell coalescence and structural maturation. The kinomorphs may significantly advance organ-scale tissue construction by extending the spatial range of cell self-organization in emerging model systems such as organoids.