The effect of concentration, thermal history and cell seeding density on the initial mechanical properties of agarose hydrogels

Development of Improved Confined Compression Testing Setups for Use in Stress Relaxation Testing of Viscoelastic Biomaterials

The development of cell-based biomaterial alternatives holds significant promise in tissue engineering applications, but it requires accurate mechanical assessment. Herein, we present the development of a novel 3D-printed confined compression apparatus, fabricated using clear resin, designed to cater to the unique demands of biomaterial developers. Our objective was to enhance the precision of force measurements and improve sample visibility during compression testing. We compared the performance of our innovative 3D-printed confined compression setup to a conventional setup by performing stress relaxation testing on hydrogels with variable degrees of crosslinking. We assessed equilibrium force, aggregate modulus, and peak force. This study demonstrates that our revised setup can capture a larger range of force values while simultaneously improving accuracy. We were able to detect significant differences in force and aggregate modulus measurements of hydrogels with variable degrees of crosslinking using our revised setup, whereas these were indistinguishable with the convectional apparatus. Further, by incorporating a clear resin in the fabrication of the compression chamber, we improved sample visibility, thus enabling real-time monitoring and informed assessment of biomaterial behavior under compressive testing.

Ionic Strength Impacts the Physical Properties of Agarose Hydrogels

Agarose is a natural polysaccharide known for its ability to form thermoreversible hydrogels. While the effects of curing temperature and polysaccharide concentration on mechanical properties have been discussed in the literature, the role of ionic strength has been less studied. In the present manuscript, we investigate the effects of supporting salt concentration and the role of cation (i.e. Na+ or Li+, neighbors in the Hofmeister series), on the setting and performance of agarose hydrogels. Compressive and rheological measurements show that the supporting salts reduce the immediate elastic response of agarose hydrogels, with Li+ showing a stronger effect than Na+ at high ionic strength, while they significantly increase the extent of linear stress-strain response (i.e., linear elasticity). The presence of increasing amounts of added supporting salt also leads to a reduction in hysteresis during mechanical deformation due to loading and unloading cycles, which is more pronounced with Li+ than with Na+. The combination of rheological measurements and NMR relaxometry shows a mesh size in agarose hydrogels in the order of 6-17 nm, with a thickness of the water layer bound to the biopolymer of about 3 nm. Of note, the different structuring of the water within the hydrogel network due to the different alkali seems to play a role for the final performance of the hydrogels.

Synthesis, Characterization and Catechol-Based Bioinspired Adhesive Properties in Wet Medium of Poly(2-Hydroxyethyl Methacrylate-co-Acrylamide) Hydrogels

Hydrogels consist of crosslinked hydrophilic polymers from which their mechanical properties can be modulated for a wide variety of applications. In the last decade, many catechol-based bioinspired adhesives have been developed following the strategy of incorporating catechol moieties into polymeric backbones. In this work, in order to further investigate the adhesive properties of hydrogels and their potential advantages, several hydrogels based on poly(2-hydroxyethyl methacrylate-co-acrylamide) with N'N-methylene-bisacrylamide (MBA), without/with L-3,4-dihydroxyphenylalanine (DOPA) as a catecholic crosslinker, were prepared via free radical copolymerization. 2-Hydroxyethyl methacrylate (HEMA) and acrylamide (AAm) were used as comonomers and MBA and DOPA both as crosslinking agents at 0.1, 0.3, and 0.5 mol.-%, respectively. The polymeric hydrogels were characterized by Fourier transform infrared spectroscopy (FT-IR), thermal analysis and swelling behavior analysis. Subsequently, the mechanical properties of hydrogels were determined. The elastic properties of the hydrogels were quantified using Young's modulus (stress-strain curves). According to the results herein, the hydrogel with a feed monomer ratio of 1:1 at 0.3 mol.-% of MBA and DOPA displayed the highest rigidity and higher failure shear stress (greater adhesive properties). In addition, the fracture lap shear strength of the biomimetic polymeric hydrogel was eight times higher than the initial one (only containing MBA); however at 0.5 mol.-% MBA/DOPA, it was only two times higher. It is understood that when two polymer surfaces are brought into close contact, physical self-bonding (Van der Waals forces) at the interface may occur in an -OH interaction with wet contacting surfaces. The hydrogels with DOPA provided an enhancement in the flexibility compared to unmodified hydrogels, alongside reduced swelling behavior on the biomimetic hydrogels. This approach expands the possible applications of hydrogels as adhesive materials, in wet conditions, within scaffolds that are commonly used as biomaterials in cartilage tissue engineering.

One-Pot Approach to Synthesize Tough and Cell Adhesive Double-Network Hydrogels Consisting of Fully Synthetic Materials of Self-Assembling Peptide and Poly(ethylene glycol)

Hydrogels with a double network (DN) structure are compelling biomaterials, holding potential for use as artificial extracellular matrices. Generally, the DN approach imparts hydrogels with high mechanical strength and cell-adhesive properties. However, achieving this often demands a complex multistep process involving potentially hazardous free-radical polymerization, which can result in toxicity. This limits their broad biological applications. In this work, we introduce a straightforward yet biocompatible method to fabricate tough and cell-adhesive DN hydrogels using entirely synthetic materials: the self-assembling peptide (RADA16) and poly(ethylene glycol) (PEG). An in situ mixing of these components leads to the sequential formation of DN hydrogels─first through the self-assembly of the RADA16 peptide and then via chemical cross-linking between PEG molecules. Hydrogels produced this way exhibited up to a 10-fold increase in fracture energy, and cells seeded on their surfaces showcased good attachment. Our design underscores the efficacy of the DN approach and the promising applications of peptides in tissue engineering.

High-Throughput Gel Microbeads as Incubators for Bacterial Competition Study

Bacteria primarily live in structured environments, such as colonies and biofilms, attached to surfaces or growing within soft tissues. They are engaged in local competitive and cooperative interactions impacting our health and well-being, for example, by affecting population-level drug resistance. Our knowledge of bacterial competition and cooperation within soft matrices is incomplete, partly because we lack high-throughput tools to quantitatively study their interactions. Here, we introduce a method to generate a large amount of agarose microbeads that mimic the natural culture conditions experienced by bacteria to co-encapsulate two strains of fluorescence-labeled Escherichia coli. Focusing specifically on low bacterial inoculum (1-100 cells/capsule), we demonstrate a study on the formation of colonies of both strains within these 3D scaffolds and follow their growth kinetics and interaction using fluorescence microscopy in highly replicated experiments. We confirmed that the average final colony size is inversely proportional to the inoculum size in this semi-solid environment as a result of limited available resources. Furthermore, the colony shape and fluorescence intensity per colony are distinctly different in monoculture and co-culture. The experimental observations in mono- and co-culture are compared with predictions from a simple growth model. We suggest that our high throughput and small footprint microbead system is an excellent platform for future investigation of competitive and cooperative interactions in bacterial communities under diverse conditions, including antibiotics stress.

Automated analysis of soft material microindentation

An understanding of the mechanical properties of soft hydrogel materials over multiple length scales is important for their application in many fields. Typical measurement methods provide either bulk mechanical properties (compression, tensile, rheology) or probing of nano or microscale properties and heterogeneity (nanoindentation, AFM). In this work we demonstrate the complementarity of instrumented microindentation to these techniques, as it provides representative Young's moduli for soft materials with minimal influence of the experimental parameters chosen, and allows mechanical property mapping across macroscopic areas. To enable automated analysis of the large quantities of data required for these measurements, we develop a new fitting algorithm to process indentation data. This method allows for the determination of Young's moduli from imperfect data by automatic selection of a region of the indentation curve which does not display inelastic deformation or substrate effects. We demonstrate the applicability of our approach with a range of hydrogels, including materials with patterns and gradients in stiffness, and expect the techniques described here to be useful developments for the mechanical analysis of a wide range of soft and biological systems.

Systematic review on the application of 3D-bioprinting technology in orthoregeneration: current achievements and open challenges

BACKGROUND

Joint degeneration and large or complex bone defects are a significant source of morbidity and diminished quality of life worldwide. There is an unmet need for a functional implant with near-native biomechanical properties. The potential for their generation using 3D bioprinting (3DBP)-based tissue engineering methods was assessed. We systematically reviewed the current state of 3DBP in orthoregeneration.

METHODS

This review was performed using PubMed and Web of Science. Primary research articles reporting 3DBP of cartilage, bone, vasculature, and their osteochondral and vascular bone composites were considered. Full text English articles were analyzed.

RESULTS

Over 1300 studies were retrieved, after removing duplicates, 1046 studies remained. After inclusion and exclusion criteria were applied, 114 articles were analyzed fully. Bioink material types and combinations were tallied. Cell types and testing methods were also analyzed. Nearly all papers determined the effect of 3DBP on cell survival. Bioink material physical characterization using gelation and rheology, and construct biomechanics were performed. In vitro testing methods assessed biochemistry, markers of extracellular matrix production and/or cell differentiation into respective lineages. In vivo proof-of-concept studies included full-thickness bone and joint defects as well as subcutaneous implantation in rodents followed by histological and µCT analyses to demonstrate implant growth and integration into surrounding native tissues.

CONCLUSIONS

Despite its relative infancy, 3DBP is making an impact in joint and bone engineering. Several groups have demonstrated preclinical efficacy of mechanically robust constructs which integrate into articular joint defects in small animals. However, notable obstacles remain. Notably, researchers encountered pitfalls in scaling up constructs and establishing implant function and viability in long term animal models. Further, to translate from the laboratory to the clinic, standardized quality control metrics such as construct stiffness and graft integration metrics should be established with investigator consensus. While there is much work to be done, 3DBP implants have great potential to treat degenerative joint diseases and provide benefit to patients globally.

Agarose hydrogel composite supports microgreen cultivation with enhanced porosity and continuous water supply under terrestrial and microgravitational conditions

Hydrogels are attractive soilless media for plant cultivation with strong water and nutrient retention. However, pristine hydrogels contain mostly ultra-micro pores and lack air-filled porosity for root zone aeration. Herein we report a porous hydrogel composite comprising an agarose network and porous growing mix particle (GMP) fillers. The agarose backbone allowed the composite to sustain a 12-d growth cycle for red cabbage microgreens without the need for watering or crew interaction. Moreover, the GMP induced greater total pore volume and increased the prevalence of pores >30 μm by 8-fold. Further investigation suggested that the nutrients from GMP accounted for a 54 % increase in microgreen yield over pristine hydrogel, while the porous structure introduced by GMP improved the yield by another 44 %. Increased air-filled porosity accelerated the water transport and loss of hydrogel but maintained favorable water potential levels for plant extraction. Finally, the hydrogel composite supported microgreen growth satisfyingly under simulated microgravity despite some morphological changes. Results of this study reveal a novel growth substrate that is lightweight, convenient, and water-efficient, while effectively sustaining plant growth for multiple applications including indoor farming and space farming.

Rapid and Economical Drug-Eluting IOL Preparation via Thermoresponsive Agarose Coating for Effective Posterior Capsular Opacification Prevention

Posterior capsular opacification (PCO), the highest incidence complication after cataract surgery, is mainly due to the attachment, proliferation, and migration of the residual lens epithelial cells (LECs). Although the drug-eluting IOLs have been proved to be an effective way to prevent PCO incidence, its preparations are time consuming and require tedious preparation steps. Herein, the thermoreversible agarose is adopted to prepare drug-eluting IOL. Such functional coating can be obtained easily by simple immersion in the antiproliferative drug containing hot agarose and taken out for cooling, which not only does not affect the optical property but also can effectively decrease the PCO incidence after intraocular implantation. As a result, the proposed agarose coating provides a rapid and economical alternative of drug-eluting IOL fabrication for PCO prevention.

Injectable three-dimensional tumor microenvironments to study mechanobiology in ovarian cancer

Epithelial ovarian cancers are among the most aggressive forms of gynecological malignancies. Despite the advent of poly adenosine diphosphate-ribose polymerase (PARP) and checkpoint inhibitors, improvement to patient survival has been modest. Limited in part by clinical translation, beneficial therapeutic strategies remain elusive in ovarian cancers. Although elevated levels of extracellular proteins, including collagens, proteoglycans, and glycoproteins, have been linked to chemoresistance, they are often missing from the processes of drug- development and screening. Biophysical and biochemical signaling from the extracellular matrix (ECM) determine cellular phenotype and affect both tumor progression and therapeutic response. However, many state-of-the-art tumor models fail to mimic the complexities of the tumor microenvironment (TME) and omit key signaling components. In this article, two interpenetrating network (IPN) hydrogel scaffold platforms, comprising of alginate-collagen or agarose-collagen, have been characterized for use as 3D in vitro models of epithelial ovarian cancer ECM. These highly tunable, injection mold compatible, and inexpensive IPNs replicate the critical governing physical and chemical signaling present within the ovarian TME. Additionally, an effective and cell-friendly live-cell retrieval method has been established to recover cells post-encapsulation. Lastly, functional mechanotransduction in ovarian cancers was demonstrated by increasing scaffold stiffness within the 3D in vitro ECM models. With these features, the agarose-collagen and alginate-collagen hydrogels provide a robust TME for the study of mechanobiology in epithelial cancers. STATEMENT OF SIGNIFICANCE: Ovarian cancer is the most lethal gynecologic cancer afflicting women today. Here we present the development, characterization, and validation of 3D interpenetrating platforms to shift the paradigm in standard in vitro modeling. These models help elucidate the roles of biophysical and biochemical cues in ovarian cancer progression. The agarose-collagen and alginate-collagen interpenetrating network (IPN) hydrogels are simple to fabricate, inexpensive, and can be modified to create custom mechanical stiffnesses and concentrations of bio-adhesive motifs. Given that investigations into the roles of biophysical characteristics in ovarian cancers have provided incongruent results, we believe that the IPN platforms will be critically important to uncovering molecular drivers. We also expect these platforms to be broadly applicable to studies involving mechanobiology in solid tumors.

pH Sensitive Pluronic Acid/Agarose-Hydrogels as Controlled Drug Delivery Carriers: Design, Characterization and Toxicity Evaluation

The objective of this study was to fabricate and evaluate a pH sensitive cross-linked polymeric network through the free radical polymerization technique for the model drug, cyclophosphamide, used in the treatment of non-Hodgkin’s lymphoma. The Hydrogels were prepared using a polymeric blend of agarose, Pluronic acid, glutaraldehyde, and methacrylic acid. The prepared hydrogels were characterized for drug loading (%), swelling pattern, release behavior, the ingredient’s compatibility, structural evaluation, thermal integrity, and toxicity evaluation in rabbits. The new polymer formation was evident from FTIR findings. The percentage loaded into the hydrogels was in the range of 58.65–75.32%. The developed hydrogels showed significant differences in swelling dynamics and drug release behavior in simulated intestinal fluid (SIF) when compared with simulated gastric fluid (SGF). The drug release was persistent and performed in a controlled manner for up to 24 h. A toxicity study was conducted on white albino rabbits. The developed hydrogels did not show any signs of ocular, skin, or oral toxicity; therefore, these hydrogels can be regarded as safe and potential carriers for controlled drug delivery in biomedical applications.

Poroelastic osmoregulation of living cell volume

Cells maintain their volume through fine intracellular osmolarity regulation. Osmotic challenges drive fluid into or out of cells causing swelling or shrinkage, respectively. The dynamics of cell volume changes depending on the rheology of the cellular constituents and on how fast the fluid permeates through the membrane and cytoplasm. We investigated whether and how poroelasticity can describe volume dynamics in response to osmotic shocks. We exposed cells to osmotic perturbations and used defocusing epifluorescence microscopy on membrane-attached fluorescent nanospheres to track volume dynamics with high spatiotemporal resolution. We found that a poroelastic model that considers both geometrical and pressurization rates captures fluid-cytoskeleton interactions, which are rate-limiting factors in controlling volume changes at short timescales. Linking cellular responses to osmotic shocks and cell mechanics through poroelasticity can predict the cell state in health, disease, or in response to novel therapeutics.

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Cell height changes can be finely captured by defocusing microscopy

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Water permeation and cellular deformability regulate dynamics of cell volume changes

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Poroelasticity describes the dynamics of cell volume changes

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The response of cell to hypo or hyperosmotic shocks are modeled by poroelasticity

Targeting Tumor-Stromal Interactions in Pancreatic Cancer: Impact of Collagens and Mechanical Traits

Pancreatic ductal adenocarcinoma (PDAC) has one of the worst outcomes among cancers with a 5-years survival rate of below 10%. This is a result of late diagnosis and the lack of effective treatments. The tumor is characterized by a highly fibrotic stroma containing distinct cellular components, embedded within an extracellular matrix (ECM). This ECM-abundant tumor microenvironment (TME) in PDAC plays a pivotal role in tumor progression and resistance to treatment. Cancer-associated fibroblasts (CAFs), being a dominant cell type of the stroma, are in fact functionally heterogeneous populations of cells within the TME. Certain subtypes of CAFs are the main producer of the ECM components of the stroma, with the most abundant one being the collagen family of proteins. Collagens are large macromolecules that upon deposition into the ECM form supramolecular fibrillar structures which provide a mechanical framework to the TME. They not only bring structure to the tissue by being the main structural proteins but also contain binding domains that interact with surface receptors on the cancer cells. These interactions can induce various responses in the cancer cells and activate signaling pathways leading to epithelial-to-mesenchymal transition (EMT) and ultimately metastasis. In addition, collagens are one of the main contributors to building up mechanical forces in the tumor. These forces influence the signaling pathways that are involved in cell motility and tumor progression and affect tumor microstructure and tissue stiffness by exerting solid stress and interstitial fluid pressure on the cells. Taken together, the TME is subjected to various types of mechanical forces and interactions that affect tumor progression, metastasis, and drug response. In this review article, we aim to summarize and contextualize the recent knowledge of components of the PDAC stroma, especially the role of different collagens and mechanical traits on tumor progression. We furthermore discuss different experimental models available for studying tumor-stromal interactions and finally discuss potential therapeutic targets within the stroma.

Meniscus cell regional phenotypes: Dedifferentiation and reversal by biomaterial embedding

Meniscus injuries are common and a major cause of long-term joint degeneration and disability. Current treatment options are limited, so novel regenerative therapies or tissue engineering strategies are urgently needed. The development of new therapies is hindered by a lack of knowledge regarding the cellular biology of the meniscus and a lack of well-established methods for studying meniscus cells in vitro. The goals of this study were to (1) establish baseline expression profiles and dedifferentiation patterns of inner and outer zone primary meniscus cells, and (2) evaluate the utility of poly(ethylene glycol) diacrylate (PEGDA) and gelatin methacrylate (GelMA) polymer hydrogels to reverse dedifferentiation trends for long-term meniscus cell culture. Using reverse transcription-quantitative polymerase chain reaction, we measured expression levels of putative meniscus phenotype marker genes in freshly isolated meniscus tissue, tissue explant culture, and monolayer culture of inner and outer zone meniscus cells from porcine knees to establish baseline dedifferentiation characteristics, and then compared these expression levels to PEGDA/GelMA embedded passaged meniscus cells. COL1A1 showed robust upregulation, while CHAD, CILP, and COMP showed downregulation with monolayer culture. Expression levels of COL2A1, ACAN, and SOX9 were surprisingly similar between inner and outer zone tissue and were found to be less sensitive as markers of dedifferentiation. When embedded in PEGDA/GelMA hydrogels, expression levels of meniscus cell phenotype genes were significantly modulated by varying the ratio of polymer components, allowing these materials to be tuned for phenotype restoration, meniscus cell culture, and tissue engineering applications.

A freeze-thaw PVA hydrogel loaded with guava leaf extract: physical and antibacterial properties

A polyvinyl alcohol (PVA) hydrogel loaded with guava leaf extract (GLE) has potential applications as a wound dressing with good antibacterial activity. This study succeeded in fabricating a PVA hydrogel containing GLE using the freeze–thaw (FT) method. By varying the GLE concentration, we can adjust the physical properties of the hydrogel. The addition of GLE results in a decrease in cross-linking during gelation and an increase in the pore size of the hydrogels. The increase of the pore size made the swelling increase and the mechanical strength decrease. The weight loss of the hydrogel also increases because the phosphate buffer saline (PBS) dissolves the GLE. Increasing the GLE concentration caused the Fourier-transform infrared (FTIR) absorbance peaks to widen due to hydrogen bonds formed during the FT process. The crystalline phase was transformed into an amorphous phase in the PVA/GLE hydrogel based on the X-ray diffraction (XRD) spectra. The differential scanning calorimetry (DSC) characterization showed a significant decrease in the hydrogel weight over temperatures of 30–150 °C due to the evaporation of water from the hydrogel matrix. The zone of inhibition of the PVA/GLE hydrogel increased with antibacterial activity against Staphylococcus aureus of 17.93% per gram and 15.79% per gram against Pseudomonas aeruginosa.

A polyvinyl alcohol (PVA) hydrogel loaded with guava leaf extract (GLE) has potential applications as a wound dressing with good antibacterial activity.

Pancreatic Ductal Adenocarcinoma: Relating Biomechanics and Prognosis

Pancreatic ductal adenocarcinoma (PDAC) is the most common form of pancreatic cancer and carries a dismal prognosis. Resectable patients are treated predominantly with surgery while borderline resectable patients may receive neoadjuvant treatment (NAT) to downstage their disease prior to possible resection. PDAC tissue is stiffer than healthy pancreas, and tissue stiffness is associated with cancer progression. Another feature of PDAC is increased tissue heterogeneity. We postulate that tumour stiffness and heterogeneity may be used alongside currently employed diagnostics to better predict prognosis and response to treatment. In this review we summarise the biomechanical changes observed in PDAC, explore the factors behind these changes and describe the clinical consequences. We identify methods available for assessing PDAC biomechanics ex vivo and in vivo, outlining the relative merits of each. Finally, we discuss the potential use of radiological imaging for prognostic use.

Enhanced articular cartilage decellularization using a novel perfusion-based bioreactor method

Current decellularization methods for articular cartilages require many steps, various and high amounts of detergents, and a relatively long time to produce decellularized scaffolds. In addition, such methods often damage the essential components and the structure of the tissue. This study aims to introduce a novel perfusion-based bioreactor (PBB) method to decellularize bovine articular cartilages efficiently while reducing the harmful physical and chemical steps as well as the duration of the process. This leads to better preservation of the structure and the essential components of the native tissue. Firstly, a certain number of channels (Ø 180 μm) were introduced into both sides of cylindrical articular bovine cartilage disks (5 mm in diameter and 1 mm in thickness). Next, the disks were decellularized in the PBB and a shaker as the control. Using the PBB method resulted in ∼90% reduction of DNA content in the specimens, which was significantly higher than those of the shaker results with ∼60%. Also, ∼50% sulfated glycosaminoglycan (sGAG) content and ∼92% of the compression properties were maintained implying the efficient preservation of the structure and components of the scaffolds. Moreover, the current study indicated that the PBB specimens supported the adherence and proliferation of the new cells effectively. In conclusion, the results show that the use of PBB method increases the efficiency of producing decellularized cartilage scaffolds with a better maintenance of essential components and structure, while reducing the chemicals and steps required for the process. This will pave the way for producing close-to-natural scaffolds for cartilage tissue engineering.

Cell-Laden Nanocellulose/Chitosan-Based Bioinks for 3D Bioprinting and Enhanced Osteogenic Cell Differentiation

3D bioprinting has recently emerged as a very useful tool in tissue engineering and regenerative medicine. However, developing suitable bioinks to fabricate specific tissue constructs remains a challenging task. Herein, we report on a nanocellulose/chitosan-based bioink, which is compatible with a 3D extrusion-based bioprinting technology, to design and engineer constructs for bone tissue engineering and regeneration applications. Bioinks were prepared using thermogelling chitosan, glycerophosphate, hydroxyethyl cellulose, and cellulose nanocrystals (CNCs). Formulations were optimized by varying the concentrations of glycerophosphate (80-300 mM), hydroxyethyl cellulose (0-0.5 mg/mL), and CNCs (0-2% w/v) to promote fast gelation kinetics (<7 s) at 37 °C and retain the shape integrity of constructs post 3D bioprinting. We investigated the effect of CNCs and pre-osteoblast cells (MC3T3-E1) on the rheological properties of bioinks, bioink printability, and mechanical properties of bioprinted scaffolds. We demonstrate that the addition of CNCs and cells (5 million cells/mL) significantly improved the viscosity of bioinks and the mechanical properties of chitosan scaffolds post-fabrication. The bioinks were biocompatible and printable at an optimized range of printing pressures (12-20 kPa) that did not compromise cell viability. The presence of CNCs promoted greater osteogenesis of MC3T3-E1 cells in chitosan scaffolds as shown by the upregulation of alkaline phosphatase activity, higher calcium mineralization, and extracellular matrix formation. The versatility of this CNCs-incorporated chitosan hydrogel makes it attractive as a bioink for 3D bioprinting to engineer scaffolds for bone tissue engineering and other therapeutic applications.

Design and construction of a novel measurement device for mechanical characterization of hydrogels: A case study

Natural biopolymer-based hydrogels especially agarose and collagen gels, considering their biocompatibility with cells and their capacity to mimic biological tissues, have widely been used for in-vitro experiments and tissue engineering applications in recent years; nevertheless their mechanical properties are not always optimal for these purposes. Regarding the importance of the mechanical properties of hydrogels, many mechanical characterization studies have been carried out for such biopolymers. In this work, we have focused on understanding the mechanical role of agarose and collagen concentration on the hydrogel strength and elastic behavior. In this direction, Amirkabir Magnetic Bead Rheometry (AMBR) characterization device equipped with an optimized electromagnet, was designed and constructed for the measurement of hydrogel mechanical properties. The operation of AMBR set-up is based on applying a magnetic field to actuate magnetic beads in contact with the gel surface in order to actuate the gel itself. In simple terms the magnetic beads leads give rise to mechanical shear stress on the gel surface when under magnetic influence and together with the associated bead-gel displacement it is possible to calculate the hydrogel shear modulus. Agarose and Collagen gels with respectively 0.2-0.6 wt % and 0.2-0.5 wt % percent concentrations were prepared for mechanical characterization in terms of their shear modulus. The shear modulus values for the different percent concentrations of the agarose gel were obtained in the range 250-650 Pa, indicating the shear modulus increases by increasing in the agar gel concentration. In addition to this, the values of shear modulus for the collagen gel increase as function of concentration in the range 240-520 Pa in accordance with an approximately linear relationship between collagen concentration and gel strength.

Inhibition of hypertrophy and improving chondrocyte differentiation by MMP-13 inhibitor small molecule encapsulated in alginate-chondroitin sulfate-platelet lysate hydrogel

BACKGROUND

Mesenchymal stem cells are a promising cell source for chondrogenic differentiation and have been widely used in several preclinical and clinical studies. However, they are prone to an unwanted differentiation process towards hypertrophy that limits their therapeutic efficacy. Matrix metallopeptidase 13 (MMP-13) is a well-known factor regulated during this undesirable event. MMP-13 is a collagen degrading enzyme, which is also highly expressed in the hypertrophic zone of the growth plate and in OA cartilage. Accordingly, we investigated the effect of MMP-13 inhibition on MSC hypertrophy.

METHODS

In this study, 5-bromoindole-2-carboxylic acid (BICA) was used as an inhibitory agent for MMP-13 expression. After identifying its optimal concentration, BICA was mixed into a hydrogel and the release rate was studied. To prepare the ideal hydrogel, chondroitin sulfate (CS) and platelet lysate (PL) were mixed with sodium alginate (Alg) at concentrations selected based on synergistic mechanical and rheometric properties. Then, four hydrogels were prepared by combining alginate (1.5%w/v) and/or CS (1%w/v) and/or PL (20%v/v). The chondrogenic potential and progression to hypertrophy of human bone marrow-derived mesenchymal stem cell (hBM-MSC)-loaded hydrogels were investigated under free swelling and mechanical loading conditions, in the presence and absence of BICA.

RESULTS

Viability of hBM-MSCs seeded in the four hydrogels was similar. qRT-PCR revealed that BICA could successfully inhibit MMP-13 expression, which led to an inhibition of Coll X and induction of Coll-II, in both free swelling and loading conditions. The GAG deposition was higher in the group combining BICA and mechanical stimulation.

CONCLUSIONS

It is concluded that BICA inhibition of MMP-13 reduces MSC hypertrophy during chondrogenesis.

In vitro maturation on an agarose matrix improves the developmental competence of porcine oocytes

Oocytes in vivo generally mature in ovarian follicles that are soft, whereas oocytes that mature in vitro are on the hard surface of culture dishes. Embryonic ontogeny through organogenesis has greater ability in in vivo matured oocytes than it does in in vitro matured oocytes, indicating the importance of a soft culture matrix. In this study, we report the effect of using an agarose matrix as a culture substrate on the development of pig oocytes derived from medium antral follicles. The cumulus-oocyte complexes (COCs) retrieved from medium antral follicles were matured on noncoated (control) culture dishes or dishes coated with 1% and 2% (w/v) agarose matrices. Subsequently, the effect of the soft culture matrix on the developmental competence of porcine oocytes was assessed by analyzing cumulus expansion, blastocyst formation after parthenogenetic activation (PA), gene expression levels (ACTN4, BMP15, BAX, HIF1A, PFKP and VEGFA), TUNEL indices, BMP15 protein expression levels, cortical granule (CG) distribution, and intraoocyte ATP levels. In vitro maturation (IVM) of pig COCs using a 1% (w/v) agarose matrix resulted in significantly higher blastocyst formation, cumulus expansion, gene expression of BMP15, HIF1A and VEGFA, protein expression of BMP15, and intraoocyte ATP levels, and there was significantly reduced expression of a pro-apoptotic gene and ACTN4 gene and a reduction in TUNEL indices. These results demonstrate that the developmental competence of porcine oocytes can be effectively improved through IVM on a soft culture matrix made of agarose over what is observed using hard culture dishes.

Experimental study on the mechanical properties of biological hydrogels of different concentrations

BACKGROUND

Biological hydrogels provide a conducive three-dimensional extracellular matrix environment for encapsulating and cultivating living cells. Microenvironmental modulus of hydrogels dictates several characteristics of cell functions such as proliferation, adhesion, self-renewal, differentiation, migration, cell morphology and fate. Precise measurement of the mechanical properties of gels is necessary for investigating cellular mechanobiology in a variety of applications in tissue engineering. Elastic properties of gels are strongly influenced by the amount of crosslinking density.

OBJECTIVE

The main purpose of the present study was to determine the elastic modulus of two types of well-known biological hydrogels: Agarose and Gelatin Methacryloyl.

METHODS

Mechanical properties such as Young's modulus, fracture stress and failure strain of the prescribed gels with a wide range of concentrations were determined using tension and compression tests.

RESULTS

The elastic modulus, failure stress and strain were found to be strongly influenced when the amount of concentration in the hydrogels was changed. The elastic modulus for a lower level of concentration, not considered in this study, was also predicted using statistical analysis.

CONCLUSIONS

Closed matching of the mechanical properties of the gels revealed that the bulk tension and compression tests could be confidently used for assessing mechanical properties of delicate biological hydrogels.

A lab-on-chip ultrasonic platform for real-time and nondestructive assessment of extracellular matrix stiffness

Extracellular matrix (ECM) mechanical stiffness and its dynamic change is one of the main cues that directly affects the differentiation and proliferation of normal cells as well as the progression of disease processes such as fibrosis and cancer. Recent advancements in biomaterials have enabled a wide range of polymer matrices that could mimic the ECM of different tissues for a wide range of in vitro basic research and drug discovery. However, most of the technologies utilized to quantify the stiffness of such ECM are either destructive or expensive, and therefore are unsuitable for the in situ, long-term monitoring of variations in ECM stiffness for on-chip cell culture applications. This work demonstrates a novel noninvasive on-chip platform for characterization of ECM stiffness in vitro, by monitoring ultrasonic wave attenuation through the targeted material. The device is composed of a pair of millimeter scale ultrasonic transmitter and receiver transducers with the test medium placed in between them. The transmitter generates an ultrasonic wave that propagates through the material, triggers the piezoelectric receiver and generates a corresponding electrical signal. The characterization reveals a linear (r² = 0.86) decrease in the output voltage of the piezoelectric receiver with an average sensitivity of -15.86 μV kPa^-1 by increasing the stiffnesses of hydrogels (from 4.3 kPa to 308 kPa made with various dry-weight concentrations of agarose and gelatin). The ultrasonic stiffness sensing is also demonstrated to successfully monitor dynamic changes in a simulated in vitro tissue by gradually changing the polymerization density of an agarose gel, as a proof-of-concept towards future use for 3D cell culture and drug screening. In situ long-term ultrasonic signal stability and thermal assessment of the device demonstrates its high robust performance even after two days of continuous operation, with negligible (<0.5 °C) heating of the hydrogel in contact with the piezoelectric transducers. In vitro biocompatibility assessment of the device with mammary fibroblasts further assures that the materials used in the platform did not produce a toxic response and cells remained viable under the applied ultrasound signals in the device.

A multilayered valve leaflet promotes cell-laden collagen type I production and aortic valve hemodynamics

Patients with aortic heart valve disease are limited to valve replacements that lack the ability to grow and remodel. This presents a major challenge for pediatric patients who require a valve capable of somatic growth and at a smaller size. A patient-specific heart valve capable of growth and remodeling while maintaining proper valve function would address this major issue. Here, we recreate the native valve leaflet structure composed of poly-ε-caprolactone (PCL) and cell-laden gelatin-methacrylate/poly (ethylene glycol) diacrylate (GelMA/PEGDA) hydrogels using 3D printing and molding, and then evaluate the ability of the multilayered scaffold to produce collagen matrix under physiological shear stress conditions. We also characterized the valve hemodynamics under aortic physiological flow conditions. The valve's fibrosa layer was replicated by 3D printing PCL in a circumferential direction similar to collagen alignment in the native leaflet, and GelMA/PEGDA sustained and promoted cell viability in the spongiosa/ventricularis layers. We found that collagen type I production can be increased in the multilayered scaffold when it is exposed to pulsatile shear stress conditions over static conditions. When the PCL component was mounted onto a valve ring and tested under physiological aortic valve conditions, the hemodynamics were comparable to commercially available valves. Our results demonstrate that a structurally representative valve leaflet can be generated using 3D printing and that the PCL layer of the leaflet can sustain proper valve function under physiological aortic valve conditions.

Synthesis and Applications of Hydrogels in Cancer Therapy

Hydrogels are water-insoluble, hydrophilic, cross-linked, three-dimensional networks of polymer chains having the ability to swell and absorb water but do not dissolve in it, that comprise the major difference between gels and hydrogels. The mechanical strength, physical integrity and solubility are offered by the crosslinks. The different applications of hydrogels can be derived based on the methods of their synthesis, response to different stimuli, and their different kinds. Hydrogels are highly biocompatible and have properties similar to human tissues that make it suitable to be used in various biomedical applications, including drug delivery and tissue engineering. The role of hydrogels in cancer therapy is highly emerging in recent years. In the present review, we highlighted different methods of synthesis of hydrogels and their classification based on different parameters. Distinctive applications of hydrogels in the treatment of cancer are also discussed.

A new framework for characterization of poroelastic materials using indentation

To characterize a poroelastic material, typically an indenter is pressed onto the surface of the material with a ramp of a finite approach velocity followed by a hold where the indenter displacement is kept constant. This leads to deformation of the porous matrix, pressurization of the interstitial fluid and relaxation due to redistribution of fluid through the pores. In most studies the poroelastic properties, including elastic modulus, Poisson ratio and poroelastic diffusion coefficient, are extracted by assuming an instantaneous step indentation. However, exerting step like indentation is not experimentally possible and usually a ramp indentation with a finite approach velocity is applied. Moreover, the poroelastic relaxation time highly depends on the approach velocity in addition to the poroelastic diffusion coefficient and the contact area. Here, we extensively studied the effect of indentation velocity using finite element simulations which has enabled the formulation of a new framework based on a master curve that incorporates the finite rise time. To verify our novel framework, the poroelastic properties of two types of hydrogels were extracted experimentally using indentation tests at both macro and micro scales. Our new framework that is based on consideration of finite approach velocity is experimentally easy to implement and provides a more accurate estimation of poroelastic properties. STATEMENT OF SIGNIFICANCE: Hydrogels, tissues and living cells are constituted of a sponge-like porous elastic matrix bathed in an interstitial fluid. It has been shown that these materials behave according to the theory of 'poroelasticity' when mechanically stimulated in a way similar to that experienced in organs within the body. In this theory, the rate at which the fluid-filled sponge can be deformed is limited by how fast interstitial fluid can redistribute within the sponge in response to deformation. Here, we simulated indentation experiments at different rates and formulated a new framework that inherently captures the effects of stimulation speed on the mechanical response of poroelastic materials. We validated our framework by conducting experiments at different length-scales on agarose and polyacrylamide hydrogels.

Chemical insights into bioinks for 3D printing

Dedicated chemical strategies are required to form hydrogel networks from bioink components, allowing cell survival during 3D bioprinting processes.

Thermoresponsive polysaccharides and their thermoreversible physical hydrogel networks

Thermoresponsive polymers have been used extensively for various applications including food additives, pharmaceutical formulations, therapeutic delivery, cosmetics and environmental remediation, to mention a few. Many thermoresponsive polymers have the ability to form physical hydrogel networks in response to temperature changes, which are particularly useful for emerging biomedical applications, including cell therapies, drug delivery systems, tissue engineering, wound healing and 3D bioprinting. In particular, the use of polysaccharides with thermoresponsive properties has been of interest due to their wide availability, versatile functionality, biodegradability, and in many cases, inherent biocompatibility. Naturally thermoresponsive polysaccharides include agarose, carrageenans and gellan gum, which exhibit upper critical solution temperatures, transitioning from a solution to a gel state upon cooling. Arguably, this limits their use in biomedical applications, particularly for cell encapsulation as they require raised temperatures to maintain a solution state that may be detrimental to living systems. Conversely, significant progress has been made over recent years to develop synthetically modified polysaccharides, which tend to exhibit lower critical solution temperatures, transitioning from a solution to a gel state upon warming. Of particular interest are thermoresponsive polysaccharides with a lower critical solution temperature in between room temperature and physiological temperature, as their solutions can conveniently be manipulated at room temperature before gelling upon warming to physiological temperature, which makes them ideal candidates for many biological applications. Therefore, this review provides an introduction to the different types of thermoresponsive polysaccharides that have been developed, their resulting hydrogels and properties, and the exciting applications that have emerged as a result of these properties.

Hydrogels of agarose, and methacrylated gelatin and hyaluronic acid are more supportive for in vitro meniscus regeneration than three dimensional printed polycaprolactone scaffolds

In this study, porcine fibrochondrocyte-seeded agarose, methacrylated gelatin (GelMA), methacrylated hyaluronic acid (MeHA) and GelMA-MeHA blend hydrogels, and 3D printed PCL scaffolds were tested under dynamic compression for potential meniscal regeneration in vitro. Cell-carrying hydrogels produced higher levels of extracellular matrix (ECM) components after a 35-day incubation than the 3D printed PCL. Cells on GelMA exhibited strong cell adhesion (evidenced with intense paxillin staining) and dendritic cell morphology, and produced an order of magnitude higher level of collagen (p < 0.05) than other materials. On the other hand, cells in agarose exhibited low cell adhesion and round cell morphology, and produced higher levels of glycosaminoglycans (GAGs) (p < 0.05) than other materials. A low level of ECM production and a high level of cell proliferation were observed on the 3D printed PCL. Dynamic compression at 10% strain enhanced GAG production in agarose (p < 0.05), and collagen production in GelMA. These results show that hydrogels have a higher potential for meniscal regeneration than the 3D printed PCL, and depending on the material used, fibrochondrocytes could be directed to proliferate or produce cartilaginous or fibrocartilaginous ECM. Agarose and MeHA could be used for the regeneration of the inner region of meniscus, while GelMA for the outer region.

Multifunctional smart hydrogels: potential in tissue engineering and cancer therapy

In recent years, clinical applications have been proposed for various hydrogel products. Hydrogels can be derived from animal tissues, plant extracts and/or adipose tissue extracellular matrices; each type of hydrogel presents significantly different functional properties and may be used for many different applications, including medical therapies, environmental pollution treatments, and industrial materials. Due to complicated preparation techniques and the complexities associated with the selection of suitable materials, the applications of many host-guest supramolecular polymeric hydrogels are limited. Thus, improvements in the design and construction of smart materials are highly desirable in order to increase the lifetimes of functional materials. Here, we summarize different functional hydrogels and their varied preparation methods and source materials. The multifunctional properties of hydrogels, particularly their unique ability to adapt to certain environmental stimuli, are chiefly based on the incorporation of smart materials. Smart materials may be temperature sensitive, pH sensitive, pH/temperature dual sensitive, photoresponsive or salt responsive and may be used for hydrogel wound repair, hydrogel bone repair, hydrogel drug delivery, cancer therapy, and so on. This review focuses on the recent development of smart hydrogels for tissue engineering applications and describes some of the latest advances in using smart materials to create hydrogels for cancer therapy.

Decoration of RGD-mimetic porous scaffolds with engineered and devitalized extracellular matrix for adipose tissue regeneration

Fat grafting is emerging as a promising alternative to silicon implants in breast reconstruction surgery. Unfortunately, this approach does not provide a proper mechanical support and is affected by drawbacks such as tissue resorption and donor site morbidity. Synthetic scaffolds can offer a valuable alternative to address these challenges, but poorly recapitulate the biochemical stimuli needed for tissue regeneration. Here, we aim at combining the positive features of a structural, synthetic polymer to an engineered, devitalized extracellular matrix (ECM) to generate a hybrid construct that can provide a mix of structural and biological stimuli needed for adipose tissue regeneration. A RGD-mimetic synthetic scaffold OPAAF, designed for soft tissue engineering, was decorated with ECM deposited by human adipose stromal cells (hASCs). The adipoinductive potential of the hybrid ECM-OPAAF construct was validated in vitro, by culture with hASC in a perfusion bioreactor system, and in vivo, by subcutaneous implantation in nude mouse. Our findings demonstrate that the hybrid ECM-OPAAF provides proper mechanical support and adipoinductive stimuli, with potential applicability as off-the-shelf material for adipose tissue reconstruction.

STATEMENT OF SIGNIFICANCE

In this study we combined the functionalities of a synthetic polymer with those of an engineered and subsequently devitalized extracellular matrix (ECM) to generate a hybrid material for adipose tissue regeneration. The developed hybrid ECM-OPAAF was demonstrated to regulate human adipose stromal cells adipogenic commitment in vitro and adipose tissue infiltration in vivo. Our findings demonstrate that the hybrid ECM-OPAAF provide proper mechanical support and adipoinductive stimuli and represents a promising off-the-shelf material for adipose tissue reconstruction. We believe that our approach could offer an alternative strategy for adipose tissue reconstruction in case of mastectomy or congenital abnormalities, overcoming the current limitations of autologous fat based strategies such as volume resorption and donor site morbidity.

Generation and characterization of monodisperse deformable alginate and pNIPAM microparticles with a wide range of shear moduli

Monodisperse particles of varying size, shape, and deformability were produced using two microfluidic strategies. For both strategies, monodisperse emulsion droplets of a crosslinkable solution were generated via flow-focusing. Subsequently, droplets were crosslinked either on chip or in an external bath. On-chip gelation resulted in spherical particles; varying the degree of crosslinking varied the deformability systematically. The optimized flow-focusing device design separated the production of monodisperse aqueous alginate droplets and the on-chip introduction of crosslinking ions. Two features were then adapted to target softer particles: the dispersed phase design and the polymer choice. The alternative design used a sheathed dispersed phase, with the polymer solution surrounding an unreactive viscous core, which generated alginate particles with a softer core. Poly(N-isopropylacrylamide) (pNIPAM) allowed access to a broad range of moduli. The resulting spherical particles were characterized using capillary micromechanics to determine the shear (G) and compressive (K) moduli. Particles with G = 0.013 kPa to 26 kPa and K = 0.221 kPa to 34.9 kPa were obtained; the softest particles are an order of magnitude softer than those previously reported. The second approach, based on earlier work by Hu et al., produced axisymmetric, non-spherical particles with fore-aft asymmetry. Alginate drops were again formed in a flow-focusing device but were crosslinked off-chip in an external gelation bath. By changing the bath viscosity, crosslinker concentration, and outlet height, the falling droplets deformed differently during gelation, resulting in a variety of shapes, such as teardrop, mushroom, and bowl shapes.

* Constrained Cage Culture Improves Engineered Cartilage Functional Properties by Enhancing Collagen Network Stability

When cultured with sufficient nutrient supply, engineered cartilage synthesizes proteoglycans rapidly, producing an osmotic swelling pressure that destabilizes immature collagen and prevents the development of a robust collagen framework, a hallmark of native cartilage. We hypothesized that mechanically constraining the proteoglycan-induced tissue swelling would enhance construct functional properties through the development of a more stable collagen framework. To test this hypothesis, we developed a novel "cage" growth system to mechanically prevent tissue constructs from swelling while ensuring adequate nutrient supply to the growing construct. The effectiveness of constrained culture was examined by testing constructs embedded within two different scaffolds: agarose and cartilage-derived matrix hydrogel (CDMH). Constructs were seeded with immature bovine chondrocytes and cultured under free swelling (FS) conditions for 14 days with transforming growth factor-β before being placed into a constraining cage for the remainder of culture. Controls were cultured under FS conditions throughout. Agarose constructs cultured in cages did not expand after the day 14 caging while FS constructs expanded to 8 × their day 0 weight after 112 days of culture. In addition to the physical differences in growth, by day 56, caged constructs had higher equilibrium (agarose: 639 ± 179 kPa and CDMH: 608 ± 257 kPa) and dynamic compressive moduli (agarose: 3.4 ± 1.0 MPa and CDMH 2.8 ± 1.0 MPa) than FS constructs (agarose: 193 ± 74 kPa and 1.1 ± 0.5 MPa and CDMH: 317 ± 93 kPa and 1.8 ± 1.0 MPa for equilibrium and dynamic properties, respectively). Interestingly, when normalized to final day wet weight, cage and FS constructs did not exhibit differences in proteoglycan or collagen content. However, caged culture enhanced collagen maturation through the increased formation of pyridinoline crosslinks and improved collagen matrix stability as measured by α-chymotrypsin solubility. These findings demonstrate that physically constrained culture of engineered cartilage constructs improves functional properties through improved collagen network maturity and stability. We anticipate that constrained culture may benefit other reported engineered cartilage systems that exhibit a mismatch in proteoglycan and collagen synthesis.

Hybrid microscaffold-based 3D bioprinting of multi-cellular constructs with high compressive strength: A new biofabrication strategy

A hybrid 3D bioprinting approach using porous microscaffolds and extrusion-based printing method is presented. Bioink constitutes of cell-laden poly(D,L-lactic-co-glycolic acid) (PLGA) porous microspheres with thin encapsulation of agarose-collagen composite hydrogel (AC hydrogel). Highly porous microspheres enable cells to adhere and proliferate before printing. Meanwhile, AC hydrogel allows a smooth delivery of cell-laden microspheres (CLMs), with immediate gelation of construct upon printing on cold build platform. Collagen fibrils were formed in the AC hydrogel during culture at body temperature, improving the cell affinity and spreading compared to pure agarose hydrogel. Cells were proven to proliferate in the bioink and the bioprinted construct. High cell viability up to 14 days was observed. The compressive strength of the bioink is more than 100 times superior to those of pure AC hydrogel. A potential alternative in tissue engineering of tissue replacements and biological models is made possible by combining the advantages of the conventional solid scaffolds with the new 3D bioprinting technology.

Bioink properties before, during and after 3D bioprinting

Bioprinting is a process based on additive manufacturing from materials containing living cells. These materials, often referred to as bioink, are based on cytocompatible hydrogel precursor formulations, which gel in a manner compatible with different bioprinting approaches. The bioink properties before, during and after gelation are essential for its printability, comprising such features as achievable structural resolution, shape fidelity and cell survival. However, it is the final properties of the matured bioprinted tissue construct that are crucial for the end application. During tissue formation these properties are influenced by the amount of cells present in the construct, their proliferation, migration and interaction with the material. A calibrated computational framework is able to predict the tissue development and maturation and to optimize the bioprinting input parameters such as the starting material, the initial cell loading and the construct geometry. In this contribution relevant bioink properties are reviewed and discussed on the example of most popular bioprinting approaches. The effect of cells on hydrogel processing and vice versa is highlighted. Furthermore, numerical approaches were reviewed and implemented for depicting the cellular mechanics within the hydrogel as well as for prediction of mechanical properties to achieve the desired hydrogel construct considering cell density, distribution and material-cell interaction.

Biologically and mechanically driven design of an RGD-mimetic macroporous foam for adipose tissue engineering applications

Despite clinical treatments for adipose tissue defects, in particular breast tissue reconstruction, have certain grades of efficacy, many drawbacks are still affecting the long-term survival of new formed fat tissue. To overcome this problem, in the last decades, several scaffolding materials have been investigated in the field of adipose tissue engineering. However, a strategy able to recapitulate a suitable environment for adipose tissue reconstruction and maintenance is still missing. To address this need, we adopted a biologically and mechanically driven design to fabricate an RGD-mimetic poly(amidoamine) oligomer macroporous foam (OPAAF) for adipose tissue reconstruction. The scaffold was designed to fulfil three fundamental criteria: capability to induce cell adhesion and proliferation, support of in vivo vascularization and match of native tissue mechanical properties. Poly(amidoamine) oligomers were formed into soft scaffolds with hierarchical porosity through a combined free radical polymerization and foaming reaction. OPAAF is characterized by a high water uptake capacity, progressive degradation kinetics and ideal mechanical properties for adipose tissue reconstruction. OPAAF's ability to support cell adhesion, proliferation and adipogenesis was assessed in vitro using epithelial, fibroblast and endothelial cells (MDCK, 3T3L1 and HUVEC respectively). In addition, in vivo subcutaneous implantation in murine model highlighted OPAAF potential to support both adipogenesis and vessels infiltration. Overall, the reported results support the use of OPAAF as a scaffold for engineered adipose tissue construct.

Experimental validation of a flat punch indentation methodology calibrated against unconfined compression tests for determination of soft tissue biomechanics

Mechanical characterisation of soft biological tissues using standard compression or tensile testing presents a significant challenge due to specimen geometrical irregularities, difficulties in cutting intact and appropriately sized test samples, and issues with slippage or damage at the grips. Indentation can overcome these problems but requires fitting a model to the resulting load-displacement data in order to calculate moduli. Despite the widespread use of this technique, few studies experimentally validate their chosen model or compensate for boundary effects. In this study, viscoelastic hydrogels of different concentrations and dimensions were used to calibrate an indentation technique performed at large specimen-strain deformation (20%) and analysed with a range of routinely used mathematical models. A rigid, flat-ended cylindrical indenter was applied to each specimen from which 'indentation moduli' and relaxation properties were calculated and compared against values obtained from unconfined compression. Only one indentation model showed good agreement (<10% difference) with all moduli values obtained from compression. A sample thickness to indenter diameter ratio ≥1:1 and sample diameter to indenter diameter ratio ≥4:1 was necessary to achieve the greatest accuracy. However, it is not always possible to use biological samples within these limits, therefore we developed a series of correction factors. The approach was validated using human diseased omentum and bovine articular cartilage resulting in mechanical properties closely matching compression values. We therefore present a widely useable indentation analysis method to allow more accurate calculation of material mechanics which is important in the study of soft tissue development, ageing, health and disease.

Bio-functionalized silk hydrogel microfluidic systems

Bio-functionalized microfluidic systems were developed based on a silk protein hydrogel elastomeric materials. A facile multilayer fabrication method using gelatin sacrificial molding and layer-by-layer assembly was implemented to construct interconnected, three dimensional (3D) microchannel networks in silk hydrogels at 100 μm minimum feature resolution. Mechanically activated valves were implemented to demonstrate pneumatic control of microflow. The silk hydrogel microfluidics exhibit controllable mechanical properties, long-term stability in various environmental conditions, tunable in vitro and in vivo degradability in addition to optical transparency, providing unique features for cell/tissue-related applications than conventional polydimethylsiloxane (PDMS) and existing hydrogel-based microfluidic options. As demonstrated in the work here, the all aqueous-based fabrication process at ambient conditions enabled the incorporation of active biological substances in the bulk phase of these new silk microfluidic systems during device fabrication, including enzymes and living cells, which are able to interact with the fluid flow in the microchannels. These silk hydrogel-based microfluidic systems offer new opportunities in engineering active diagnostic devices, tissues and organs that could be integrated in vivo, and for on-chip cell sensing systems.

Methods of high integrity RNA extraction from cell/agarose construct

Background

Agarose hydrogels are widely used for three-dimensional cell scaffolding in tissue engineering and cell biology. Recently, molecular profiles have been obtained with extraction of a minimal volume of RNA using fluorescent-tagged quantitative polymerase chain reaction (qPCR), which requires high integrity RNA. However, the agarose interferes considerably with the quantity and quality of the extracted RNA. Moreover, little is known about RNA integrity when the RNA is extracted from cell/agarose construct. Thus, in order to obtain RNA of sufficient integrity, we examined various extraction methods and addressed reproducible methodologies for RNA extraction from cell/agarose constructs using spectrophotometry and microfluidic capillary electrophoresis.

Results

With various extraction methods using a mono-phasic solution of phenol and guanidine isothiocyanate, we evaluated quantity and quality of total RNA from cell/agarose construct. Extraction with solution of phenol and guanidine isothiocyanate followed by a silica based membrane filter column gave sufficient RNA integrity number, which allowed us to proceed to fluorescent-tagged qPCR for evaluating various cellular activities.

Conclusions

The RNA extraction methods using phenol and guanidine isothiocyanate solution and a silica membrane column can be useful for obtaining high integrity RNA from cell/agarose constructs rich in polysaccharide and extracellular matrix. Our study contributes to further investigation using agarose hydrogels and other materials rich in polysaccharide in the field of cellular and tissue engineering.