Mechanical Characterization by Mesoscale Indentation: Advantages and Pitfalls for Tissue and Scaffolds

Durable lymph-node expansion is associated with the efficacy of therapeutic vaccination

Following immunization, lymph nodes dynamically expand and contract. The mechanical and cellular changes enabling the early-stage expansion of lymph nodes have been characterized, yet the durability of such responses and their implications for adaptive immunity and vaccine efficacy are unknown. Here, by leveraging high-frequency ultrasound imaging of the lymph nodes of mice, we report more potent and persistent lymph-node expansion for animals immunized with a mesoporous silica vaccine incorporating a model antigen than for animals given bolus immunization or standard vaccine formulations such as alum, and that durable and robust lymph-node expansion was associated with vaccine efficacy and adaptive immunity for 100 days post-vaccination in a mouse model of melanoma. Immunization altered the mechanical and extracellular-matrix properties of the lymph nodes, drove antigen-dependent proliferation of immune and stromal cells, and altered the transcriptional features of dendritic cells and inflammatory monocytes. Strategies that robustly maintain lymph-node expansion may result in enhanced vaccination outcomes.

Type III Collagen Regulates Matrix Architecture and Mechanosensing during Wound Healing

Postnatal cutaneous wound healing is characterized by development of a collagen-rich scar lacking the architecture and functional integrity of unwounded tissue. Directing cell behaviors to efficiently heal wounds while minimizing scar formation remains a major wound management goal. In this study, we demonstrate type III collagen (COL3) as a critical regulator of re-epithelialization and scar formation during healing of COL3-enriched, regenerative (Acomys), scar-permissive (CD-1 Mus and wild-type Col3B6/B6 mice) and COL3-deficient, scar-promoting (Col3F/F, a murine conditional knockdown model) cutaneous wound models. We define a scar-permissive fibrillar collagen architecture signature characterized by elongated and anisotropically aligned collagen fibers that is dose-dependently suppressed by COL3. Furthermore, loss of COL3 alters how cells interpret their microenvironment-their mechanoperception-such that COL3-deficient cells display mechanically active phenotypes in the absence of increased microenvironmental stiffness through the upregulation and engagement of the profibrotic integrin α11. Further understanding COL3's role in regulating matrix architecture and mechanoresponses may inform clinical strategies that harness proregenerative mechanisms.

Mechanical characterization of soft biomaterials: which time and spatial scale to choose?

The mechanical properties of soft gels hold significant relevance in biomedicine and biomaterial design, including the development of tissue engineering constructs and bioequivalents. It is important to adequately characterize the gel's mechanical properties since they play a role both in the overall structural properties of the construct and the physiological responses of cells. The question remains which approach for the mechanical characterization is most suitable for specific biomaterials. Our investigation is centered on the comparison of three types of gels and four distinct mechanical testing techniques: shear rheology, compression, microindentation, and nanoindentation by atomic force microscopy. While analyzing an elastic homogeneous synthetic hydrogel (a polyacrylamide gel), we observed close mechanical results across the different testing techniques. However, our findings revealed more distinct outcomes when assessing a highly viscoelastic gel (Ecoflex) and a heterogeneous biopolymer hydrogel (enzymatically crosslinked gelatin). To ensure precise data interpretation, we introduced correction factors to account for the boundary conditions inherent in many of the testing methods. The results of this study underscore the critical significance of considering both the temporal and spatial scales in mechanical measurements of biomaterials. Furthermore, they encourage the employment of a combination of diverse testing techniques, particularly in the characterization of heterogeneous viscoelastic materials such as biological samples. The obtained results will contribute to the refinement of mechanical testing protocols and advance the development of soft gels for tissue engineering.

Analysis of the Mechanical Characteristics of Human Pancreas through Indentation: Preliminary In Vitro Results on Surgical Samples

Pancreatic surgery is extremely challenging and demands an extended learning curve to be executed with a low incidence of post-operative complications. The soft consistency of the human pancreas poses a primary challenge for pancreatic surgeons. This study aimed to analyze the preliminary mechanical characteristics of the human pancreas to develop a realistic synthetic phantom for surgical simulations in the near future. Pancreatic specimens, comprehensive of the pancreatic parenchyma and main pancreatic duct, were collected during pancreatic resections and analyzed through nano-bio-indentation (BioindenterTM UNHT3 Bio, Anton Paar GmbH, Graz, Austria) to measure the elastic modulus. Comparisons were made between slow and fast loading rates, immediate and post-freezing analyses, and multipoint indentations. The results demonstrated that a slow loading rate (30 μN/min), immediate analysis, and multipoint measurements are crucial for obtaining accurate values of the elastic modulus of the human pancreas (1.40 ± 0.47 kPa). In particular, the study revealed that analysis after freezing could impact the outcomes of the indentation. Moreover, the study suggested that both the pancreatic parenchyma and the main pancreatic duct should be analyzed to achieve a more precise and comprehensive definition of the. mechanical features of the pancreas. These preliminary findings represent the initial steps toward defining the consistency and mechanical characteristics of human pancreatic tissue with the goal of creating a realistic synthetic phantom.

Engineering dense tumor constructs via cellular contraction of extracellular matrix hydrogels

Physical characteristics of solid tumors such as dense internal microarchitectures and pathological stiffness influence cancer progression and treatment. While it is routine to engineer culture substrates and scaffolds with elastic moduli that approximate tumors, these models often fail to capture characteristic internal microarchitectures such as densely compacted concentric ECM fibers at the stromal interface. Contractile mesenchymal cells can solve this engineering challenge by deforming, contracting, and compacting extracellular matrix (ECM) hydrogels to decrease tissue volume and increase tissue density. Here we demonstrate that allowing human fibroblasts of varying origins to freely contract collagen type I-containing hydrogels co-seeded with carcinoma cell spheroids produces a tissue engineered construct with structural features that mimic dense solid tumors in vivo. Morphometry and mechanical testing were conducted in tandem with biochemical analysis of proliferation and viability to confirm that dense carcinoma constructs engineered using this approach capture relevant physical characteristics of solid carcinomas in a tractable format that preserves viability and is amenable to extended culture. The reported method is adaptable to the use of multiple mesenchymal cell types and the inclusion of fibrin in the ECM combined with seeding of endothelial cells to produce prevascularized constructs. The physical dense carcinoma constructs engineered using this approach may provide more clinically relevant venues for studying cancer pathophysiology and the challenges associated with the delivery of macromolecular drugs and cellular immunotherapies to solid tumors.

An in vitro study of micromechanics, cellular proliferation and viability on both decellularized porcine dura grafts and native porcine dura grafts

Damage to the dura mater may occur during intracranial or spinal surgeries, which can result in cerebrospinal fluid leakage and other potentially fatal physiological changes. As a result, biological and synthetic derived scaffolds are typically used to repair dura mater post intracranial or spinal surgeries. The extracellular matrix of xenogeneic dura scaffolds has been shown to exhibit increased cell infiltration and regeneration than synthetic dura materials. In this study, we investigated the biocompatibility of native and decellularized porcine dura by seeding rat fibroblast cells onto the constructs. Cell proliferation, cell viability, and the mechanical properties of these dural grafts were evaluated post-re-seeding on days 3,7 and 14. Live-dead staining and resazurin salts were used to quantify cell viability and cell proliferation, respectively. Micro indentation was conducted to quantify the mechanical integrity of the native and acellular dura graft. The findings indicate that the acellular porcine dura graft creates a beneficial setting for infiltrating rat fibroblast cells. Cell viability, proliferation, and micro indentation results on the acellular grafts are comparable with the native control porcine dura tissue. In conclusion, the porcine scaffold material showed increased cell viability at each time point evaluated. The sustained mechanical response and favorable viability of the cells on the decellularized grafts provide promising insight into the potential use of porcine dura in clinical cranial dura mater graft applications.

Cell therapy biomanufacturing: integrating biomaterial and flow-based membrane technologies for production of engineered T-cells

Adoptive T-cell therapies (ATCTs) are increasingly important for the treatment of cancer, where patient immune cells are engineered to target and eradicate diseased cells. The biomanufacturing of ATCTs involves a series of time-intensive, lab-scale steps, including isolation, activation, genetic modification, and expansion of a patient's T-cells prior to achieving a final product. Innovative modular technologies are needed to produce cell therapies at improved scale and enhanced efficacy. In this work, well-defined, bioinspired soft materials were integrated within flow-based membrane devices for improving the activation and transduction of T cells. Hydrogel coated membranes (HCM) functionalized with cell-activating antibodies were produced as a tunable biomaterial for the activation of primary human T-cells. T-cell activation utilizing HCMs led to highly proliferative T-cells that expressed a memory phenotype. Further, transduction efficiency was improved by several fold over static conditions by using a tangential flow filtration (TFF) flow-cell, commonly used in the production of protein therapeutics, to transduce T-cells under flow. The combination of HCMs and TFF technology led to increased cell activation, proliferation, and transduction compared to current industrial biomanufacturing processes. The combined power of biomaterials with scalable flow-through transduction techniques provides future opportunities for improving the biomanufacturing of ATCTs.

Acoustic Force Elastography Microscopy

OBJECTIVE

Hydrogel scaffolds have attracted attention to develop cellular therapy and tissue engineering platforms for regenerative medicine applications. Among factors, local mechanical properties of scaffolds drive the functionalities of cell niche. Dynamic mechanical analysis (DMA), the standard method to characterize mechanical properties of hydrogels, restricts development in tissue engineering because the measurement provides a single elasticity value for the sample, requires direct contact, and represents a destructive evaluation preventing longitudinal studies on the same sample. We propose a novel technique, acoustic force elastography microscopy (AFEM), to evaluate elastic properties of tissue engineering scaffolds.

RESULTS

AFEM can resolve localized and two-dimensional (2D) elastic properties of both transparent and opaque materials with advantages of being non-contact and non-destructive. Gelatin hydrogels, neat synthetic oligo[poly(ethylene glycol)fumarate] (OPF) scaffolds, OPF hydroxyapatite nanocomposite scaffolds and ex vivo biological tissue were examined with AFEM to evaluate the elastic modulus. These measurements of Young's modulus range from approximately 2 kPa to over 100 kPa were evaluated and are in good agreement with finite element simulations, surface wave measurements, and DMA tests.

CONCLUSION

The AFEM can resolve localized and 2D elastic properties of hydrogels, scaffolds and thin biological tissues. These materials can either be transparent or non-transparent and their evaluation can be done in a non-contact and non-destructive manner, thereby facilitating longitudinal evaluation.

SIGNIFICANCE

AFEM is a promising technique to quantify elastic properties of scaffolds for tissue engineering and will be applied to provide new insights for exploring elastic changes of cell-laden scaffolds for tissue engineering and material science.

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.

Soft Contact Mechanics with Gradient-Stiffness Surfaces

The stiffness in the top surface of many biological entities like cornea or articular cartilage, as well as chemically cross-linked synthetic hydrogels, can be significantly lower or more compliant than the bulk. When such a heterogeneous surface comes into contact, the contacting load is distributed differently from typical contact models. The mechanical response under indentation loading of a surface with a gradient of stiffness is a complex, integrated response that necessarily includes the heterogeneity. In this work, we identify empirical contact models between a rigid indenter and gradient elastic surfaces by numerically simulating quasi-static indentation. Three key case studies revealed the specific ways in which (I) continuous gradients, (II) laminate-layer gradients, and (III) alternating gradients generate new contact mechanics at the shallow-depth limit. Validation of the simulation-generated models was done by micro- and nanoindentation experiments on polyacrylamide samples synthesized to have a softer gradient surface layer. The field of stress and stretch in the subsurface as visualized from the simulations also reveals that the gradient layers become confined, which pushes the stretch fields closer to the surface and radially outward. Thus, contact areas are larger than expected, and average contact pressures are lower than predicted by the Hertz model. The overall findings of this work are new contact models and the mechanisms by which they change. These models allow a more accurate interpretation of the plethora of indentation data on surface gradient soft matter (biological and synthetic) as well as a better prediction of the force response to gradient soft surfaces. This work provides examples of how gradient hydrogel surfaces control the subsurface stress distribution and loading response.

Measuring the elastic modulus of soft biomaterials using nanoindentation

The measurement of the elastic modulus of soft biomaterials via nanoindentation relies on the accurate determination of the zero-point of the tip-sample interaction on which the depth of penetration into the sample is based. Non-cantilever based nanoindentation systems were originally designed for hard materials, and therefore monitoring the zero-point contact presents a significant challenge for the characterisation of very soft biomaterials. This study investigates the ability of non-cantilever based nanoindentation to differentiate between hydrogels with elastic moduli on the order of single kiloPascals (kPa) using a bespoke soft contact protocol and low flexural stiffness of instrument. Polyethylene glycol (PEG) hydrogels were fabricated as a model system with a range of elastic moduli by varying the polymer concentration and degree of crosslinking. Elastic modulus values were calculated using the Oliver-Pharr method, Hertzian contact model, as well as a viscoelastic model to account for the time-dependent behaviour of the gels. The stiffness measurements were validated by measuring cantilever beams with the equivalent flexural stiffness to that of the PEG hydrogels being tested. The results demonstrated a high repeatability of the measurements, enabling differentiation between hydrogels with elastic moduli in the single kPa to hundreds of kPa range.

Decellularized peripheral nerve as an injectable delivery vehicle for neural applications

Damage to the nervous system can result in loss of sensory and motor function, paralysis, or even death. To facilitate neural regeneration and functional recovery, researchers have employed biomaterials strategies to address both peripheral and central nervous system injuries. Injectable hydrogels that recapitulate native nerve extracellular matrix are especially promising for neural tissue engineering because they offer more flexibility for minimally invasive applications and provide a growth-permissive substrate for neural cell types. Here, we explore the development of injectable hydrogels derived from decellularized rat peripheral nerves (referred to as "injectable peripheral nerve [iPN] hydrogels"), which are processed using a newly developed sodium deoxycholate and DNase (SDD) decellularization method. We assess the gelation kinetics, mechanical properties, cell bioactivity, and drug release kinetics of the iPN hydrogels. The iPN hydrogels thermally gel when exposed to 37°C in under 20 min and have mechanical properties similar to neural tissue. The hydrogels demonstrate in vitro biocompatibility through support of Schwann cell viability and metabolic activity. Additionally, iPN hydrogels promote greater astrocyte spreading compared to collagen I hydrogels. Finally, the iPN is a promising delivery vehicle of drug-loaded microparticles for a combinatorial approach to neural injury therapies.

Endocytic proteins with prion-like domains form viscoelastic condensates that enable membrane remodeling

Membrane invagination and vesicle formation are key steps in endocytosis and cellular trafficking. Here, we show that endocytic coat proteins with prion-like domains (PLDs) form hemispherical puncta in the budding yeast, Saccharomyces cerevisiae These puncta have the hallmarks of biomolecular condensates and organize proteins at the membrane for actin-dependent endocytosis. They also enable membrane remodeling to drive actin-independent endocytosis. The puncta, which we refer to as endocytic condensates, form and dissolve reversibly in response to changes in temperature and solution conditions. We find that endocytic condensates are organized around dynamic protein-protein interaction networks, which involve interactions among PLDs with high glutamine contents. The endocytic coat protein Sla1 is at the hub of the protein-protein interaction network. Using active rheology, we inferred the material properties of endocytic condensates. These experiments show that endocytic condensates are akin to viscoelastic materials. We use these characterizations to estimate the interfacial tension between endocytic condensates and their surroundings. We then adapt the physics of contact mechanics, specifically modifications of Hertz theory, to develop a quantitative framework for describing how interfacial tensions among condensates, the membrane, and the cytosol can deform the plasma membrane to enable actin-independent endocytosis.

Elastic modulus distribution in poly(N-isopopylacrylamide) and oligo(ethylene glycol methacrylate)-based microgels studied by AFM

The spatial elastic modulus distribution of microgel networks in presence and absence of bifunctional crosslinkers is studied by AFM. Thermoresponsive poly(N-isopopylacrylamide) (PNIPAM) and poly(2-(2-methoxyethoxy)ethyl methacrylate-co-oligo(ethylene glycol)methacrylate) (P(MEO2MA-co-OEGMA)) microgels are synthesized via precipitation polymerization above their lower critical solution temperature (LCST). High-resolution elastic modulus profiles are acquired using AFM force-indentation mapping of surface-deposited microgels at 25 °C. For both microgel systems, the use of a bifunctional crosslinker leads to a strong elastic modulus gradient with stiff microgel cores and soft networks toward the edge. In absence of a dedicated crosslinker (self-crosslinking), PNIPAM microgels show a homogeneous elastic modulus distribution, whereas self-crosslinked P(MEO2MA-co-OEGMA) microgels still show decreasing elastic moduli from the centre to the edge of the microgels. However, POEGMA microgels without comonomer showed no elastic modulus gradient suggesting that different incorporation rates of MEO2MA and OEGMA result in a radial variation of the polymer segment density. In addition, when varying the molecular weight of OEGMA the overall elastic modulus was affected, possibly due to molecular weight-dependent phase behavior and different reactivity. This shows that quite different microgel architectures can be obtained by the simple "one-pot" precipitation reaction of microgels which may open to new avenues toward advanced applications.

Mechanical properties of anterior lens capsule assessed with AFM and nanoindenter in relation to human aging, pseudoexfoliation syndrome, and trypan blue staining

The purpose of this study is the mechanical characterization of the mid-to- old-age human anterior lens capsules (ALCs) obtained by capsulorhexis using Atomic Force Microscopy (AFM) and a nanoindenter at different spatial scales. The dependencies on the human age, presence or absence of pseudoexfoliation syndrome (PEX), and application of trypan blue staining during the surgery were analyzed. The measurements on both the anterior (AS) and epithelial (ES) sides of the ALC were conducted and the effect of cells present on the epithelial side was carefully accounted for. The ES of the ALC had a homogenous distribution of the Young's modulus over the surface as shown by the macroscale mapping with the nanoindenter and local AFM indentations, while the AS was more heterogeneous. Age-related changes were assessed in groups ranging from the mid-age (from 48 years) to old-age (up to 93 years). We found that the ES was always stiffer than the AS, and this difference decreased with age due to a gradual decrease in the Young's modulus of the ES and an increase in the modulus of the AS. No significant changes were found in the mechanical properties of ALCs of PEX patients versus the PEX-free group, as well as in the properties of the ALC with and without trypan blue staining.

Integration of flexible polyimide arrays into soft extracellular matrix-based hydrogel materials for a tissue-engineered electronic nerve interface (TEENI)

BACKGROUND

Biomimetic hydrogels used in tissue engineering can improve tissue regeneration and enable targeted cellular behavior; there is growing interest in combining hydrogels with microelectronics to create new neural interface platforms to help patient populations. However, effective processes must be developed to integrate flexible but relatively stiff (e.g., 1-10 GPa) microelectronic arrays within soft (e.g., 1-10 kPa) hydrogels.

NEW METHOD

Here, a novel method for integrating polyimide microelectrode arrays within a biomimetic hydrogel scaffold is demonstrated for use as a tissue-engineered electronic nerve interface (TEENI). Tygon tubing and a series of 3D printed molds were used to facilitate hydrogel fabrication and device assembly.

COMPARISON WITH EXISTING METHODS

Other comparable regenerative peripheral nerve interface technologies do not utilize the flexible microelectrode array design nor the hydrogel scaffold described here. These methods typically use stiff electrode arrays that are affixed to a similarly stiff implantable tube serving as the nerve guidance conduit.

RESULTS

Our results indicate that there is a substantial mechanical mismatch between the flexible microelectronic arrays and the soft hydrogel. However, using the methods described here, there is consistent fabrication of these regenerative peripheral nerve interfaces suitable for implantation.

CONCLUSIONS

The assembly process that was developed resulted in repeatable and consistent integration of microelectrode arrays within a soft tissue-engineered hydrogel. As reported elsewhere, these devices have been successfully implanted in a rat sciatic nerve model and yielded neural recordings. This process can be adapted for other applications and hydrogels in which flexible electronic materials are combined with soft regenerative scaffolds.

Tunable methacrylated hyaluronic acid-based hydrogels as scaffolds for soft tissue engineering applications

Hyaluronic acid (HA)-based biomaterials have been explored for a number of applications in biomedical engineering, particularly as tissue regeneration scaffolds. Crosslinked forms of HA are more robust and provide tunable mechanical properties and degradation rates that are critical in regenerative medicine; however, crosslinking modalities reported in the literature vary and there are few comparisons of different scaffold properties for various crosslinking approaches. In this study, we offer direct comparison of two methacrylation techniques for HA (glycidyl methacrylate HA [GMHA] or methacrylic anhydride HA [MAHA]). The two methods for methacrylating HA provide degrees of methacrylation ranging from 2.4 to 86%, reflecting a wider range of properties than is possible using only a single methacrylation technique. We have also characterized mechanical properties for nine different tissues isolated from rat (ranging from lung at the softest to muscle at the stiffest) using indentation techniques and show that we can match the full range of mechanical properties (0.35-6.13 kPa) using either GMHA or MAHA. To illustrate utility for neural tissue engineering applications, functional hydrogels with adhesive proteins (either GMHA or MAHA base hydrogels with collagen I and laminin) were designed with effective moduli mechanically matched to rat sciatic nerve (2.47 ± 0.31 kPa). We demonstrated ability of these hydrogels to support three-dimensional axonal elongation from dorsal root ganglia cultures. Overall, we have shown that methacrylated HA provides a tunable platform with a wide range of properties for use in soft tissue engineering.

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.