Nanoindentation of Soft Biological Materials

Mechanical Properties of DNA Hydrogels: Towards Highly Programmable Biomaterials

DNA hydrogels are self-assembled biomaterials that rely on Watson–Crick base pairing to form large-scale programmable three-dimensional networks of nanostructured DNA components. The unique mechanical and biochemical properties of DNA, along with its biocompatibility, make it a suitable material for the assembly of hydrogels with controllable mechanical properties and composition that could be used in several biomedical applications, including the design of novel multifunctional biomaterials. Numerous studies that have recently emerged, demonstrate the assembly of functional DNA hydrogels that are responsive to stimuli such as pH, light, temperature, biomolecules, and programmable strand-displacement reaction cascades. Recent studies have investigated the role of different factors such as linker flexibility, functionality, and chemical crosslinking on the macroscale mechanical properties of DNA hydrogels. In this review, we present the existing data and methods regarding the mechanical design of pure DNA hydrogels and hybrid DNA hydrogels, and their use as hydrogels for cell culture. The aim of this review is to facilitate further study and development of DNA hydrogels towards utilizing their full potential as multifeatured and highly programmable biomaterials with controlled mechanical properties.

Mechanical Characterization for Cellular Mechanobiology: Current Trends and Future Prospects

Accurate mechanical characterization of adherent cells and their substrates is important for understanding the influence of mechanical properties on cells themselves. Recent mechanobiology studies outline the importance of mechanical parameters, such as stress relaxation and strain stiffening on the behavior of cells. Numerous techniques exist for probing mechanical properties and it is vital to understand the benefits of each technique and how they relate to each other. This mini review aims to guide the reader through the toolbox of mechanical characterization techniques by presenting well-established and emerging methods currently used to assess mechanical properties of substrates and cells.

Viscoelasticity in simple indentation-cycle experiments: a computational study

Instrumented indentation has become an indispensable tool for quantitative analysis of the mechanical properties of soft polymers and biological samples at different length scales. These types of samples are known for their prominent viscoelastic behavior, and attempts to calculate such properties from the indentation data are constantly made. The simplest indentation experiment presents a cycle of approach (deepening into the sample) and retraction of the indenter, with the output of the force and indentation depth as functions of time and a force versus indentation dependency (force curve). The linear viscoelastic theory based on the elastic–viscoelastic correspondence principle might predict the shape of force curves based on the experimental conditions and underlying relaxation function of the sample. Here, we conducted a computational analysis based on this theory and studied how the force curves were affected by the indenter geometry, type of indentation (triangular or sinusoidal ramp), and the relaxation functions. The relaxation functions of both traditional and fractional viscoelastic models were considered. The curves obtained from the analytical solutions, numerical algorithm and finite element simulations matched each other well. Common trends for the curve-related parameters (apparent Young’s modulus, normalized hysteresis area, and curve exponent) were revealed. Importantly, the apparent Young’s modulus, obtained by fitting the approach curve to the elastic model, demonstrated a direct relation to the relaxation function for all the tested cases. The study will help researchers to verify which model is more appropriate for the sample description without extensive calculations from the basic curve parameters and their dependency on the indentation rate.

Low Temperature Nanoindentation: Development and Applications

Nanoindentation technique at low temperatures have developed from initial micro-hardness driving method at a single temperature to modern depth-sensing indentation (DSI) method with variable temperatures over the last three decades. The technique and implementation of representative cooling systems adopted on the indentation apparatuses are discussed in detail here, with particular emphasis on pros and cons of combination with indentation technique. To obtain accurate nanoindentation curves and calculated results of material properties, several influence factors have been carefully considered and eliminated, including thermal drift and temperature induced influence on indenter and specimen. Finally, we further show some applications on typical materials and discuss the perspectives related to low temperature nanoindentation technique.

Osteoarthritis Severely Decreases the Elasticity and Hardness of Knee Joint Cartilage: A Nanoindentation Study

The nanoindentation method was applied to determine the elastic modulus and hardness of knee articular cartilage. Cartilage samples from both high weight bearing (HWB) and low weight bearing (LWB) femoral condyles were collected from patients diagnosed with osteoarthritis (OA). The mean elastic modulus of HWB cartilage was 4.46 ± 4.44 MPa in comparison to that of the LWB region (9.81 ± 8.88 MPa, p < 0.001). Similarly, the hardness was significantly lower in HWB tissue (0.317 ± 0.397 MPa) than in LWB cartilage (0.455 ± 0.434 MPa, p < 0.001). When adjusted to patients’ ages, the mean elastic modulus and hardness were both significantly lower in the age group over 70 years (p < 0.001). A statistically significant difference in mechanical parameters was also found in grade 3 and 4 OA. This study provides an insight into the nanomechanical properties of the knee articular cartilage and provides a starting point for personalized cartilage grafts that are compatible with the mechanical properties of the native tissue.

A Helmholtz resonator on elastic foundation for measurement of the elastic coefficient of human skin

A new sensing mechanism is proposed for the measurement of elasticity of human skin by utilizing Helmholtz resonator with a flexible membrane mounted at the bottom and putting on an elastic foundation. Elastic coefficient of human skin is modeled as the elastic foundation modulus, based on the assumption that human skin is equivalent to the Winkler foundation. For the Helmholtz resonator, the acoustic transmission loss (by which resonant frequency can be acquired) was derived by using the receptance coupling method, based on the theories of conventional Helmholtz resonator and fixed-edge membrane on elastic foundation. The fundamental resonant frequency of the proposed Helmholtz resonator was proved to be related with the elastic foundation modulus, and was used as the indicator of elastic foundation modulus to be measured. Theoretical derivation for measuring elastic foundation modulus and analytical example were presented. Experiments measuring the elastic foundation modulus of the phantoms were carried out by utilizing phantoms with different stiffness using gelatin with corresponding different concentrations. The analytical and experimental results verified the effectiveness of the proposed method. Nanoindentation test was conducted for comparison, and relative errors ranged from 9.24% to 20.06% were obtained, which tends to be higher with the increasing concentration of gelatin.

Finite element simulation for the effect of loading rate on visco-hyperelastic characterisation of soft materials by spherical nanoindentation

Nanoindentation test performed by atomic force microscopy is highly recommended for the characterisation of soft materials at nanoscale. The assumption proposed in the characterisation is that the material is pure elastic with no viscosity. However, this assumption does not represent the real characteristics of soft materials such as bio tissue or cell. Therefore, a parametric finite element simulation of nanoindentation by spherical tip was carried out to investigate the response of cells with different constitutive laws (elastic, hyperelastic and visco-hyperelastic). The investigation of the loading rate effect on the characterisation of cell mechanical properties was performed for different size of spherical tip. The selected dimensions of spherical tips cover commercially available products. The viscosity effects are insensitive to the varied dimensions of spherical tip in this study. A limit loading rate was found above which viscous effect has to be considered to correctly determine the mechanical properties. The method in this work can be implemented to propose a criterion for the threshold of loading rate when viscosity effect can be neglected for soft material characterisation.

Toxicity and photosensitizing assessment of gelatin methacryloyl-based hydrogels photoinitiated with lithium phenyl-2,4,6-trimethylbenzoylphosphinate in human primary renal proximal tubule epithelial cells

Gelatin methacryloyl (GelMA) and lithium phenyl-2,4,6-trimethylbenzoylphosphinate (LAP) photoinitiator are commonly used in combination to produce a photosensitive polymer but there are concerns that must be addressed: the presence of unreacted monomer is well known to be cytotoxic, and lithium salts are known to cause acute kidney injury. In this study, acellular 10% GelMA hydrogels cross-linked with different LAP concentrations and cross-linking illumination times were evaluated for their cytotoxicity, photosensitizing potential, and elastic moduli. Alamar Blue and CyQuant Direct Cell viability assays were performed on human primary renal proximal tubule epithelial cells (hRPTECs) exposed to extracts of each formulation. UV exposure during cross-linking was not found to affect extract cytotoxicity in either assay. LAP concentration did not affect extract cytotoxicity as determined by the Alamar Blue assay but reduced hRPTEC viability in the CyQuant Direct cell assay. Photocatalytic activity of formulation extracts toward NADH oxidation was used as a screening method for photosensitizing potential; longer UV exposure durations yielded extracts with less photocatalytic activity. Finally, elastic moduli determined using nanoindentation was found to plateau to approximately 20-25 kPa after exposure to 342 mJ/cm2 at 2.87 mW of UV-A exposure regardless of LAP concentration. LAP at concentrations commonly used in bioprinting (<0.5% w/w) was not found to be cytotoxic although the differences in cytotoxicity evaluation determined from the two viability assays imply cell membrane damage and should be investigated further. Complete cross-linking of all formulations decreased photocatalytic activity while maintaining predictable final elastic moduli.