3D Scaffolds to Study Basic Cell Biology

Site-Selective Biofunctionalization of 3D Microstructures Via Direct Ink Writing

Two-photon lithography has revolutionized multi-photon 3D laser printing, enabling precise fabrication of micro- and nanoscale structures. Despite many advancements, challenges still persist, particularly in biofunctionalization of 3D microstructures. This study introduces a novel approach combining two-photon lithography with scanning probe lithography for post-functionalization of 3D microstructures overcoming limitations in achieving spatially controlled biomolecule distribution. The method utilizes a diverse range of biomolecule inks, including phospholipids, and two different proteins, introducing high spatial resolution and distinct functionalization on separate areas of the same microstructure. The surfaces of 3D microstructures are treated using bovine serum albumin and/or 3-(Glycidyloxypropyl)trimethoxysilane (GPTMS) to enhance ink retention. The study further demonstrates different strategies to create binding sites for cells by integrating different biomolecules, showcasing the potential for customized 3D cell microenvironments. Specific cell adhesion onto functionalized 3D microscaffolds is demonstrated, which paves the way for diverse applications in tissue engineering, biointerfacing with electronic devices and biomimetic modeling.

Two- and three-photon processes during photopolymerization in 3D laser printing

The performance of a photoinitiator is key to control efficiency and resolution in 3D laser nanoprinting. Upon light absorption, a cascade of competing photophysical processes leads to photochemical reactions toward radical formation that initiates free radical polymerization (FRP). Here, we investigate 7-diethylamino-3-thenoylcoumarin (DETC), belonging to an efficient and frequently used class of photoinitiators in 3D laser printing, and explain the molecular bases of FRP initiation upon DETC photoactivation. Depending on the presence of a co-initiator, DETC causes radical generation either upon two-photon- or three-photon excitation, but the mechanism for these processes is not well understood so far. Here, we show that the unique three-photon based radical formation by DETC, in the absence of a co-initiator, results from its excitation to highly excited triplet states. They allow a hydrogen-atom transfer reaction from the pentaerythritol triacrylate (PETA) monomer to DETC, enabling the formation of the reactive PETA alkyl radical, which initiates FRP. The formation of active DETC radicals is demonstrated to be less spontaneous. In contrast, photoinitiation in the presence of an onium salt co-initiator proceeds via intermolecular electron transfer after the photosensitization of the photoinitiator to the lowest triplet excited state. Our quantum mechanical calculations demonstrate photophysical processes upon the multiphoton activation of DETC and explain different reactions for the radical formation upon DETC photoactivation. This investigation for the first time describes possible pathways of FRP initiation in 3D laser nanoprinting and permits further rational design of efficient photoinitiators to increase the speed and sensitivity of 3D laser nanoprinting.

Direct laser writing-enabled 3D printing strategies for microfluidic applications

Over the past decade, additive manufacturing-or "three-dimensional (3D) printing"-has attracted increasing attention in the Lab on a Chip community as a pathway to achieve sophisticated system architectures that are difficult or infeasible to fabricate via conventional means. One particularly promising 3D manufacturing technology is "direct laser writing (DLW)", which leverages two-photon (or multi-photon) polymerization (2PP) phenomena to enable high geometric versatility, print speeds, and precision at length scales down to the 100 nm range. Although researchers have demonstrated the potential of using DLW for microfluidic applications ranging from organ on a chip and drug delivery to micro/nanoparticle processing and soft microrobotics, such scenarios present unique challenges for DLW. Specifically, microfluidic systems typically require macro-to-micro fluidic interfaces (e.g., inlet and outlet ports) to facilitate fluidic loading, control, and retrieval operations; however, DLW-based 3D printing relies on a micron-to-submicron-sized 2PP volume element (i.e., "voxel") that is poorly suited for manufacturing these larger-scale fluidic interfaces. In this Tutorial Review, we highlight and discuss the four most prominent strategies that researchers have developed to circumvent this trade-off and realize macro-to-micro interfaces for DLW-enabled microfluidic components and systems. In addition, we consider the possibility that-with the advent of next-generation commercial DLW printers equipped with new dynamic voxel tuning, print field, and laser power capabilities-the overall utility of DLW strategies for Lab on a Chip fields may soon expand dramatically.

A Scaffold-Assisted 3D Cancer Cell Model for Surface-Enhanced Raman Scattering-Based Real-Time Sensing and Imaging

Despite recent advances in the development of scaffold-based three-dimensional (3D) cell models, challenges persist in imaging and monitoring cell behavior within these complex structures due to their heterogeneous cell distribution and geometries. Incorporating sensors into 3D scaffolds provides a potential solution for real-time, in situ sensing and imaging of biological processes such as cell growth and disease development. We introduce a 3D printed hydrogel-based scaffold capable of supporting both surface-enhanced Raman scattering (SERS) biosensing and imaging of 3D breast cancer cell models. The scaffold incorporates plasmonic nanoparticles and SERS tags, for sensing and imaging, respectively. We demonstrate the scaffold's adaptability and modularity in supporting breast cancer spheroids, thereby enabling spatial and temporal monitoring of tumor evolution.

Modelling cell shape in 3D structured environments: A quantitative comparison with experiments

Cell shape plays a fundamental role in many biological processes, including adhesion, migration, division and development, but it is not clear which shape model best predicts three-dimensional cell shape in structured environments. Here, we compare different modelling approaches with experimental data. The shapes of single mesenchymal cells cultured in custom-made 3D scaffolds were compared by a Fourier method with surfaces that minimize area under the given adhesion and volume constraints. For the minimized surface model, we found marked differences to the experimentally observed cell shapes, which necessitated the use of more advanced shape models. We used different variants of the cellular Potts model, which effectively includes both surface and bulk contributions. The simulations revealed that the Hamiltonian with linear area energy outperformed the elastic area constraint in accurately modelling the 3D shapes of cells in structured environments. Explicit modelling the nucleus did not improve the accuracy of the simulated cell shapes. Overall, our work identifies effective methods for accurately modelling cellular shapes in complex environments.

High-Throughput Two-Photon 3D Printing Enabled by Holographic Multi-Foci High-Speed Scanning

The emerging two-photon polymerization (TPP) technique enables high-resolution printing of complex 3D structures, revolutionizing micro/nano additive manufacturing. Various fast scanning and parallel processing strategies have been proposed to promote its efficiency. However, obtaining large numbers of uniform focal spots for parallel high-speed scanning remains challenging, which hampers the realization of higher throughput. We report a TPP printing platform that combines galvanometric mirrors and liquid crystal on silicon spatial light modulator (LCoS-SLM). By setting the target light field at LCoS-SLM's diffraction center, sufficient energy is acquired to support simultaneous polymerization of over 400 foci. With fast scanning, the maximum printing speed achieves 1.49 × 108 voxels s-1, surpassing the existing scanning-based TPP methods while maintaining high printing resolution and flexibility. To demonstrate the processing capability, functional 3D microstructure arrays are rapidly fabricated and applied in micro-optics and micro-object manipulation. Our method may expand the prospects of TPP in large-scale micro/nanomanufacturing.

Scaffold-based 3D cell culture models in cancer research

Three-dimensional (3D) cell cultures have emerged as valuable tools in cancer research, offering significant advantages over traditional two-dimensional (2D) cell culture systems. In 3D cell cultures, cancer cells are grown in an environment that more closely mimics the 3D architecture and complexity of in vivo tumors. This approach has revolutionized cancer research by providing a more accurate representation of the tumor microenvironment (TME) and enabling the study of tumor behavior and response to therapies in a more physiologically relevant context. One of the key benefits of 3D cell culture in cancer research is the ability to recapitulate the complex interactions between cancer cells and their surrounding stroma. Tumors consist not only of cancer cells but also various other cell types, including stromal cells, immune cells, and blood vessels. These models bridge traditional 2D cell cultures and animal models, offering a cost-effective, scalable, and ethical alternative for preclinical research. As the field advances, 3D cell cultures are poised to play a pivotal role in understanding cancer biology and accelerating the development of effective anticancer therapies. This review article highlights the key advantages of 3D cell cultures, progress in the most common scaffold-based culturing techniques, pertinent literature on their applications in cancer research, and the ongoing challenges.

Evaluation of Osteogenic Potential for Rat Adipose-Derived Stem Cells under Xeno-Free Environment

This study aimed to develop a novel culture method for rat adipose-derived stem cells (rADSC) and evaluate their osteogenic potential. The rADSC cultured in xeno-free culture medium (XF-rADSCs) or conventional culture medium containing fetal bovine serum (FBS-rADSCs) were combined with micropieces of xeno-free recombinant collagen peptide to form 3-dimensional aggregates (XF-rADSC-CellSaic or FBS-rADSC-CellSaic). Both FBS-rADSC and XF-ADSC in CellSaic exhibited multilineage differentiation potential. Compared to FBS-rADSC-CellSaic, XF-rADSC-CellSaic accelerated and promoted osteogenic differentiation in vitro. When transplanted into rat mandibular congenital bone defects, the osteogenically differentiated XF-rADSC-CellSaic induced regeneration of bone tissue with a highly maturated structure compared to FBS-rADSC-CellSaic. In conclusion, XF-rADSC-CellSaic is a feasible 3-dimensional platform for efficient bone formation.

Fabrication and characterization of ceramic-polymer composite 3D scaffolds and demonstration of osteoinductive propensity with gingival mesenchymal stem cells

The fabrication of biomaterial 3D scaffolds for bone tissue engineering applications involves the usage of metals, polymers, and ceramics as the base constituents. Notwithstanding, the composite materials facilitating enhanced osteogenic differentiation/regeneration are endorsed as the ideally suited bone grafts for addressing critical-sized bone defects. Here, we report the successful fabrication of 3D composite scaffolds mimicking the ECM of bone tissue by using ∼30 wt% of collagen type I (Col-I) and ∼70 wt% of different crystalline phases of calcium phosphate (CP) nanomaterials [hydroxyapatite (HAp), beta-tricalcium phosphate (βTCP), biphasic hydroxyapatite (βTCP-HAp or BCP)], where pH served as the sole variable for obtaining these CP phases. The different Ca/P ratio and CP nanomaterials orientation in these CP/Col-I composite scaffolds not only altered the microstructure, surface area, porosity with randomly oriented interconnected pores (80-450 μm) and mechanical strength similar to trabecular bone but also consecutively influenced the bioactivity, biocompatibility, and osteogenic differentiation potential of gingival-derived mesenchymal stem cells (gMSCs). In fact, BCP/Col-I, as determined from micro-CT analysis, achieved the highest surface area (∼42.6 m2 g-1) and porosity (∼85%), demonstrated improved bioactivity and biocompatibility and promoted maximum osteogenic differentiation of gMSCs among the three. Interestingly, the released Ca2+ ions, as low as 3 mM, from these scaffolds could also facilitate the osteogenic differentiation of gMSCs without even subjecting them to osteoinduction, thereby attesting these CP/Col-I 3D scaffolds as ideally suited bone graft materials.

Synthesis and Cytotoxicity Studies of Poly(1,4-butanediol citrate) Gels for Cell Culturing

One of the main branches of regenerative medicine is biomaterials research, which is designed to develop and study materials for regenerative therapies, controlled drug delivery systems, wound dressings, etc. Research is continually being conducted to find biomaterials-especially polymers-with better biocompatibility, broader modification possibilities and better application properties. This study describes a potential biomaterial, poly(1,4-butanediol citrate). The gelation time of poly(1,4-butanediol citrate) was estimated. Based on this, the limiting reaction time and temperature were determined to avoid gelling of the reaction mixture. Experiments with different process conditions were carried out, and the products were characterised through NMR spectra analysis. Using statistical methods, the functions were defined, describing the dependence of the degree of esterification of the acid groups on the following process parameters: temperature and COOH/OH group ratio. Polymer films from the synthesised polyester were prepared and characterised. The main focus was assessing the initial biocompatibility of the materials.

Auxeticity as a Mechanobiological Tool to Create Meta-Biomaterials

Mechanical and morphological design parameters, such as stiffness or porosity, play important roles in creating orthopedic implants and bone substitutes. However, we have only a limited understanding of how the microarchitecture of porous scaffolds contributes to bone regeneration. Meta-biomaterials are increasingly used to precisely engineer the internal geometry of porous scaffolds and independently tailor their mechanical properties (e.g., stiffness and Poisson's ratio). This is motivated by the rare or unprecedented properties of meta-biomaterials, such as negative Poisson's ratios (i.e., auxeticity). It is, however, not clear how these unusual properties can modulate the interactions of meta-biomaterials with living cells and whether they can facilitate bone tissue engineering under static and dynamic cell culture and mechanical loading conditions. Here, we review the recent studies investigating the effects of the Poisson's ratio on the performance of meta-biomaterials with an emphasis on the relevant mechanobiological aspects. We also highlight the state-of-the-art additive manufacturing techniques employed to create meta-biomaterials, particularly at the micrometer scale. Finally, we provide future perspectives, particularly for the design of the next generation of meta-biomaterials featuring dynamic properties (e.g., those made through 4D printing).

How 3D scaffolds with different mechanical properties affect the activity of neuronal networks in in vitro models. ANNUAL INTERNATIONAL CONFERENCE OF THE IEEE ENGINEERING IN MEDICINE AND BIOLOGY SOCIETY. IEEE ENGINEERING IN MEDICINE AND BIOLOGY SOCIETY. ANNUAL INTERNATIONAL CONFERENCE 2023;2023:1-4. [PMID: 38083594 DOI: 10.1109/embc40787.2023.10340624] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Three-dimensionality has been proven extensively to be critical in the development of a reliable model for different anatomical compartments and for many diseases. Currently, we can produce implantable structures that help in the regeneration of different tissues such as bone and heart. Different is the situation when we consider the neuronal compartment. As it is still difficult to understand exactly how the brain computes, to conceive how the complex chain of neuronal events can generate conscious behavior, a comprehensive and workable model of neuronal tissue still has to be found. In this perspective, in the present work, we developed and compared different 3D scaffolds to understand the effects produced by the mechanical and material properties of four different scaffolds on a 3D neuronal network. To help in preclinical testing procedure, the scalability and ease-of-use of the different approaches were also taken into consideration.Clinical Relevance- By comparing different 3D scaffolds for the creation of neuronal constructs, the results in this paper move towards understanding the best strategy to develop functional 3D neuronal units for reliable pre-clinical studies.

Type-Independent 3D Writing and Nano-Patterning of Confined Biopolymers

Biopolymers are essential building blocks that constitute cells and tissues with well-defined molecular structures and diverse biological functions. Their three-dimensional (3D) complex architectures are used to analyze, control, and mimic various cells and their ensembles. However, the free-form and high-resolution structuring of various biopolymers remain challenging because their structural and rheological control depend critically on their polymeric types at the submicron scale. Here, direct 3D writing of intact biopolymers is demonstrated using a systemic combination of nanoscale confinement, evaporation, and solidification of a biopolymer-containing solution. A femtoliter solution is confined in an ultra-shallow liquid interface between a fine-tuned nanopipette and a chosen substrate surface to achieve directional growth of biopolymer nanowires via solvent-exclusive evaporation and concurrent solution supply. The evaporation-dependent printing is biopolymer type-independent, therefore, the 3D motor-operated precise nanopipette positioning allows in situ printing of nucleic acids, polysaccharides, and proteins with submicron resolution. By controlling concentrations and molecular weights, several different biopolymers are reproducibly patterned with desired size and geometry, and their 3D architectures are biologically active in various solvents with no structural deformation. Notably, protein-based nanowire patterns exhibit pin-point localization of spatiotemporal biofunctions, including target recognition and catalytic peroxidation, indicating their application potential in organ-on-chips and micro-tissue engineering.

Selective Positioning of Different Cell Types on 3D Scaffolds via DNA Hybridization

Three-dimensional (3D) microscaffolds for cell biology have shown their potential in mimicking physiological environments and simulating complex multicellular constructs. However, controlling the localization of cells precisely on microfabricated structures is still complex and usually limited to two-dimensional assays. Indeed, the implementation of an efficient method to selectively target different cell types to specific regions of a 3D microscaffold would represent a decisive step toward cell-by-cell assembly of complex cellular arrangements. Here, we use two-photon lithography (2PL) to fabricate 3D microarchitectures with functional photoresists. UV-mediated click reactions are used to functionalize their surfaces with single-stranded DNA oligonucleotides, using sequential repetition to decorate different scaffold regions with individual DNA addresses. Various immortalized cell lines and stem cells modified by grafting complementary oligonucleotides onto the phospholipid membranes can then be immobilized onto complementary regions of the 3D structures by selective hybridization. This allows controlled cocultures to be established with spatially separated arrays of eukaryotic cells in 3D.

Development of osteon-like scaffold-cell construct by quadruple coaxial extrusion-based 3D bioprinting of nanocomposite hydrogel

Despite advances in bone tissue engineering, fabricating a scaffold which can be used as an implant for large bone defects remains challenge. One of the great importance in fabricating a biomimetic bone implant is considering the possibility of the integration of the structure and function of implants with hierarchical structure of bone. Herein, we propose a method to mimic the structural unit of compact bone, osteon, with spatial pattern of human umbilical vein endothelial cells (HUVECs) and human mesenchymal stem cells (hMSCs) in the adjacent layers that mimic Haversian canal and lamella, respectively. To this end, coaxial extrusion-based bioprinting technique via a customized quadruple-layer core-shell nozzle was employed. 3D implant scaffold-cell construct was fabricated by using polyethylene glycol as a hollowing agent in the first layer, gelatin methacryloyl (GelMA) and alginate blended hydrogel encapsulating HUVEC cells with vascular endothelial growth factor nanoparticles in the second layer (vasculogenic layer) to mimic vascular vessel, and GelMA and alginate blended hydrogel containing hMSCs cells in the outer osteogenic layer to imitate lamella. Two types of bone minerals, whitlockite and hydroxyapatite, were incorporated in osteogenic layer to induce osteoblastic differentiation and enhance mechanical properties (the young's modules of nanocomposite increased from 35 kPa to 80 kPa). In-vitro evaluations demonstrated high cell viability (94 % within 10 days) and proliferation. Furthermore, ALP enzyme activity increased considerably within 2 weeks and mineralized extra cellular matrix considerably produced within 3 weeks. Also, a significant increase in osteogenic markers was observed indicating the presence of differentiated osteoblast cells. Therefore, the work indicates the potential of single step 3D bioprinting process to fabricate biomimetic osteons to use as bone grafts for regeneration.

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.

Visible-Light-Degradable 3D Microstructures in Aqueous Environments

The additive manufacturing technique direct laser writing (DLW), also known as two-photon laser lithography, is becoming increasingly established as a technique capable of fabricating functional 3D microstructures. Recently, there has been an increasing effort to impart microstructures fabricated using DLW with advanced functionalities by introducing responsive chemical entities into the underpinning photoresists. Herein, a novel photoresist based on the photochemistry of the bimane group is introduced that can be degraded upon exposure to very mild conditions, requiring only water and visible light (λ_max  = 415-435 nm) irradiation. The degradation of the microstructures is tracked and quantified using AFM measurements of their height. The influence of the writing parameters as well as the degradation conditions is investigated, unambiguously evidencing effective visible light degradation in aqueous environments. Finally, the utility of the photodegradable resist system is demonstrated by incorporating it into multimaterial 3D microstructures, serving as a model for future applications.

In-situ mineralized homogeneous collagen-based scaffolds for potential guided bone regeneration

For guided bone regeneration (GBR) in clinical orthopedics, the importance of a suitable scaffold which can provide the space needed for bone regeneration and simultaneously promotes the new bone formation cannot be overemphasized. Due to its excellent biocompatibility, mechanical strength, and similarity in structure and composition to natural bone, the mineralized collagen-based scaffolds have been increasingly considered as promising GBR scaffolds. Herein, we propose a novel method to fabricate an in-situ mineralized homogeneous collagen-based scaffold (IMHCS) with excellent osteogenic capability for GBR by electrospinning the collagen solution in combination with essential mineral ions. The IMHCS exhibited homogeneous distribution of apatite crystals in electrospun fibers, which helped to achieve a significantly higher tensile strength than the pure collagen scaffold (CS) and the scaffold with directly added nano-hydroxyapatite particles (HAS). Furthermore, the IMHCS had significantly better cell compatibility, cell migration ratio, and osteogenic differentiation property than the HAS and CS. Therefore, the IMHCS not only retains traditional function of inhibiting fibroblast invasion, but also possesses excellent osteogenic differentiation property, indicating a robust alternative for GBR applications.

Adaptation of cell spreading to varying fibronectin densities and topographies is facilitated by β1 integrins

Cells mechanical behaviour in physiological environments is mediated by interactions with the extracellular matrix (ECM). In particular, cells can adapt their shape according to the availability of ECM proteins, e.g., fibronectin (FN). Several in vitro experiments usually simulate the ECM by functionalizing the surfaces on which cells grow with FN. However, the mechanisms underlying cell spreading on non-uniformly FN-coated two-dimensional substrates are not clarified yet. In this work, we studied cell spreading on variously functionalized substrates: FN was either uniformly distributed or selectively patterned on flat surfaces, to show that A549, BRL, B16 and NIH 3T3 cell lines are able to sense the overall FN binding sites independently of their spatial arrangement. Instead, only the total amount of available FN influences cells spreading area, which positively correlates to the FN density. Immunocytochemical analysis showed that β1 integrin subunits are mainly responsible for this behaviour, as further confirmed by spreading experiments with β1-deficient cells. In the latter case, indeed, cells areas do not show a dependency on the amount of available FN on the substrates. Therefore, we envision for β1 a predominant role in cells for sensing the number of ECM ligands with respect to other focal adhesion proteins.

Hydrogels for Exosome Delivery in Biomedical Applications

Hydrogels, which are hydrophilic polymer networks, have attracted great attention, and significant advances in their biological and biomedical applications, such as for drug delivery, tissue engineering, and models for medical studies, have been made. Due to their similarity in physiological structure, hydrogels are highly compatible with extracellular matrices and biological tissues and can be used as both carriers and matrices to encapsulate cellular secretions. As small extracellular vesicles secreted by nearly all mammalian cells to mediate cell–cell interactions, exosomes play very important roles in therapeutic approaches and disease diagnosis. To maintain their biological activity and achieve controlled release, a strategy that embeds exosomes in hydrogels as a composite system has been focused on in recent studies. Therefore, this review aims to provide a thorough overview of the use of composite hydrogels for embedding exosomes in medical applications, including the resources for making hydrogels and the properties of hydrogels, and strategies for their combination with exosomes.

Challenges and limits of mechanical stability in 3D direct laser writing

Direct laser writing is an effective technique for fabrication of complex 3D polymer networks using ultrashort laser pulses. Practically, it remains a challenge to design and fabricate high performance materials with different functions that possess a combination of high strength, substantial ductility, and tailored functionality, in particular for small feature sizes. To date, it is difficult to obtain a time-resolved microscopic picture of the printing process in operando. To close this gap, we herewith present a molecular dynamics simulation approach to model direct laser writing and investigate the effect of writing condition and aspect ratio on the mechanical properties of the printed polymer network. We show that writing conditions provide a possibility to tune the mechanical properties and an optimum writing condition can be applied to fabricate structures with improved mechanical properties. We reveal that beyond the writing parameters, aspect ratio plays an important role to tune the stiffness of the printed structures.

Direct laser writing is an effective technique for fabrication of complex 3D polymer networks using ultrashort laser pulses but to date it is difficult to obtain a time-resolved microscopic picture of the printing process in operando. Here, the use molecular dynamics simulation to model direct laser writing and investigate the effect of writing condition and aspect ratio on the mechanical properties of the printed polymer network.

Integration of Biofunctional Molecules into 3D-Printed Polymeric Micro-/Nanostructures

Three-dimensional printing at the micro-/nanoscale represents a new challenge in research and development to achieve direct printing down to nanometre-sized objects. Here, FluidFM, a combination of microfluidics with atomic force microscopy, offers attractive options to fabricate hierarchical polymer structures at different scales. However, little is known about the effect of the substrate on the printed structures and the integration of (bio)functional groups into the polymer inks. In this study, we printed micro-/nanostructures on surfaces with different wetting properties, and integrated molecules with different functional groups (rhodamine as a fluorescent label and biotin as a binding tag for proteins) into the base polymer ink. The substrate wetting properties strongly affected the printing results, in that the lateral feature sizes increased with increasing substrate hydrophilicity. Overall, ink modification only caused minor changes in the stiffness of the printed structures. This shows the generality of the approach, as significant changes in the mechanical properties on chemical functionalization could be confounders in bioapplications. The retained functionality of the obtained structures after UV curing was demonstrated by selective binding of streptavidin to the printed structures. The ability to incorporate binding tags to achieve specific interactions between relevant proteins and the fabricated micro-/nanostructures, without compromising the mechanical properties, paves a way for numerous bio and sensing applications. Additional flexibility is obtained by tuning the substrate properties for feature size control, and the option to obtain functionalized printed structures without post-processing procedures will contribute to the development of 3D printing for biological applications, using FluidFM and similar dispensing techniques.

Extracellular Vesicles from 3D Engineered Microtissues Harbor Disease-Related Cargo Absent in EVs from 2D Cultures

Engineered microtissues that recapitulate key properties of the tumor microenvironment can induce clinically relevant cancer phenotypes in vitro. However, their effect on molecular cargo of secreted extracellular vesicles (EVs) has not yet been investigated. Here, the impact of hydrogel-based 3D engineered microtissues on EVs secreted by benign and malignant prostate cells is assessed. Compared to 2D cultures, yield of EVs per cell is significantly increased for cancer cells cultured in 3D. Whole transcriptome sequencing and proteomics of 2D-EV and 3D-EV samples reveal stark contrasts in molecular cargo. For one cell type in particular, LNCaP, enrichment is observed exclusively in 3D-EVs of GDF15, FASN, and TOP1, known drivers of prostate cancer progression. Using imaging flow cytometry in a novel approach to validate a putative EV biomarker, colocalization in single EVs of GDF15 with CD9, a universal EV marker, is demonstrated. Finally, in functional assays it is observed that only 3D-EVs, unlike 2D-EVs, confer increased invasiveness and chemoresistance to cells in 2D. Collectively, this study highlights the value of engineered 3D microtissue cultures for the study of bona fide EV cargoes and their potential to identify biomarkers that are not detectable in EVs secreted by cells cultured in standard 2D conditions.

Dynamics of Endothelial Engagement and Filopodia Formation in Complex 3D Microscaffolds

The understanding of endothelium–extracellular matrix interactions during the initiation of new blood vessels is of great medical importance; however, the mechanobiological principles governing endothelial protrusive behaviours in 3D microtopographies remain imperfectly understood. In blood capillaries submitted to angiogenic factors (such as vascular endothelial growth factor, VEGF), endothelial cells can transiently transdifferentiate in filopodia-rich cells, named tip cells, from which angiogenesis processes are locally initiated. This protrusive state based on filopodia dynamics contrasts with the lamellipodia-based endothelial cell migration on 2D substrates. Using two-photon polymerization, we generated 3D microstructures triggering endothelial phenotypes evocative of tip cell behaviour. Hexagonal lattices on pillars (“open”), but not “closed” hexagonal lattices, induced engagement from the endothelial monolayer with the generation of numerous filopodia. The development of image analysis tools for filopodia tracking allowed to probe the influence of the microtopography (pore size, regular vs. elongated structures, role of the pillars) on orientations, engagement and filopodia dynamics, and to identify MLCK (myosin light-chain kinase) as a key player for filopodia-based protrusive mode. Importantly, these events occurred independently of VEGF treatment, suggesting that the observed phenotype was induced through microtopography. These microstructures are proposed as a model research tool for understanding endothelial cell behaviour in 3D fibrillary networks.

Comparison of direct and inverse methods for 2.5D traction force microscopy

Essential cellular processes such as cell adhesion, migration and division strongly depend on mechanical forces. The standard method to measure cell forces is traction force microscopy (TFM) on soft elastic substrates with embedded marker beads. While in 2D TFM one only reconstructs tangential forces, in 2.5D TFM one also considers normal forces. Here we present a systematic comparison between two fundamentally different approaches to 2.5D TFM, which in particular require different methods to deal with noise in the displacement data. In the direct method, one calculates strain and stress tensors directly from the displacement data, which in principle requires a divergence correction. In the inverse method, one minimizes the difference between estimated and measured displacements, which requires some kind of regularization. By calculating the required Green's functions in Fourier space from Boussinesq-Cerruti potential functions, we first derive a new variant of 2.5D Fourier Transform Traction Cytometry (FTTC). To simulate realistic traction patterns, we make use of an analytical solution for Hertz-like adhesion patches. We find that FTTC works best if only tangential forces are reconstructed, that 2.5D FTTC is more precise for small noise, but that the performance of the direct method approaches the one of 2.5D FTTC for larger noise, before both fail for very large noise. Moreover we find that a divergence correction is not really needed for the direct method and that it profits more from increased resolution than the inverse method.

Action Plots in Action: In-Depth Insights into Photochemical Reactivity

Predicting wavelength-dependent photochemical reactivity is challenging. Herein, we revive the well-established tool of measuring action spectra and adapt the technique to map wavelength-resolved covalent bond formation and cleavage in what we term "photochemical action plots". Underpinned by tunable lasers, which allow excitation of molecules with near-perfect wavelength precision, the photoinduced reactivity of several reaction classes have been mapped in detail. These include photoinduced cycloadditions and bond formation based on photochemically generated o-quinodimethanes and 1,3-dipoles such as nitrile imines as well as radical photoinitiator cleavage. Organized by reaction class, these data demonstrate that UV/vis spectra fail to act as a predictor for photochemical reactivity at a given wavelength in most of the examined reactions, with the photochemical reactivity being strongly red shifted in comparison to the absorption spectrum. We provide an encompassing perspective of the power of photochemical action plots for bond-forming reactions and their emerging applications in the design of wavelength-selective photoresists and photoresponsive soft-matter materials.

Development of 3D culture scaffolds for directional neuronal growth using 2-photon lithography

Conventional applications of transplant technology, applied to severe traumatic injuries of the nervous system, have met limited success in the clinics due to the complexity of restoring function to the damaged tissue. Neural tissue engineering aims to deploy scaffolds mimicking the physiological properties of the extracellular matrix to facilitate the elongation of axons and the repair of damaged nerves. However, the fabrication of ideal scaffolds with precisely controlled thickness, texture, porosity, alignment, and with the required mechanical strength, features needed for effective clinical applications, remains technically challenging. We took advantage of state-of-the-art 2-photon photolithography to fabricate highly ordered and biocompatible 3D nanogrid structures to enhance neuronal directional growth. First, we characterized the physical and chemical properties and proved the biocompatibility of said scaffolds by successfully culturing primary sensory and motor neurons on their surface. Interestingly, axons extended along the fibers with a high degree of alignment to the pattern of the nanogrid, as opposed to the lack of directionality observed on flat glass or polymeric surfaces, and could grow in 3D between different layers of the scaffold. The axonal growth pattern observed is highly desirable for the treatment of traumatic nerve damage occurring during peripheral and spinal cord injuries. Thus, our findings provide a proof of concept and explore the possibility of deploying aligned fibrous 3D scaffold/implants for the directed growth of axons, and could be used in the design of scaffolds targeted towards the restoration and repair of lost neuronal connections.

Three-Dimensional RAW264.7 Cell Model on Electrohydrodynamic Printed Poly(ε-Caprolactone) Scaffolds for In Vitro Study of Anti-Inflammatory Compounds

Inflammation plays an essential role in the human immune system, and anti-inflammatory compounds are important to promote health. However, the in vitro screening of these compounds is largely dependent on flat biology. Herein, we report our efforts in establishing a 3D inflammation murine macrophage model. Murine macrophage RAW 264.7 cells were cultured on poly(ε-caprolactone) (PCL) scaffolds fabricated through an electrohydrodynamic jetting 3D printer and their behavior were examined. Cells on PCL scaffolds showed a 3D shape and morphology with multilayers and a lower proliferation rate. Moreover, macrophages were not activated by scaffold material PCL and 3D microenvironment. The 3D cells showed greater sensitivity to lipopolysaccharide stimulation with higher production activity of nitric oxide (NO), nitric oxide synthases (iNOS), and cyclooxygenase-2 (COX-2). Additionally, the 3D macrophage model showed lower drug sensitivity to commercial anti-inflammatory drugs including aspirin, ibuprofen, and dexamethasone, and natural flavones apigenin and luteolin with higher IC₅₀ for NO production and lower iNOS and COX-2 inhibition efficacy. Overall, the 3D macrophage model showed promise for higher accurate screening of anti-inflammatory compounds. We developed, for the first time, a 3D macrophage model based on a 3D-printed PCL scaffold that provides an extracellular matrix environment for cells to grow in the 3D dimension. 3D-grown RAW 264.7 cells showed different sensitivities and responses to anti-inflammatory compounds from its 2D model. The 3D cells have lower sensitivity to both commercial and natural anti-inflammatory compounds. Consequently, our 3D macrophage model could be applied to screen anti-inflammatory compounds more accurately and thus holds great potential in next-generation drug screening applications.

A rapidly magnetically assembled stem cell microtissue with "hamburger" architecture and enhanced vascularization capacity

With the development of magnetic manipulation technology based on magnetic nanoparticles (MNPs), scaffold-free microtissues can be constructed utilizing the magnetic attraction of MNP-labeled cells. The rapid in vitro construction and in vivo vascularization of microtissues with complex hierarchical architectures are of great importance to the viability and function of stem cell microtissues. Endothelial cells are indispensable for the formation of blood vessels and can be used in the prevascularization of engineered tissue constructs. Herein, safe and rapid magnetic labeling of cells was achieved by incubation with MNPs for 1 h, and ultrathick scaffold-free microtissues with different sophisticated architectures were rapidly assembled, layer by layer, in 5 min intervals. The in vivo transplantation results showed that in a stem cell microtissue with trisection architecture, the two separated human umbilical vein endothelial cell (HUVEC) layers would spontaneously extend to the stem cell layers and connect with each other to form a spatial network of functional blood vessels, which anastomosed with the host vasculature. The "hamburger" architecture of stem cell microtissues with separated HUVEC layers could promote vascularization and stem cell survival. This study will contribute to the construction and application of structural and functional tissues or organs in the future.

Fused-Silica 3D Chiral Metamaterials via Helium-Assisted Microcasting Supporting Topologically Protected Twist Edge Resonances with High Mechanical Quality Factors

It is predicted theoretically that a 1D diatomic chain of 3D chiral cells can support a topological bandgap that allows for translating a small time-harmonic axial movement at one end of the chain into a resonantly enhanced large rotation of an edge state at the other end. This edge state is topologically protected such that an arbitrary mass of a mirror at the other end does not shift the eigenfrequency out of the bandgap. Herein, this complex 3D laser-beam-scanner microstructure is realized in fused-silica form. A novel microcasting approach is introduced that starts from a hollow polymer cast made by standard 3D laser nanoprinting. The cast is evacuated and filled with helium, such that a highly viscous commercial glass slurry is sucked in. After UV curing and thermal debinding of the polymer, the fused-silica glass is sintered at 1225 °C under vacuum. Detailed optical measurements reveal a mechanical quality factor of the twist-edge resonance of 2850 at around 278 kHz resonance frequency under ambient conditions. The microcasting approach can likely be translated to many other glasses, to metals and ceramics, and to complex architectures that are not or not yet amenable to direct 3D laser printing.

Feedback-based positioning and diffusion suppression of particles via optical control of thermoviscous flows

The ability to control the position of micron-size particles with high precision using tools such as optical tweezers has led to major advances in fields such as biology, physics and material science. In this paper, we present a novel optical strategy to confine particles in solution with high spatial control using feedback-controlled thermoviscous flows. We show that this technique allows micron-size particles to be positioned and confined with subdiffraction precision (24 nm), effectively suppressing their diffusion. Due to its physical characteristics, our approach might be particular attractive where laser exposure is of concern or materials are inherently incompatible with optical tweezing since it does not rely on contrast in the refractive index.

Utility of TPP-manufactured biophysical restrictions to probe multiscale cellular dynamics

Biophysical restrictions regulate protein diffusion, nucleus deformation, and cell migration, which are all universal and important processes for cells to perform their biological functions. However, current technologies addressing these multiscale questions are extremely limited. Herein, through two-photon polymerization (TPP), we present the precise, low-cost, and multiscale microstructures (micro-fences) as a versatile investigating platform. With nanometer-scale printing resolution and multiscale scanning capacity, TPP is capable of generating micro-fences with sizes of 0.5–1000 μm. These micro-fences are utilized as biophysical restrictions to determine the fluidity of supported lipid bilayers (SLB), to investigate the restricted diffusion of Src family kinase protein Lck on SLB, and also to reveal the mechanical bending of cell nucleus and T cell climbing ability. Taken together, the proposed versatile and low-cost micro-fences have great potential in probing the restricted dynamics of molecules, organelles, and cells to understand the basics of physical biology.

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Fiber-tip polymer clamped-beam probe for high-sensitivity nanoforce measurements

Micromanipulation and biological, material science, and medical applications often require to control or measure the forces asserted on small objects. Here, we demonstrate for the first time the microprinting of a novel fiber-tip-polymer clamped-beam probe micro-force sensor for the examination of biological samples. The proposed sensor consists of two bases, a clamped beam, and a force-sensing probe, which were developed using a femtosecond-laser-induced two-photon polymerization (TPP) technique. Based on the finite element method (FEM), the static performance of the structure was simulated to provide the basis for the structural design. A miniature all-fiber micro-force sensor of this type exhibited an ultrahigh force sensitivity of 1.51 nm μN-1, a detection limit of 54.9 nN, and an unambiguous sensor measurement range of ~2.9 mN. The Young's modulus of polydimethylsiloxane, a butterfly feeler, and human hair were successfully measured with the proposed sensor. To the best of our knowledge, this fiber sensor has the smallest force-detection limit in direct contact mode reported to date, comparable to that of an atomic force microscope (AFM). This approach opens new avenues towards the realization of small-footprint AFMs that could be easily adapted for use in outside specialized laboratories. As such, we believe that this device will be beneficial for high-precision biomedical and material science examination, and the proposed fabrication method provides a new route for the next generation of research on complex fiber-integrated polymer devices.

Facile Surface Functionalization Strategy for Two-Photon Lithography Microstructures

Two-photon lithography (TPL) is a powerful tool to construct small-scale objects with complex and precise 3D architectures. While the limited selection of chemical functionalities on the printed structures has restricted the application of this method in fabricating functional objects and devices, this study presents a facile, efficient, and extensively applicable method to functionalize the surfaces of the objects printed by TPL. TPL-printed objects, regardless of their compositions, can be efficiently functionalized by combining trichlorovinylsilane treatment and thiol-ene chemistry. Various functionalities can be introduced on the printed objects, without affecting their micro-nano topographies. Hence, microstructures with diverse functions can be generated using non-functional photoresists. Compared to existed strategies, this method is fast, highly efficient, and non photoresist-dependent. In addition, this method can be applied to various materials, such as metals, metal oxides, and plastics that can be potentially utilized in TPL or other 3D printing technologies. The applications of this method on the biofunctionalization of microrobots and cell scaffolds are also demonstrated.

In Vitro Osteoinductivity Assay of Hydroxylapatite Scaffolds, Obtained with Biomorphic Transformation Processes, Assessed Using Human Adipose Stem Cell Cultures

In this study, the in vitro biocompatibility and osteoinductive ability of a recently developed biomorphic hydroxylapatite ceramic scaffold (B-HA) derived from transformation of wood structures were analyzed using human adipose stem cells (hASCs). Cell viability and metabolic activity were evaluated in hASCs, parental cells and in recombinant genetically engineered hASC-eGFP cells expressing the green fluorescence protein. B-HA osteoinductivity properties, such as differentially expressed genes (DEG) involved in the skeletal development pathway, osteocalcin (OCN) protein expression and mineral matrix deposition in hASCs, were evaluated. In vitro induction of osteoblastic genes, such as Alkaline phosphatase (ALPL), Bone gamma-carboxyglutamate (gla) protein (BGLAP), SMAD family member 3 (SMAD3), Sp7 transcription factor (SP7) and Transforming growth factor, beta 3 (TGFB3) and Tumor necrosis factor (ligand) superfamily, member 11 (TNFSF11)/Receptor activator of NF-κB (RANK) ligand (RANKL), involved in osteoclast differentiation, was undertaken in cells grown on B-HA. Chondrogenic transcription factor SRY (sex determining region Y)-box 9 (SOX9), tested up-regulated in hASCs grown on the B-HA scaffold. Gene expression enhancement in the skeletal development pathway was detected in hASCs using B-HA compared to sintered hydroxylapatite (S-HA). OCN protein expression and calcium deposition were increased in hASCs grown on B-HA in comparison with the control. This study demonstrates the biocompatibility of the novel biomorphic B-HA scaffold and its potential use in osteogenic differentiation for hASCs. Our data highlight the relevance of B-HA for bone regeneration purposes.

Membrane-coated 3D architectures for bottom-up synthetic biology

One of the great challenges of bottom-up synthetic biology is to recreate the cellular geometry and surface functionality required for biological reactions. Of particular interest are lipid membrane interfaces where many protein functions take place. However, cellular 3D geometries are often complex, and custom-shaping stable lipid membranes on relevant spatial scales in the micrometer range has been hard to accomplish reproducibly. Here, we use two-photon direct laser writing to 3D print microenvironments with length scales relevant to cellular processes and reactions. We formed lipid bilayers on the surfaces of these printed structures, and we evaluated multiple combinatorial scenarios, where physiologically relevant membrane compositions were generated on several different polymer surfaces. Functional dynamic protein systems were reconstituted in vitro and their self-organization was observed in response to the 3D geometry. This method proves very useful to template biological membranes with an additional spatial dimension, and thus allows a better understanding of protein function in relation to the complex morphology of cells and organelles.

Multifunctional Scaffolds and Synergistic Strategies in Tissue Engineering and Regenerative Medicine

The increasing demand for organ replacements in a growing world with an aging population as well as the loss of tissues and organs due to congenital defects, trauma and diseases has resulted in rapidly evolving new approaches for tissue engineering and regenerative medicine (TERM). The extracellular matrix (ECM) is a crucial component in tissues and organs that surrounds and acts as a physical environment for cells. Thus, ECM has become a model guide for the design and fabrication of scaffolds and biomaterials in TERM. However, the fabrication of a tissue/organ replacement or its regeneration is a very complex process and often requires the combination of several strategies such as the development of scaffolds with multiple functionalities and the simultaneous delivery of growth factors, biochemical signals, cells, genes, immunomodulatory agents, and external stimuli. Although the development of multifunctional scaffolds and biomaterials is one of the most studied approaches for TERM, all these strategies can be combined among them to develop novel synergistic approaches for tissue regeneration. In this review we discuss recent advances in which multifunctional scaffolds alone or combined with other strategies have been employed for TERM purposes.

Rapid Prototyping of 3D Biochips for Cell Motility Studies Using Two-Photon Polymerization

The study of cellular migration dynamics and strategies plays a relevant role in the understanding of both physiological and pathological processes. An important example could be the link between cancer cell motility and tumor evolution into metastatic stage. These strategies can be strongly influenced by the extracellular environment and the consequent mechanical constrains. In this framework, the possibility to study the behavior of single cells when subject to specific topological constraints could be an important tool in the hands of biologists. Two-photon polymerization is a sub-micrometric additive manufacturing technique that allows the fabrication of 3D structures in biocompatible resins, enabling the realization of ad hoc biochips for cell motility analyses, providing different types of mechanical stimuli. In our work, we present a new strategy for the realization of multilayer microfluidic lab-on-a-chip constructs for the study of cell motility which guarantees complete optical accessibility and the possibility to freely shape the migration area, to tailor it to the requirements of the specific cell type or experiment. The device includes a series of micro-constrictions that induce different types of mechanical stress on the cells during their migration. We show the realization of different possible geometries, in order to prove the versatility of the technique. As a proof of concept, we present the use of one of these devices for the study of the motility of murine neuronal cancer cells under high physical confinement, highlighting their peculiar migration mechanisms.

Engineering Advanced In Vitro Models of Systemic Sclerosis for Drug Discovery and Development

Systemic sclerosis (SSc) is a complex multisystem disease with the highest case-specific mortality among all autoimmune rheumatic diseases, yet without any available curative therapy. Therefore, the development of novel therapeutic antifibrotic strategies that effectively decrease skin and organ fibrosis is needed. Existing animal models are cost-intensive, laborious and do not recapitulate the full spectrum of the disease and thus commonly fail to predict human efficacy. Advanced in vitro models, which closely mimic critical aspects of the pathology, have emerged as valuable platforms to investigate novel pharmaceutical therapies for the treatment of SSc. This review focuses on recent advancements in the development of SSc in vitro models, sheds light onto biological (e.g., growth factors, cytokines, coculture systems), biochemical (e.g., hypoxia, reactive oxygen species) and biophysical (e.g., stiffness, topography, dimensionality) cues that have been utilized for the in vitro recapitulation of the SSc microenvironment, and highlights future perspectives for effective drug discovery and validation.

3D-Printed Multi-Stimuli-Responsive Mobile Micromachines

Magnetically actuated and controlled mobile micromachines have the potential to be a key enabler for various wireless lab-on-a-chip manipulations and minimally invasive targeted therapies. However, their embodied, or physical, task execution capabilities that rely on magnetic programming and control alone can curtail their projected performance and functional diversity. Integration of stimuli-responsive materials with mobile magnetic micromachines can enhance their design toolbox, enabling independently controlled new functional capabilities to be defined. To this end, here, we show three-dimensional (3D) printed size-controllable hydrogel magnetic microscrews and microrollers that respond to changes in magnetic fields, temperature, pH, and divalent cations. We show two-way size-controllable microscrews that can reversibly swell and shrink with temperature, pH, and divalent cations for multiple cycles. We present the spatial adaptation of these microrollers for penetration through narrow channels and their potential for controlled occlusion of small capillaries (30 μm diameter). We further demonstrate one-way size-controllable microscrews that can swell with temperature up to 65% of their initial length. These hydrogel microscrews, once swollen, however, can only be degraded enzymatically for removal. Our results can inspire future applications of 3D- and 4D-printed multifunctional mobile microrobots for precisely targeted obstructive interventions (e.g., embolization) and lab- and organ-on-a-chip manipulations.

Red-Light Driven Photocatalytic Oxime Ligation for Bioorthogonal Hydrogel Design

Light-mediated polymer cross-linking is frequently employed for the preparation of hydrogels for biomedical applications. However, most photopolymerization processes require activation by UV light or short wavelength visible light, which are highly absorbed by skin and tissue, limiting their uses in transdermal initiation. Herein, we introduce red light-enabled oxime ligation by the in situ photogeneration of aldehydes, which rapidly react with hydroxylamines. We demonstrate efficient polymer cross-linking behind a dermal tissue model by red light initiation. Optimization of the photopolymerization conditions allows for 3D encapsulation of human foreskin fibroblasts with good cell viability postencapsulation.

Biointerface Materials for Cellular Adhesion: Recent Progress and Future Prospects

While many natural instances of adhesion between cells and biological macromolecules have been elucidated, understanding how to mimic these adhesion events remains to be a challenge. Discovering new biointerface materials that can provide an appropriate environment, and in some cases, also providing function similar to the body’s own extracellular matrix, would be highly beneficial to multiple existing applications in biomedical and biological engineering, and provide the necessary insight for the advancement of new technology. Such examples of current applications that would benefit include biosensors, high-throughput screening and tissue engineering. From a mechanical perspective, these biointerfaces would function as bioactuators that apply focal adhesion points onto cells, allowing them to move and migrate along a surface, making biointerfaces a very relevant application in the field of actuators. While it is evident that great strides in progress have been made in the area of synthetic biointerfaces, we must also acknowledge their current limitations as described in the literature, leading to an inability to completely function and dynamically respond like natural biointerfaces. In this review, we discuss the methods, materials and, possible applications of biointerface materials used in the current literature, and the trends for future research in this area.

Precision 3D-Printed Cell Scaffolds Mimicking Native Tissue Composition and Mechanics

Cellular dynamics are modeled by the 3D architecture and mechanics of the extracellular matrix (ECM) and vice versa. These bidirectional cell-ECM interactions are the basis for all vital tissues, many of which have been investigated in 2D environments over the last decades. Experimental approaches to mimic in vivo cell niches in 3D with the highest biological conformity and resolution can enable new insights into these cell-ECM interactions including proliferation, differentiation, migration, and invasion assays. Here, two-photon stereolithography is adopted to print up to mm-sized high-precision 3D cell scaffolds at micrometer resolution with defined mechanical properties from protein-based resins, such as bovine serum albumin or gelatin methacryloyl. By modifying the manufacturing process including two-pass printing or post-print crosslinking, high precision scaffolds with varying Young's moduli ranging from 7-300 kPa are printed and quantified through atomic force microscopy. The impact of varying scaffold topographies on the dynamics of colonizing cells is observed using mouse myoblast cells and a 3D-lung microtissue replica colonized with primary human lung fibroblast. This approach will allow for a systematic investigation of single-cell and tissue dynamics in response to defined mechanical and bio-molecular cues and is ultimately scalable to full organs.

Characterization of Mesenchymal Stem Cell Differentiation within Miniaturized 3D Scaffolds through Advanced Microscopy Techniques

Three-dimensional culture systems and suitable substrates topographies demonstrated to drive stem cell fate in vitro by mechanical conditioning. For example, the Nichoid 3D scaffold remodels stem cells and shapes nuclei, thus promoting stem cell expansion and stemness maintenance. However, the mechanisms involved in force transmission and in biochemical signaling at the basis of fate determination are not yet clear. Among the available investigation systems, confocal fluorescence microscopy using fluorescent dyes enables the observation of cell function and shape at the subcellular scale in vital and fixed conditions. Contrarily, nonlinear optical microscopy techniques, which exploit multi-photon processes, allow to study cell behavior in vital and unlabeled conditions. We apply confocal fluorescence microscopy, coherent anti-Stokes Raman scattering (CARS), and second harmonic generation (SHG) microscopy to characterize the phenotypic expression of mesenchymal stem cells (MSCs) towards adipogenic and chondrogenic differentiation inside Nichoid scaffolds, in terms of nuclear morphology and specific phenotypic products, by comparing these techniques. We demonstrate that the Nichoid maintains a rounded nuclei during expansion and differentiation, promoting MSCs adipogenic differentiation while inhibiting chondrogenesis. We show that CARS and SHG techniques are suitable for specific estimation of the lipid and collagenous content, thus overcoming the limitations of using unspecific fluorescent probes.

3D Printing and NIR Fluorescence Imaging Techniques for the Fabrication of Implants

Three-dimensional (3D) printing technology holds great potential to fabricate complex constructs in the field of regenerative medicine. Researchers in the surgical fields have used 3D printing techniques and their associated biomaterials for education, training, consultation, organ transplantation, plastic surgery, surgical planning, dentures, and more. In addition, the universal utilization of 3D printing techniques enables researchers to exploit different types of hardware and software in, for example, the surgical fields. To realize the 3D-printed structures to implant them in the body and tissue regeneration, it is important to understand 3D printing technology and its enabling technologies. This paper concisely reviews 3D printing techniques in terms of hardware, software, and materials with a focus on surgery. In addition, it reviews bioprinting technology and a non-invasive monitoring method using near-infrared (NIR) fluorescence, with special attention to the 3D-bioprinted tissue constructs. NIR fluorescence imaging applied to 3D printing technology can play a significant role in monitoring the therapeutic efficacy of 3D structures for clinical implants. Consequently, these techniques can provide individually customized products and improve the treatment outcome of surgeries.

Mechanical stimulation of single cells by reversible host-guest interactions in 3D microscaffolds

Many essential cellular processes are regulated by mechanical properties of their microenvironment. Here, we introduce stimuli-responsive composite scaffolds fabricated by three-dimensional (3D) laser lithography to simultaneously stretch large numbers of single cells in tailored 3D microenvironments. The key material is a stimuli-responsive photoresist containing cross-links formed by noncovalent, directional interactions between β-cyclodextrin (host) and adamantane (guest). This allows reversible actuation under physiological conditions by application of soluble competitive guests. Cells adhering in these scaffolds build up initial traction forces of ~80 nN. After application of an equibiaxial stretch of up to 25%, cells remodel their actin cytoskeleton, double their traction forces, and equilibrate at a new dynamic set point within 30 min. When the stretch is released, traction forces gradually decrease until the initial set point is retrieved. Pharmacological inhibition or knockout of nonmuscle myosin 2A prevents these adjustments, suggesting that cellular tensional homeostasis strongly depends on functional myosin motors.