Cellular responses to beating hydrogels to investigate mechanotransduction

Advances and Trends of Photoresponsive Hydrogels for Bone Tissue Engineering

The development of static hydrogels as an optimal choice for bone tissue engineering (BTE) remains a difficult challenge primarily due to the intricate nature of bone healing processes, continuous physiological functions, and pathological changes. Hence, there is an urgent need to exploit smart hydrogels with programmable properties that can effectively enhance bone regeneration. Increasing evidence suggests that photoresponsive hydrogels are promising bioscaffolds for BTE due to their advantages such as controlled drug release, cell fate modulation, and the photothermal effect. Here, we review the current advances in photoresponsive hydrogels. The mechanism of photoresponsiveness and its advanced applications in bone repair are also elucidated. Future research would focus on the development of more efficient, safer, and smarter photoresponsive hydrogels for BTE. This review is aimed at offering comprehensive guidance on the trends of photoresponsive hydrogels and shedding light on their potential clinical application in BTE.

Porous Anisometric PNIPAM Microgels: Tailored Porous Structure and Thermal Response

The porous structure of microgels significantly influences their properties and, thus, their suitability for various applications, in particular as building blocks for tissue scaffolds. Porosity is one of the crucial features for microgel-cell interactions and significantly increases the cells' accumulation and proliferation. Consequently, tailoring the porosity of microgels in an effortless way is important but still challenging, especially for nonspherical microgels. This work presents a straightforward procedure to fabricate complex-shaped poly(N-isopropyl acrylamide) (PNIPAM) microgels with tuned porous structures using the so-called cononsolvency effect during microgel polymerization. Therefore, the classical solvent in the reaction solution is exchanged from water to water-methanol mixtures in a stop-flow lithography process. For cylindrical microgels with a higher methanol content during fabrication, a greater degree of collapsing is observed, and their aspect ratio increases. Furthermore, the collapsing and swelling velocities change with the methanol content, indicating a modified porous structure, which is confirmed by electron microscopy micrographs. Furthermore, swelling patterns of the microgel variants occur during cooling, revealing their thermal response as a highly heterogeneous process. These results show a novel procedure to fabricate PNIPAM microgels of any elongated 2D shape with tailored porous structure and thermoresponsiveness by introducing the cononsolvency effect during stop-flow lithography polymerization.

A multiscale approach to assess thermomechanical performance and force generation in nanorobotic microgels

We present a multiscale approach to characterize the performance of photothermally powered, nanorobotic 3D microgels. Optically triggered nanoactuators, consisting of a gold nanorod core and thermoresponsive pNIPMAM shell, are used as building blocks to generate the nanorobotic 3D microgels. We use microfluidic encapsulation to physically embed the nanoactuators in an alginate network, to form the microgel droplets. The nanoactuators respond to near-infrared light owing to the synergistic effects of plasmonic and thermoresponsive components, and the nanorobotic 3D microgels generate compressive force under the same light stimulus. We use a multiscale approach to characterize this behavior for both the nanoactuators and the assembled microgels via dynamic light scattering and fluorescence microscopy, respectively. A thermoresponsive fluorescent molecule, Rhodamine B, is integrated into alginate chains to monitor the temperature of the microgels (22-59 °C) during actuation at laser intensities up to 6.4 μW μm-2. Our findings show that nanoactuators and the microgels exhibit reversible deformation above the lower critical solution temperature of the thermoresponsive polymer at 42 °C. 785 nm laser light triggers the generation of 2D radial strain in nanoactuators at a maximum of 44%, which translates to an average 2D radial strain of 2.1% in the nanorobotic microgels at 26.4 vol% nanoactuator loading. We then use a semi-experimental approach to quantify the photothermally generated forces in the microgels. Finite element modeling coupled with experimental measurements shows that nanorobotic microgels generate up to 8.5 nN of force over encapsulated single cells. Overall, our method provides a comprehensive approach to characterizing the mechanical performance of nanorobotic hydrogel networks.

The Role of Mechanotransduction in Contact Inhibition of Locomotion and Proliferation

Contact inhibition (CI) represents a crucial tumor-suppressive mechanism responsible for controlling the unbridled growth of cells, thus preventing the formation of cancerous tissues. CI can be further categorized into two distinct yet interrelated components: CI of locomotion (CIL) and CI of proliferation (CIP). These two components of CI have historically been viewed as separate processes, but emerging research suggests that they may be regulated by both distinct and shared pathways. Specifically, recent studies have indicated that both CIP and CIL utilize mechanotransduction pathways, a process that involves cells sensing and responding to mechanical forces. This review article describes the role of mechanotransduction in CI, shedding light on how mechanical forces regulate CIL and CIP. Emphasis is placed on filamin A (FLNA)-mediated mechanotransduction, elucidating how FLNA senses mechanical forces and translates them into crucial biochemical signals that regulate cell locomotion and proliferation. In addition to FLNA, trans-acting factors (TAFs), which are proteins or regulatory RNAs capable of directly or indirectly binding to specific DNA sequences in distant genes to regulate gene expression, emerge as sensitive players in both the mechanotransduction and signaling pathways of CI. This article presents methods for identifying these TAF proteins and profiling the associated changes in chromatin structure, offering valuable insights into CI and other biological functions mediated by mechanotransduction. Finally, it addresses unanswered research questions in these fields and delineates their possible future directions.

In silico analysis shows that dynamic changes in curvature guide cell migration over long distances

In vitro experiments have shown that cell scale curvatures influence cell migration; cells avoid convex hills and settle in concave valleys. However, it is not known whether dynamic changes in curvature can guide cell migration. This study extends a previous in-silico model to explore the effects over time of changing the substrate curvature on cell migration guidance. By simulating a dynamic surface curvature using traveling wave patterns, we investigate the influence of wave height and speed, and find that long-distance cell migration guidance can be achieved on specific wave patterns. We propose a mechanistic explanation of what we call dynamic curvotaxis and highlight those cellular features that may be involved. Our results open a new area of study for understanding cell mobility in dynamic environments, from single-cell in vitro experiments to multi-cellular in vivo mechanisms.

Actuation of Soft Thermoresponsive Hydrogels Mechanically Stimulates Osteogenesis in Human Mesenchymal Stem Cells without Biochemical Factors

Mesenchymal stem cells (MSCs) have the potential to differentiate into multiple lineages and can be harvested relatively easily from adults, making them a promising cell source for regenerative therapies. While it is well-known how to consistently differentiate MSCs into adipose, chondrogenic, and osteogenic lineages by treatment with biochemical factors, the number of studies exploring how to achieve this with mechanical signals is limited. A relatively unexplored area is the effect of cyclic forces on the MSC differentiation. Recently, our group developed a thermoresponsive N-ethyl acrylamide/N-isopropylacrylamide (NIPAM/NEAM) hydrogel supplemented with gold nanorods that are able to convert near-infrared light into heat. Using light pulses allows for local hydrogel collapse and swelling with physiologically relevant force and frequency. In this study, MSCs are cultured on this hydrogel system with a patterned surface and exposed to intermittent or continuous actuation of the hydrogel for 3 days to study the effect of actuation on MSC differentiation. First, cells are harvested from the bone marrow of three donors and tested for their MSC phenotype, meeting the following criteria: the harvested cells are adherent and demonstrate a fibroblast-like bipolar morphology. They lack the expression of CD34 and CD45 but do express CD73, CD90, and CD105. Additionally, their differentiation potential into adipogenic, chondrogenic, and osteogenic lineages is validated by the addition of standardized differentiation media. Next, MSCs are exposed to intermittent or continuous actuation, which leads to a significantly enhanced cell spreading compared to nonactuated cells. Moreover, actuation results in nuclear translocation of Runt-related transcription factor 2 and the Yes-associated protein. Together, these results indicate that cyclic mechanical stimulation on a soft, ridged substrate modulates the MSC fate commitment in the direction of osteogenesis.

Dynamic Stiffening Hydrogel with Instructive Stiffening Timing Modulates Stem Cell Fate In Vitro and Enhances Bone Remodeling In Vivo

Biomechanical stimuli derived from the extracellular matrix (ECM) extremely tune stem cell fate through 3D and spatiotemporal changes in vivo. The matrix stiffness is a crucial factor during bone tissue development. However, most in vitro models to study the osteogenesis of mesenchymal stem cells (MSCs) are static or stiffening in a 2D environment. Here, a dynamic and controllable stiffening 3D biomimetic model is created to regulate the osteogenic differentiation of MSCs with a dual-functional gelatin macromer that can generate a double-network hydrogel by sequential enzymatic and light-triggered crosslinking reactions. The findings show that these dynamic hydrogels allowed cells to spread and expand prior to the secondary crosslinking and to sense high stiffness after stiffening. The MSCs in the dynamic hydrogels, especially the hydrogel stiffened at the late period, present significantly elevated osteogenic ECM secretion, gene expression, and nuclear localization of Yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ). In vivo evaluation of animal experiments further indicates that the enhancement of dynamic stiffening on osteogenesis of MSCs substantially promotes bone remodeling. Consequently, this work reveals that the 3D dynamic stiffening microenvironment as a critical biophysical cue not only mediates the stem cell fate in vitro, but also augments bone restoration in vivo.

Varying the Stiffness and Diffusivity of Rod-Shaped Microgels Independently through Their Molecular Building Blocks

Microgels are water-swollen, crosslinked polymers that are widely used as colloidal building blocks in scaffold materials for tissue engineering and regenerative medicine. Microgels can be controlled in their stiffness, degree of swelling, and mesh size depending on their polymer architecture, crosslink density, and fabrication method-all of which influence their function and interaction with the environment. Currently, there is a lack of understanding of how the polymer composition influences the internal structure of soft microgels and how this morphology affects specific biomedical applications. In this report, we systematically vary the architecture and molar mass of polyethylene glycol-acrylate (PEG-Ac) precursors, as well as their concentration and combination, to gain insight in the different parameters that affect the internal structure of rod-shaped microgels. We characterize the mechanical properties and diffusivity, as well as the conversion of acrylate groups during photopolymerization, in both bulk hydrogels and microgels produced from the PEG-Ac precursors. Furthermore, we investigate cell-microgel interaction, and we observe improved cell spreading on microgels with more accessible RGD peptide and with a stiffness in a range of 20 kPa to 50 kPa lead to better cell growth.

The reversibility of cellular mechano-activation

The cellular microenvironment is highly heterogeneous and dynamic. Therefore, cells must be equipped with molecular tools to adapt and respond to constantly fluctuating inputs. One such input is mechanical force, which activates signalling and regulates cell behaviour in the process of mechanotransduction. Whereas the mechanisms activating mechanotransduction are well studied, the reversibility of this process, whereby cells disassemble and reverse force-activated signalling pathways upon cessation of mechanical stimulation is far less understood. In this review we will outline some of the key experimental techniques to investigate the reversibility of mechanical signalling, and key discoveries arising from them.

Polysaccharides, proteins, and synthetic polymers based multimodal hydrogels for various biomedical applications: A review

Nature-derived or biologically encouraged hydrogels have attracted considerable interest in numerous biomedical applications owing to their multidimensional utility and effectiveness. The internal architecture of a hydrogel network, the chemistry of the raw materials involved, interaction across the interface of counter ions, and the ability to mimic the extracellular matrix (ECM) govern the clinical efficacy of the designed hydrogels. This review focuses on the mechanistic viewpoint of different biologically driven/inspired biomacromolecules that encourages the architectural development of hydrogel networks. In addition, the advantage of hydrogels by mimicking the ECM and the significance of the raw material selection as an indicator of bioinertness is deeply elaborated in the review. Furthermore, the article reviews and describes the application of polysaccharides, proteins, and synthetic polymer-based multimodal hydrogels inspired by or derived from nature in different biomedical areas. The review discusses the challenges and opportunities in biomaterials along with future prospects in terms of their applications in biodevices or functional components for human health issues. This review provides information on the strategy and inspiration from nature that can be used to develop a link between multimodal hydrogels as the main frame and its utility in biomedical applications as the primary target.

Transformative Materials to Create 3D Functional Human Tissue Models In Vitro in a Reproducible Manner

Recreating human tissues and organs in the petri dish to establish models as tools in biomedical sciences has gained momentum. These models can provide insight into mechanisms of human physiology, disease onset, and progression, and improve drug target validation, as well as the development of new medical therapeutics. Transformative materials play an important role in this evolution, as they can be programmed to direct cell behavior and fate by controlling the activity of bioactive molecules and material properties. Using nature as an inspiration, scientists are creating materials that incorporate specific biological processes observed during human organogenesis and tissue regeneration. This article presents the reader with state-of-the-art developments in the field of in vitro tissue engineering and the challenges related to the design, production, and translation of these transformative materials. Advances regarding (stem) cell sources, expansion, and differentiation, and how novel responsive materials, automated and large-scale fabrication processes, culture conditions, in situ monitoring systems, and computer simulations are required to create functional human tissue models that are relevant and efficient for drug discovery, are described. This paper illustrates how these different technologies need to converge to generate in vitro life-like human tissue models that provide a platform to answer health-based scientific questions.

Volume adaptation of neonatal cardiomyocyte spheroids in 3D stiffness gradient GelMA

Present understandings of cardiomyocyte mechanobiology have primarily been developed using 2-dimensional, monocellular cell cultures, however the emergence of 3-dimensional (3D) multicellular cardiac constructs has enabled us to develop more sophisticated recapitulations of the cardiac microenvironment. Several of these strategies have illustrated that incorporating elements of the extracellular matrix (ECM) can promote greater maturation and enhance desirable cardiac functions, such as contractility, but the responses of these cardiac constructs to biophysically aberrant conditions, such as in the post-infarct heart, has remained relatively unexplored. In our study, we employ a stiffness gradient gelatin methacryloyl (GelMA) hydrogel platform to unpack the mechanobiology of cardiac spheroids. We encapsulated neonatal rat cardiac cell spheroids in a 4.4-18.7 kPa linear stiffness gradient up to 120 h. We found the proportion of viable cells within the spheroids increased over time, but the cell number per spheroid decreased. Spheroids expand more in softer matrices while stiffer matrices promote larger nuclei without changing nuclei shape. Volume expansion came primarily from cells expressing vimentin. We did not observe any correlations between stiffness and mechanomarker expression, however we found that after 120 h post-encapsulation, the localization of YAP, the localization of MRTF-A and the expression of Lamin-A was correlated with spheroid morphology. The same trends were not observed 24 h post-encapsulation, indicating that volume adaptation can take a relatively long time. Our data demonstrates that cardiac spheroids are mechanosensitive and that their capacity to respond to ECM-based cues depends on their capacity to adapt their volume with a 3D microenvironment.

Engineering Strategies to Move from Understanding to Steering Renal Tubulogenesis

Rebuilding the kidney in the context of tissue engineering offers a major challenge as the organ is structurally complex and has a high variety of specific functions. Recreation of kidney function is inherently connected to the formation of tubules since the functional subunit of the kidney, the nephron, is based on tubular structures. In vivo, tubulogenesis culminates in a perfectly shaped, patterned, and functional renal tubule via different morphogenic processes that depend on delicately orchestrated chemical, physical, and mechanical interactions between cells and between cells and their microenvironment. This review summarizes the current understanding of the role of the microenvironment in the morphogenic processes involved in in vivo renal tubulogenesis. We highlight the current state-of-the-art of renal tubular engineering and provide a view on the design elements that can be extracted from these studies. Next, we discuss how computational modeling can aid in specifying and identifying design parameters and provide directions on how these design parameters can be incorporated in biomaterials for the purpose of engineering renal tubulogenesis. Finally, we propose that a step-by-step reciprocal interaction between understanding and engineering is necessary to effectively guide renal tubulogenesis. Impact statement Tubular tissue engineering lies at the foundation of regenerating kidney tissue function, as the functional subunit of the kidney, the nephron, is based on tubular structures. Guiding renal tubulogenesis toward functional renal tubules requires in-depth knowledge of the developmental processes that lead to the formation of native tubules as well as engineering approaches to steer these processes. In this study, we review the role of the microenvironment in the developmental processes that lead to functional renal tubules and give directions how this knowledge can be harnessed for biomaterial-based tubular engineering using computational models.

Buckling and Interfacial Deformation of Fluorescent Poly(N-isopropylacrylamide) Microgel Capsules

Hollow microgels are fascinating model systems at the crossover between polymer vesicles, emulsions, and colloids as they deform, interpenetrate, and eventually shrink at higher volume fraction or when subjected to an external stress. Here, we introduce a system consisting of microgels with a micrometer-sized cavity enabling a straightforward characterization in situ using fluorescence microscopy techniques. Similarly to elastic capsules, these systems are found to reversibly buckle above a critical osmotic pressure, conversely to smaller hollow microgels, which were previously reported to deswell at high volume fraction. Simulations performed on monomer-resolved in silico hollow microgels confirm the buckling transition and show that the presented microgels can be described with a thin shell model theory. When brought to an interface, these microgels, that we define as microgel capsules, strongly deform and we thus propose to utilize them to locally probe interfacial properties within a theoretical framework adapted from the Johnson-Kendall-Roberts (JKR) theory. Besides their capability to sense their environment and to address fundamental questions on the elasticity and permeability of microgel systems, microgel capsules can be further envisioned as model systems mimicking anisotropic responsive biological systems such as red blood and epithelial cells thanks to the possibility offered by microgels to be synthesized with custom-designed properties.

Thermally and Magnetically Programmable Hydrogel Microactuators

The rapid development in micro-machinery enabled the investigation of smart materials that can embody fast response, programmable actuation, and flexibility to perform mechanical work. Soft magnetic actuators represent an interesting platform toward combining those properties. This study focuses on the synthesis of micro-actuators that respond to thermal and magnetic stimuli using micro-molding with a soft template as a fabrication technique. These microsystems consist of a hydrogel matrix loaded with anisotropic magnetic nanospindles. When a homogeneous magnetic field is applied, the nanospindles initially dispersed in monomer solution, align and assemble into dipolar chains. The ensuing UV-polymerization creates a network and conveniently arrests these nanostructures. Consequently, the magnetic dipole moment is coplanar with the microgel. Varying the shape, volume, and composition of the micro-actuators during synthesis provides a temperature-dependent control over the magnetic response and the polarizability. Beyond isotropic swelling, shaping the hydrogel as long thin ribbons with a passive layer on one side allows for differential swelling leading to bending and twisting deformations, for example, 2D- or 3D-spiral. These deformations involve a reversible amplification of the magnetic response and orientation of the hydrogels under magnetic field. Temperature control herewith determines the conformation and simultaneously the magnetic response of the micro-actuators.

Reconfigurable scaffolds for adaptive tissue regeneration

Tissue engineering and regenerative medicine have offered promising alternatives for clinical treatment of body tissue traumas, losses, dysfunctions, or diseases, where scaffold-based strategies are particularly popular and effective. Over the decades, scaffolds for tissue regeneration have been remarkably evolving. Nevertheless, conventional scaffolds still confront grand challenges in bio-adaptions in terms of both tissue-scaffold and cell-scaffold interplays, for example complying with complicated three-dimensional (3D) shapes of biological tissues and recapitulating the ordered cell regulation effects of native cell microenvironments. Benefiting from the recent advances in "intelligent" biomaterials, reconfigurable scaffolds have been emerging, demonstrating great promise in addressing the bio-adaption challenges through altering their macro-shapes and/or micro-structures. This mini-review article presents a brief overview of the cutting-edge research on reconfigurable scaffolds, summarizing the materials for forming reconfigurable scaffolds and highlighting their applications for adaptive tissue regeneration. Finally, the challenges and prospects of reconfigurable scaffolds are also discussed, shedding light on the bright future of next-generation reconfigurable scaffolds with upgrading adaptability.

A machine learning based approach for quantitative evaluation of cell migration in Transwell assays based on deformation characteristics

Many pathological and physiological processes, including embryonic development, immune response and cancer metastasis, involve studies on cell migration, and especially detection methods, for which it is difficult to satisfy the requirements for rapid and quantitative evaluation and analysis. In view of the shortcomings in simultaneously quantifying the number of migrated cells and non-migrated cells using Transwell assays, we propose a novelty approach for the evaluation of cell migration by distinguishing whether the cells have migrated based on the regularity of the cell morphology changes. Traditionally, the status of living cells and dead cells are detected and analyzed by machine learning using some common morphological characteristics, e.g., area and perimeter of the cells. However, the accuracy of detecting whether cells have migrated or not using these common characteristics is not high, and the characteristics are not appropriate for our studies. Therefore, from the point of view of mechanism analysis for the migration behavior, we examined the regularity of different morphology changes of migrated cells and non-migrated cells, and thus discovered the distinguishable morphological characteristics. Then, two deformation characteristics, deformation index and taper index are proposed. Then, a machine learning based algorithm that can identify migrated cells according to the proposed deformation characteristics was devised. In addition, images of migrated cells and non-migrated cells were obtained from the Transwell assays. This algorithm was trained, and was able to successfully identify migrated cells with an accuracy of 84% using the proposed morphological characteristics. This method greatly improves the identification accuracy when compared with the identification of traditional characteristics of which the accuracy was about 54.7%. This machine learning based method might be employed as a potential tool for cell counting and evaluation of cell migration with the aim of reducing time and improving automation compared with the traditional method. This method is effective, rapid, and incorporate advances in artificial intelligence which could be used for adapting the current evaluation methods.

Static and Dynamic: Evolving Biomaterial Mechanical Properties to Control Cellular Mechanotransduction

The extracellular matrix (ECM) is a highly dynamic system that constantly offers physical, biological, and chemical signals to embraced cells. Increasing evidence suggests that mechanical signals derived from the dynamic cellular microenvironment are essential controllers of cell behaviors. Conventional cell culture biomaterials, with static mechanical properties such as chemistry, topography, and stiffness, have offered a fundamental understanding of various vital biochemical and biophysical processes, such as cell adhesion, spreading, migration, growth, and differentiation. At present, novel biomaterials that can spatiotemporally impart biophysical cues to manipulate cell fate are emerging. The dynamic properties and adaptive traits of new materials endow them with the ability to adapt to cell requirements and enhance cell functions. In this review, an introductory overview of the key players essential to mechanobiology is provided. A biophysical perspective on the state-of-the-art manipulation techniques and novel materials in designing static and dynamic ECM-mimicking biomaterials is taken. In particular, different static and dynamic mechanical cues in regulating cellular mechanosensing and functions are compared. This review to benefit the development of engineering biomechanical systems regulating cell functions is expected.

Nanocomposite Hydrogels as Functional Extracellular Matrices

Over recent years, nano-engineered materials have become an important component of artificial extracellular matrices. On one hand, these materials enable static enhancement of the bulk properties of cell scaffolds, for instance, they can alter mechanical properties or electrical conductivity, in order to better mimic the in vivo cell environment. Yet, many nanomaterials also exhibit dynamic, remotely tunable optical, electrical, magnetic, or acoustic properties, and therefore, can be used to non-invasively deliver localized, dynamic stimuli to cells cultured in artificial ECMs in three dimensions. Vice versa, the same, functional nanomaterials, can also report changing environmental conditions-whether or not, as a result of a dynamically applied stimulus-and as such provide means for wireless, long-term monitoring of the cell status inside the culture. In this review article, we present an overview of the technological advances regarding the incorporation of functional nanomaterials in artificial extracellular matrices, highlighting both passive and dynamically tunable nano-engineered components.

A Near-Infrared Mechanically Switchable Elastomeric Film as a Dynamic Cell Culture Substrate

Commercial static cell culture substrates can usually not change their physical properties over time, resulting in a limited representation of the variation in biomechanical cues in vivo. To overcome this limitation, approaches incorporating gold nanoparticles to act as transducers to external stimuli have been employed. In this work, gold nanorods were embedded in an elastomeric matrix and used as photothermal transducers to fabricate biocompatible light-responsive substrates. The nanocomposite films analysed by lock-in thermography and nanoindentation show a homogeneous heat distribution and a greater stiffness when irradiated with NIR light. After irradiation, the initial stiffness values were recovered. In vitro experiments performed during NIR irradiation with NIH-3T3 fibroblasts demonstrated that these films were biocompatible and cells remained viable. Cells cultured on the light stiffened nanocomposite exhibited a greater proliferation rate and stronger focal adhesion clustering, indicating increased cell-surface binding strength.

Does the Extracellular Matrix Support Cell-Cell Communication by Elastic Wave Packets?

The extracellular matrix (ECM) is a fibrous network supporting biological cells and provides them a medium for interaction. Cells modify the ECM by applying traction forces, and these forces can propagate to long ranges and establish a mechanism of mechanical communication between neighboring cells. Previous studies have mainly focused on analysis of static force transmission across the ECM. In this study, we explore the plausibility of dynamic mechanical interaction, expressed as vibrations or abrupt fluctuations, giving rise to elastic waves propagating along ECM fibers. We use a numerical mass-spring model to simulate the longitudinal and transversal waves propagating along a single ECM fiber and across a 2D random fiber network. The elastic waves are induced by an active contracting cell (signaler) and received by a passive neighboring cell (receiver). We show that dynamic wave propagation may amplify the signal at the receiver end and support up to an order of magnitude stronger mechanical cues and longer-ranged communication relative to static transmission. Also, we report an optimal impulse duration corresponding to the most effective transmission, as well as extreme fast impulses, in which the waves are encaged around the active cell and do not reach the neighboring cell, possibly due to the Anderson localization effect. Finally, we also demonstrate that extracellular fluid viscosity reduces, but still allows, dynamic propagation along embedded ECM fibers. Our results motivate future biological experiments in mechanobiology to investigate, on the one hand, the mechanosensitivity of cells to dynamic forces traveling and guided by the ECM and, on the other hand, the impact of ECM architecture and remodeling on dynamic force transmission and its spectral filtering, dispersion, and decay.

Soft nano and microstructures for the photomodulation of cellular signaling and behavior

Photoresponsive soft materials are everywhere in the nature, from human's retina tissues to plants, and have been the inspiration for engineers in the development of modern biomedical materials. Light as an external stimulus is particularly attractive because it is relatively cheap, noninvasive to superficial biological tissues, can be delivered contactless and offers high spatiotemporal control. In the biomedical field, soft materials that respond to long wavelength or that incorporate a photon upconversion mechanism are desired to overcome the limited UV-visible light penetration into biological tissues. Upon light exposure, photosensitive soft materials respond through mechanisms of isomerization, crosslinking or cleavage, hyperthermia, photoreactions, electrical current generation, among others. In this review, we discuss the most recent applications of photosensitive soft materials in the modulation of cellular behavior, for tissue engineering and regenerative medicine, in drug delivery and for phototherapies.

Mechanobiology Platform Realized Using Photomechanical Mxene Nanocomposites: Bilayer Photoactuator Design and In Vitro Mechanical Forces Stimulation

Mechanotransduction is the process by which cells convert external forces and physical constraints into biochemical signals that control several aspects of cellular behavior. A number of approaches have been proposed to investigate the mechanisms of mechanotransduction; however, it remains a great challenge to develop a platform for dynamic multivariate mechanical stimulation of single cells and small colonies of cells. In this study, we combined polydimethylsiloxane (PDMS) and PDMS/Mxene nanoplatelets (MNPs) to construct a soft bilayer nanocomposite for extracellular mechanical stimulation. Fast backlash actuation of the bilayer as a result of near-infrared irradiation caused mechanical force stimulation of cells in a controllable manner. The excellent controllability of the light intensity and frequency allowed backlash bending acceleration and frequency to be manipulated. As gastric gland carcinoma cell line MKN-45 was the research subject, mechanical force loading conditions could trigger apoptosis of the cells in a stimulation duration time-dependent manner. Cell apoptotic rates were positively related to the duration time. In the case of 6 min mechanical force loading, apoptotic cell percentage rose to 34.46% from 5.5% of the control. This approach helps apply extracellular mechanical forces, even with predesigned loading cycles, and provides a solution to study cell mechanotransduction in complex force conditions. It is also a promising therapeutic technique for combining physical therapy and biomechanics.

A bead-based method for high-throughput mapping of the sequence- and force-dependence of T cell activation

Adaptive immunity relies on T lymphocytes that use αβ T cell receptors (TCRs) to discriminate among peptides presented by major histocompatibility complex molecules (pMHCs). Identifying pMHCs capable of inducing robust T cell responses will not only enable a deeper understanding of the mechanisms governing immune responses but could also have broad applications in diagnosis and treatment. T cell recognition of sparse antigenic pMHCs in vivo relies on biomechanical forces. However, in vitro screening methods test potential pMHCs without force and often at high (nonphysiological) pMHC densities and thus fail to predict potent agonists in vivo. Here, we present a technology termed BATTLES (biomechanically assisted T cell triggering for large-scale exogenous-pMHC screening) that uses biomechanical force to initiate T cell triggering for peptides and cells in parallel. BATTLES displays candidate pMHCs on spectrally encoded beads composed of a thermo-responsive polymer capable of applying shear loads to T cells, facilitating exploration of the force- and sequence-dependent landscape of T cell responses. BATTLES can be used to explore basic T cell mechanobiology and T cell-based immunotherapies.

Magnetically Driven Hierarchical Alignment in Biomimetic Fibrous Hydrogels

In vivo, natural biomaterials are frequently anisotropic, exhibiting directional microstructures and mechanical properties. It remains challenging to develop such anisotropy in synthetic materials. Here, a facile one-step approach for in situ fabrication of hydrogels with hierarchically anisotropic architectures and direction-dependent mechanical properties is proposed. The anisotropic hydrogels, composed of a fibrous gel network (0.1 wt%), cross-linked with magnetic nanoparticles (spheres, rods, and wires, <0.1 wt%) are readily formed in the presence of very low magnetic fields (<20 mT). The anisotropy of the nanoparticles is transduced to the polymer network, leading to macroscopic anisotropy, for instance, in mechanical properties. Electrostatic repulsion by the negatively charged nanoparticles induces an additional layer of order in the material, perpendicular to the magnetic field direction. The straightforward fabrication strategy allows for stepwise deposition of layers with different degrees or directions of anisotropy, which enables the formation of complex structures that are able to mimic some of the complex hierarchical architectures found in biology. It is anticipated that this approach of hydrogel alignment may serve as a guide for designing advanced biomaterials in tissue engineering.

Functions and clinical significance of mechanical tumor microenvironment: cancer cell sensing, mechanobiology and metastasis

Dynamic and heterogeneous interaction between tumor cells and the surrounding microenvironment fuels the occurrence, progression, invasion, and metastasis of solid tumors. In this process, the tumor microenvironment (TME) fractures cellular and matrix architecture normality through biochemical and mechanical means, abetting tumorigenesis and treatment resistance. Tumor cells sense and respond to the strength, direction, and duration of mechanical cues in the TME by various mechanotransduction pathways. However, far less understood is the comprehensive perspective of the functions and mechanisms of mechanotransduction. Due to the great therapeutic difficulties brought by the mechanical changes in the TME, emerging studies have focused on targeting the adverse mechanical factors in the TME to attenuate disease rather than conventionally targeting tumor cells themselves, which has been proven to be a potential therapeutic approach. In this review, we discussed the origins and roles of mechanical factors in the TME, cell sensing, mechano‐biological coupling and signal transduction, in vitro construction of the tumor mechanical microenvironment, applications and clinical significance in the TME.

Translating Therapeutic Microgels into Clinical Applications

Microgels are crosslinked, water-swollen networks with a 10 nm to 100 µm diameter and can be modified chemically or biologically to render them biocompatible for advanced clinical applications. Depending on their intended use, microgels require different mechanical and structural properties, which can be engineered on demand by altering the biochemical composition, crosslink density of the polymer network, and the fabrication method. Here, the fundamental aspects of microgel research and development, as well as their specific applications for theranostics and therapy in the clinic, are discussed. A detailed overview of microgel fabrication techniques with regards to their intended clinical application is presented, while focusing on how microgels can be employed as local drug delivery materials, scavengers, and contrast agents. Moreover, microgels can act as scaffolds for tissue engineering and regeneration application. Finally, an overview of microgels is given, which already made it into pre-clinical and clinical trials, while future challenges and chances are discussed. This review presents an instructive guideline for chemists, material scientists, and researchers in the biomedical field to introduce them to the fundamental physicochemical properties of microgels and guide them from fabrication methods via characterization techniques and functionalization of microgels toward specific applications in the clinic.

Cell monolayer deformation microscopy reveals mechanical fragility of cell monolayers following EMT

Tissue and cell mechanics are crucial factors in maintaining homeostasis and in development, with aberrant mechanics contributing to many diseases. During the epithelial-to-mesenchymal transition (EMT), a highly conserved cellular program in organismal development and cancer metastasis, cells gain the ability to detach from their original location and autonomously migrate. While a great deal of biochemical and biophysical changes at the single-cell level have been revealed, how the physical properties of multicellular assemblies change during EMT, and how this may affect disease progression, is unknown. Here we introduce cell monolayer deformation microscopy (CMDM), a new methodology to measure the planar mechanical properties of cell monolayers by locally applying strain and measuring their resistance to deformation. We employ this new method to characterize epithelial multicellular mechanics at early and late stages of EMT, finding the epithelial monolayers to be relatively compliant, ductile, and mechanically homogeneous. By comparison, the transformed mesenchymal monolayers, while much stiffer, were also more brittle, mechanically heterogeneous, displayed more viscoelastic creep, and showed sharp yield points at significantly lower strains. Here, CMDM measurements identify specific biophysical functional states of EMT and offer insight into how cell aggregates fragment under mechanical stress. This mechanical fingerprinting of multicellular assemblies using new quantitative metrics may also offer new diagnostic applications in healthcare to characterize multicellular mechanical changes in disease.

Engineered Extracellular Matrices with Integrated Wireless Microactuators to Study Mechanobiology

Mechanobiology explores how forces regulate cell behaviors and what molecular machinery are responsible for the sensing, transduction, and modulation of mechanical cues. To this end, probing of cells cultured on planar substrates has served as a primary experimental setting for many decades. However, native extracellular matrices (ECMs) consist of fibrous protein assemblies where the physical properties spanning from the individual fiber to the network architecture can influence the transmission of forces to and from the cells. Here, a robotic manipulation platform that allows wireless, localized, and programmable deformation of an engineered fibrous ECM is introduced. A finite-element-based digital twin of the fiber network calibrated against measured local and global parameters enables the calculation of deformations and stresses generated by different magnetic actuation schemes across a range of network properties. Physiologically relevant mechanical forces are applied to cells cultured on the fiber network, statically or dynamically, revealing insights into the effects of matrix-borne forces and deformations as well as force-mediated matrix remodeling on cell migration and intracellular signaling. These capabilities are not matched by any existing approach, and this versatile platform has the potential to uncover fundamental mechanisms of mechanobiology in settings with greater relevance to living tissues.

Modulation of hydrogel stiffness by external stimuli: soft materials for mechanotransduction studies

Mechanotransduction is an important process in determining cell survival, proliferation, migration and differentiation. The extracellular matrix (ECM) is the component of natural tissue that provides structural support and biochemical signals to adhering cells. The ECM is dynamic and undergoes physical and biochemical changes in response to various stimuli and there is an interest in understanding the effect of dynamic changes in stiffness on cell behaviour and fate. Therefore, stimuli-responsive hydrogels have been developed to mimic the cells' microenvironment in a controlled fashion. Herein, we review strategies for dynamic modulation of stiffness using various stimuli, such as light, temperature and pH. Special emphasis is placed on conducting polymer (CP) hydrogels and their fabrication procedures. We believe that the redox properties of CPs and hydrogels' biological properties make CPs hydrogels a promising substrate to investigate the effect of dynamic stiffness changes and mechanical actuation on cell fate in future studies.

Multiphoton Laser Fabrication of Hybrid Photo-Activable Biomaterials

The possibility to shape stimulus-responsive optical polymers, especially hydrogels, by means of laser 3D printing and ablation is fostering a new concept of “smart” micro-devices that can be used for imaging, thermal stimulation, energy transducing and sensing. The composition of these polymeric blends is an essential parameter to tune their properties as actuators and/or sensing platforms and to determine the elasto-mechanical characteristics of the printed hydrogel. In light of the increasing demand for micro-devices for nanomedicine and personalized medicine, interest is growing in the combination of composite and hybrid photo-responsive materials and digital micro-/nano-manufacturing. Existing works have exploited multiphoton laser photo-polymerization to obtain fine 3D microstructures in hydrogels in an additive manufacturing approach or exploited laser ablation of preformed hydrogels to carve 3D cavities. Less often, the two approaches have been combined and active nanomaterials have been embedded in the microstructures. The aim of this review is to give a short overview of the most recent and prominent results in the field of multiphoton laser direct writing of biocompatible hydrogels that embed active nanomaterials not interfering with the writing process and endowing the biocompatible microstructures with physically or chemically activable features such as photothermal activity, chemical swelling and chemical sensing.

Mechanotransduction assays for neural regeneration strategies: A focus on glial cells

Glial cells are mechanosensitive, and thus, engineered systems have taken a step forward to design mechanotransduction platforms in order to impart diverse mechanical stresses to cells. Mechanical strain encountered in the central nervous system can arise from diverse mechanisms, such as tissue reorganization, fluid flow, and axon growth, as well as pathological events including axon swelling or mechanical trauma. Biomechanical relevance of the in vitro mechanical testing requires to be placed in line with the physiological and mechanical changes in central nervous tissues that occur during the progression of neurodegenerative diseases. Mechanotransduction signaling utilized by glial cells and the recent approaches intended to model altered microenvironment adapted to pathological context are discussed in this review. New insights in systems merging substrate's stiffness and topography should be considered for further glial mechanotransduction studies, while testing platforms for drug discoveries promise great advancements in pharmacotherapy. Potential leads and strategies for clinical outcomes are expected to be developed following the exploration of these glial mechanosensitive signaling pathways.

Programming Colloidal Self-Assembled Patterns (cSAPs) into Thermo-Responsible Hybrid Surfaces for Controlling Human Stem Cells and Macrophages

Hybrid surfaces with tunable topography, chemistry, and stiffness have potential to rebuild native extracellular matrix (ECM) and manipulate cell behavior in vitro. However, the fabrication of controllable hybrid surfaces is still challenging. In this study, colloidal self-assembly technology was used to program particles into highly ordered structures with hybrid chemistry and stiffness at biointerfaces. These colloidal self-assembled patterns (cSAPs), including unary, binary, and ternary cSAPs, composed of silicon (Si), polystyrene (PS), and/or poly(N-isopropylacrylamide) (pNIPAM) nanogels (PNGs), were fabricated using either coassembly or layer-by-layer (LBL) methods. The selected binary cSAPs (i.e., PS/PNG and PNG/PS) have a tunable surface topography and wettability between 25 and 37 °C; thus, they can be used as dynamic cell culture substrates. Human adipose-derived mesenchymal stem cells (hASCs), bone marrow-derived mesenchymal stem cells (hBMSCs), and macrophages (THP-1) were investigated on these hybrid cSAPs under a static or dynamic system. The results showed that hybrid cSAPs significantly influenced the focal adhesions, cell morphology, cell migration, and gene expressions of stem cells. In general, stem cells had more vinculin puncta, smaller spreading size, and faster migration speed than the TCPS control. Hybrid cSAPs up-regulated gene expressions of focal adhesion kinase (FAK) and chondrocytes (AGG and SOX9) under static culture, while they also up-regulated osteocytes (COL1 and RUNX2) under dynamic culture. THP-1 macrophages were at M0 state on all cSAPs under static culture. However, cells became sensitive under dynamic culture. For example, some M1 genes (i.e., IL6, CD68, and TNFα) and M2 genes (i.e., IL10 and CD206) were down-regulated, while other M1 genes (i.e., IL1β) and M2 genes (i.e., TGF-β and IL1ra) were up-regulated, depending on the particle combinations. In conclusion, new hybrid cSAPs with thermoresponsive surface properties are versatile materials for stem cells and macrophages manipulation.

Harnessing Mechanobiology for Tissue Engineering

A primary challenge in tissue engineering is to recapitulate both the structural and functional features of whole tissues and organs. In vivo, patterning of the body plan and constituent tissues emerges from the carefully orchestrated interactions between the transcriptional programs that give rise to cell types and the mechanical forces that drive the bending, twisting, and extensions critical to morphogenesis. Substantial recent progress in mechanobiology-understanding how mechanics regulate cell behaviors and what cellular machineries are responsible-raises the possibility that one can begin to use these insights to help guide the strategy and design of functional engineered tissues. In this perspective, we review and propose the development of different approaches, from providing appropriate extracellular mechanical cues to interfering with cellular mechanosensing machinery, to aid in controlling cell and tissue structure and function.

Active biomaterials for mechanobiology

Active biomaterials offer novel approaches to study mechanotransduction in mammalian cells. These material systems probe cellular responses by dynamically modulating their resistance to endogenous forces or applying exogenous forces on cells in a temporally controlled manner. Stimuli-responsive molecules, polymers, and nanoparticles embedded inside cytocompatible biopolymer networks transduce external signals such as light, heat, chemicals, and magnetic fields into changes in matrix elasticity (few kPa to tens of kPa) or forces (few pN to several μN) at the cell-material interface. The implementation of active biomaterials in mechanobiology has generated scientific knowledge and therapeutic potential relevant to a variety of conditions including but not limited to cancer metastasis, fibrosis, and tissue regeneration. We discuss the repertoire of cellular responses that can be studied using these platforms including receptor signaling as well as downstream events namely, cytoskeletal organization, nuclear shuttling of mechanosensitive transcriptional regulators, cell migration, and differentiation. We highlight recent advances in active biomaterials and comment on their future impact.

[Skin aging: Pathophysiology and innovative therapies]

One of the major challenges of the 21^st century is the fight against aging, defined as a set of physiological mechanisms altering the physical and intellectual capacities of human beings. Aging of the skin is only one visible part of this process. It is associated with major healing defects linked in part to the alteration of the biomechanical properties of skin cells, mainly dermal fibroblasts. The immune system, another key component in maintaining skin homeostasis and the efficient healing of wounds, also suffers the effects of time: the consequent skin immunosenescence would limit the anti-infectious and vaccine response, while promoting a pro-tumor environment. The main skin damages due to aging, whether intrinsic or extrinsic, will be detailed before listing the effective anti-aging strategies to combat age-related dermal and epidermal stigmas.

Bioinstructive Micro-Nanotextured Zirconia Ceramic Interfaces for Guiding and Stimulating an Osteogenic Response In Vitro

Osseous implantology’s material requirements include a lack of potential for inducing allergic disorders and providing both functional and esthetic features for the patient’s benefit. Despite being bioinert, Zirconia ceramics have become a candidate of interest to be used as an alternative to titanium dental and cochlear bone-anchored hearing aid (BAHA) implants, implying the need for endowing the surface with biologically instructive properties by changing basic parameters such as surface texture. Within this context, we propose anisotropic and isotropic patterns (linear microgroove arrays, and superimposed crossline microgroove arrays, respectively) textured in zirconia substrates, as bioinstructive interfaces to guide the cytoskeletal organization of human mesenchymal stem cells (hMSCs). The designed textured micro-nano interfaces with either steep ridges and microgratings or curved edges, and nanoroughened walls obtained by direct femtosecond laser texturing are used to evaluate the hMSC response parameters and osteogenic differentiation to each topography. Our results show parallel micro line anisotropic surfaces are able to guide cell growth only for the steep surfaces, while the curved ones reduce the initial response and show the lowest osteogenic response. An improved osteogenic phenotype of hMSCs is obtained when grown onto isotropic grid/pillar-like patterns, showing an improved cell coverage and Ca/P ratio, with direct implications for BAHA prosthetic development, or other future applications in regenerating bone defects.

Motion magnification analysis of microscopy videos of biological cells

It is well recognized that isolated cardiac muscle cells beat in a periodic manner. Recently, evidence indicates that other, non-muscle cells, also perform periodic motions that are either imperceptible under conventional lab microscope lens or practically not easily amenable for analysis of oscillation amplitude, frequency, phase of movement and its direction. Here, we create a real-time video analysis tool to visually magnify and explore sub-micron rhythmic movements performed by biological cells and the induced movements in their surroundings. Using this tool, we suggest that fibroblast cells perform small fluctuating movements with a dominant frequency that is dependent on their surrounding substrate and its stiffness.

Mechanical Stimulation of Adhesion Receptors Using Light-Responsive Nanoparticle Actuators Enhances Myogenesis

The application of cyclic strain is known to enhance myoblast differentiation and muscle growth in vitro and in vivo. However, current techniques apply strain to full tissues or cell monolayers, making it difficult to evaluate whether mechanical stimulation at the subcellular or single-cell scales would drive myoblast differentiation. Here, we report the use of optomechanical actuator (OMA) particles, comprised of a ∼0.6 μm responsive hydrogel coating a gold nanorod (100 × 20 nm) core, to mechanically stimulate the integrin receptors in myoblasts. When illuminated with near-infrared (NIR) light, OMA nanoparticles rapidly collapse, exerting mechanical forces to cell receptors bound to immobilized particles. Using a pulsed illumination pattern, we applied cyclic integrin forces to C2C12 myoblasts cultured on a monolayer of OMA particles and then measured the cellular response. We found that 20 min of OMA actuation resulted in cellular elongation in the direction of the stimulus and enhancement of nuclear YAP1 accumulation, an effector of ERK phosphorylation. Cellular response was dependent on direct conjugation of RGD peptides to the OMA particles. Repeated OMA mechanical stimulation for 5 days led to enhanced myogenesis as quantified using cell alignment, fusion, and sarcomeric myosin expression in myotubes. OMA-mediated myogenesis was sensitive to the geometry of stimulation but not to MEK1/2 inhibition. Finally, we found that OMA stimulation in regions proximal to the nucleus resulted in localization of the transcription activator YAP-1 to the nucleus, further suggesting the role of YAP1 in mechanotransduction in C2C12 cells. These findings demonstrate OMAs as a novel tool for studying the role of spatially localized forces in influencing myogenesis.

Fast Switching of Bright Whiteness in Channeled Hydrogel Networks

Beside pigment absorption and reflection by periodic photonic structures, natural species often use light scattering to achieve whiteness. Synthetic hydrogels offer opportunities in stimuli-responsive materials and devices; however, they are not conventionally considered as ideal materials to achieve high whiteness by scattering due to the ill-defined porosities and the low refractive index contrast between the polymer and water. Herein, a poly(N-isopropylacrylamide) hydrogel network with percolated empty channels (ch-PNIPAm) is demonstrated to possess switchable bright whiteness upon temperature changes, obtained by removing the physical agarose gel in a semi-interpenetrating network of agarose and PNIPAm. The hydrogel is highly transparent at room temperature and becomes brightly white above 35 °C. Compared to conventional PNIPAm, the ch-PNIPAm hydrogel exhibits 80% higher reflectance at 800 nm and 18 times faster phase transition kinetics. The nanoscopic channels in the ch-PNIPAm facilitate water diffusion upon phase transition, thus enabling the formation of smaller pores and enhanced whiteness in the gel. Furthermore, fast photothermally triggered response down to tens of milliseconds can be achieved. This unique property of the ch-PNIPAm hydrogel to efficiently scatter visible light can be potentially used for, e.g., smart windows, optical switches, and, as demonstrated in this report, thermoresponsive color displays.

Integrating Biophysics in Toxicology

Integration of biophysical stimulation in test systems is established in diverse branches of biomedical sciences including toxicology. This is largely motivated by the need to create novel experimental setups capable of reproducing more closely in vivo physiological conditions. Indeed, we face the need to increase predictive power and experimental output, albeit reducing the use of animals in toxicity testing. In vivo, mechanical stimulation is essential for cellular homeostasis. In vitro, diverse strategies can be used to model this crucial component. The compliance of the extracellular matrix can be tuned by modifying the stiffness or through the deformation of substrates hosting the cells via static or dynamic strain. Moreover, cells can be cultivated under shear stress deriving from the movement of the extracellular fluids. In turn, introduction of physical cues in the cell culture environment modulates differentiation, functional properties, and metabolic competence, thus influencing cellular capability to cope with toxic insults. This review summarizes the state of the art of integration of biophysical stimuli in model systems for toxicity testing, discusses future challenges, and provides perspectives for the further advancement of in vitro cytotoxicity studies.

Mechanical tumor microenvironment and transduction: cytoskeleton mediates cancer cell invasion and metastasis

Metastasis is a complicated, multistep process that is responsible for over 90% of cancer-related death. Metastatic disease or the movement of cancer cells from one site to another requires dramatic remodeling of the cytoskeleton. The regulation of cancer cell migration is determined not only by biochemical factors in the microenvironment but also by the biomechanical contextual information provided by the extracellular matrix (ECM). The responses of the cytoskeleton to chemical signals are well characterized and understood. However, the mechanisms of response to mechanical signals in the form of externally applied force and forces generated by the ECM are still poorly understood. Furthermore, understanding the way cellular mechanosensors interact with the physical properties of the microenvironment and transmit the signals to activate the cytoskeletal movements may help identify an effective strategy for the treatment of cancer. Here, we will discuss the role of tumor microenvironment during cancer metastasis and how physical forces remodel the cytoskeleton through mechanosensing and transduction.

Soft temperature-responsive microgels of complex shape in stop-flow lithography

Stop-flow lithography (SFL) has emerged as a facile high-throughput fabrication method for μm-sized anisometric particles; yet, the fabrication of soft, anisometric microgels has not frequently been addressed in the literature. Furthermore, and to the best of the authors' knowledge, no soft, complex-shaped microgels with temperature-responsive behavior have been fabricated with this technology before. However, such microgels have tremendous potential as building blocks and actuating elements in rapidly developing fields, such as tissue engineering and additive manufacturing of soft polymeric building blocks, bio-hybrid materials, or soft micro-robotics. Given their great potential, we prove in this work that SFL is a viable method for the fabrication of soft, temperature-responsive, and complex-shaped microgels. The microgels, fabricated in this work, consist of poly(N-isopropylacrylamide) (pNIPAm), which is crosslinked with N,N'-methylenebis(acrylamide). The results confirm that the shape of the pNIPAm microgels is determined by the transparency mask, used in SFL. Furthermore, it is shown that, in order to realize stable microgels, a minimum threshold of crosslinker concentration of 2 wt% is required. Above this threshold, the stiffness of pNIPAm microgels can be deliberately altered by adjusting the concentration of the crosslinker. The fabricated pNIPAm microgels show the targeted temperature-responsive behavior. Within this context, temperature-dependent reversible swelling is confirmed, even for fractal-like geometries, such as micro snowflakes. Thus, these microgels provide the targeted unique combination of softness, shape complexity, and temperature responsiveness and increase the freedom of design for actuated building blocks.

Advancing Versatile Ferroelectric Materials Toward Biomedical Applications

Ferroelectric materials (FEMs), possessing piezoelectric, pyroelectric, inverse piezoelectric, nonlinear optic, ferroelectric-photovoltaic, and many other properties, are attracting increasing attention in the field of biomedicine in recent years. Because of their versatile ability of interacting with force, heat, electricity, and light to generate electrical, mechanical, and optical signals, FEMs are demonstrating their unique advantages for biosensing, acoustics tweezer, bioimaging, therapeutics, tissue engineering, as well as stimulating biological functions. This review summarizes the current-available FEMs and their state-of-the-art fabrication techniques, as well as provides an overview of FEMs-based applications in the field of biomedicine. Challenges and prospects for future development of FEMs for biomedical applications are also outlined.