Strain rate viscoelastic analysis of soft and highly hydrated biomaterials

Experimental & numerical investigations of ultra-high-speed dynamics of optically induced droplet cavitation in soft materials

Perfluorocarbon (PFC) droplets represent a novel class of phase-shift contrast agent with promise in applications in biomedical and bioengineering fields. PFC droplets undergo a fast liquid-gas transition upon exposure to acoustic or optical triggering, offering a potential adaptable and versatile tool as contrast agent in diagnostic imaging and localized drug delivery vehicles in therapeutics systems. In this paper, we utilize advanced imaging techniques to investigate ultra-high-speed inertial dynamics and rectified quasi-static (low-speed) diffusion evolution of optically induced PFC droplet vaporization within three different hydrogels, each of different concentrations, examining effects such as droplet size and PFC core on bubble dynamics and material viscoelastic properties. Gelatin hydrogels reveal concentration-dependent impacts on bubble expansion and material elasticity. Embedding PFC droplets in gelatin increases internal pressure, resulting in higher equilibrium radius and continuous bubble growth during quasi-static evolution. Similar trends are observed in fibrin and polyacrylamide matrices, with differences in bubble behavior attributed to matrix properties and droplet presence. Interestingly, droplet size exhibits minimal impact on bubble expansion during inertial dynamics but influences quasi-static evolution, with larger droplets leading to continuous growth beyond 60 s. Furthermore, the core composition of PFC droplets significantly affects bubble behavior, with higher boiling point droplets exhibiting higher maximum expansion and faster quasi-static dissolution rates. Overall, the study sheds light on the intricate interplay between droplet characteristics, matrix properties, and multi-timescale bubble dynamics, offering valuable insights into their behavior within biomimetic hydrogels.

Being thin-skinned can still reduce damage from dynamic puncture

The integumentary system in animals serves as an important line of defence against physiological and mechanical external forces. Over time, integuments have evolved layered structures (scales, cuticle and skin) with high toughness and strength to resist damage and prevent wound expansion. While previous studies have examined their defensive performance under low-rate conditions, the failure response and damage resistance of these thin layers under dynamic biological puncture remain underexplored. Here, we utilize a novel experimental framework to investigate the mechanics of dynamic puncture in both bilayer structures of synthetic tissue-mimicking composite materials and natural skin tissues. Our findings reveal the remarkable efficiency of a thin outer skin layer in reducing the overall extent of dynamic puncture damage. This enhanced damage resistance is governed by interlayer properties through puncture energetics and diminishes in strength at higher puncture rates due to rate-dependent effects in silicone tissue simulants. In addition, natural skin tissues exhibit unique material properties and failure behaviours, leading to superior damage reduction capability compared with synthetic counterparts. These findings contribute to a deeper understanding of the inherent biomechanical complexity of biological puncture systems with layered composite material structures. They lay the groundwork for future comparative studies and bio-inspired applications.

Liver click dECM hydrogels for engineering hepatic microenvironments

Decellularized extracellular matrix (dECM) hydrogels provide tissue-specific microenvironments which accommodate physiological cellular phenotypes in 3D in vitro cell cultures. However, their formation hinges on collagen fibrillogenesis, a complex process which limits regulation of physicochemical properties. Hence, achieving reproducible results with dECM hydrogels poses as a challenge. Here, we demonstrate that thiolation of solubilized liver dECM enables rapid formation of covalently crosslinked hydrogels via Michael-type addition, allowing for precise control over mechanical properties and superior organotypic biological activity. Investigation of various decellularization methodologies revealed that treatment of liver tissue with Triton X-100 and ammonium hydroxide resulted in near complete DNA removal with significant retention of the native liver proteome. Chemical functionalization of pepsin-solubilized liver dECMs via 1-ethyl-3(3-dimethylamino)propyl carbodiimide (EDC)/N-hydroxysuccinimide (NHS) coupling of l-Cysteine created thiolated liver dECM (dECM-SH), which rapidly reacted with 4-arm polyethylene glycol (PEG)-maleimide to form optically clear hydrogels under controlled conditions. Importantly, Young's moduli could be precisely tuned between 1 - 7 kPa by varying polymer concentrations, enabling close replication of healthy and fibrotic liver conditions in in vitro cell cultures. Click dECM-SH hydrogels were cytocompatible, supported growth of HepG2 and HepaRG liver cells, and promoted liver-specific functional phenotypes as evidenced by increased metabolic activity, as well CYP1A2 and CYP3A4 activity and excretory function when compared to monolayer culture and collagen-based hydrogels. Our findings demonstrate that click-functionalized dECM hydrogels offer a highly controlled, reproducible alternative to conventional tissue-derived hydrogels for in vitro cell culture applications. STATEMENT OF SIGNIFICANCE: Traditional dECM hydrogels face challenges in reproducibility and mechanical property control due to variable crosslinking processes. We introduce a click hydrogel based on porcine liver decellularized extracellular matrix (dECM) that circumnavigates these challenges. After optimizing liver decellularization for ECM retention, we integrated thiol-functionalized liver dECM with polyethylene-glycol derivatives through Michael-type addition click chemistry, enabling rapid, room-temperature gelation. This offers enhanced control over the hydrogel's mechanical and biochemical properties. The resultant click dECM hydrogels mimic the liver's natural ECM and exhibit greater mechanical tunability and handling ease, facilitating their application in high-throughput and industrial settings. Moreover, these hydrogels significantly improve the function of HepaRG-derived hepatocytes in 3D culture, presenting an advancement for liver tissue cell culture models for drug testing applications.

Exploring Mixed Ionic-Electronic-Conducting PVA/PEDOT:PSS Hydrogels as Channel Materials for Organic Electrochemical Transistors

Here, we report the preparation and evaluation of PVA/PEDOT:PSS-conducting hydrogels working as channel materials for OECT applications, focusing on the understanding of their charge transport and transfer properties. Our conducting hydrogels are based on crosslinked PVA with PEDOT:PSS interacting via hydrogen bonding and exhibit an excellent swelling ratio of ~180-200% w/w. Our electrochemical impedance studies indicate that the charge transport and transfer processes at the channel material based on conducting hydrogels are not trivial compared to conducting polymeric films. The most relevant feature is that the ionic transport through the swollen hydrogel is clearly different from the transport through the solution, and the charge transfer and diffusion processes govern the low-frequency regime. In addition, we have performed in operando Raman spectroscopy analyses in the OECT devices supported by first-principle computational simulations corroborating the doping/de-doping processes under different applied gate voltages. The maximum transconductance (gm~1.05 μS) and maximum volumetric capacitance (C*~2.3 F.cm-3) values indicate that these conducting hydrogels can be promising candidates as channel materials for OECT devices.

Biogenic polymer-based patches for congenital cardiac surgery: a feasibility study

Objective

Currently used patch materials in congenital cardiac surgery do not grow, renew, or remodel. Patch calcification occurs more rapidly in pediatric patients eventually leading to reoperations. Bacterial cellulose (BC) as a biogenic polymer offers high tensile strength, biocompatibility, and hemocompatibility. Thus, we further investigated the biomechanical properties of BC for use as patch material.

Methods

The BC-producing bacteria Acetobacter xylinum were cultured in different environments to investigate optimal culturing conditions. For mechanical characterization, an established method of inflation for biaxial testing was used. The applied static pressure and deflection height of the BC patch were measured. Furthermore, a displacement and strain distribution analysis was performed and compared to a standard xenograft pericardial patch.

Results

The examination of the culturing conditions revealed that the BC became homogenous and stable when cultivated at 29°C, 60% oxygen concentration, and culturing medium exchange every third day for a total culturing period of 12 days. The estimated elastic modulus of the BC patches ranged from 200 to 530 MPa compared to 230 MPa for the pericardial patch. The strain distributions, calculated from preloaded (2 mmHg) to 80 mmHg inflation, show BC patch strains ranging between 0.6% and 4%, which was comparable to the pericardial patch. However, the pressure at rupture and peak deflection height varied greatly, ranging from 67 to around 200 mmHg and 0.96 to 5.28 mm, respectively. The same patch thickness does not automatically result in the same material properties indicating that the manufacturing conditions have a significant impact on durability.

Conclusions

BC patches can achieve comparable results to pericardial patches in terms of strain behavior as well as in the maximum applied pressure that can be withstood without rupture. Bacterial cellulose patches could be a promising material worth further research.

An unbroken network of interactions connecting flagellin domains is required for motility in viscous environments

In its simplest form, bacterial flagellar filaments are composed of flagellin proteins with just two helical inner domains, which together comprise the filament core. Although this minimal filament is sufficient to provide motility in many flagellated bacteria, most bacteria produce flagella composed of flagellin proteins with one or more outer domains arranged in a variety of supramolecular architectures radiating from the inner core. Flagellin outer domains are known to be involved in adhesion, proteolysis and immune evasion but have not been thought to be required for motility. Here we show that in the Pseudomonas aeruginosa PAO1 strain, a bacterium that forms a ridged filament with a dimerization of its flagellin outer domains, motility is categorically dependent on these flagellin outer domains. Moreover, a comprehensive network of intermolecular interactions connecting the inner domains to the outer domains, the outer domains to one another, and the outer domains back to the inner domain filament core, is required for motility. This inter-domain connectivity confers PAO1 flagella with increased stability, essential for its motility in viscous environments. Additionally, we find that such ridged flagellar filaments are not unique to Pseudomonas but are, instead, present throughout diverse bacterial phyla.

Dynamic Ocular Response to Mechanical Loading: The Role of Viscoelasticity in Energy Dissipation by the Cornea

We have used vibrational optical coherence tomography (VOCT) to measure the resonant frequency, elastic modulus, and loss modulus of components of the anterior segment of pig eyes in vitro. Such basic biomechanical properties of the cornea have been shown to be abnormal not only in diseases of the anterior segment but also in posterior segment diseases as well. This information is needed to better understand corneal biomechanics in health and disease and to be able to diagnose the early stages of corneal pathologies. Results of dynamic viscoelastic studies on whole pig eyes and isolated corneas indicate that at low strain rates (30 Hz or less), the viscous loss modulus is as high as 0.6 times the elastic modulus for both whole eyes and corneas. This large viscous loss is similar to that of skin, which has been hypothesized to be dependent upon the physical association of proteoglycans with collagenous fibers. The energy dissipation properties of the cornea provide a mechanism to dissipate energy associated with blunt trauma, thereby preventing delamination and failure. The cornea possesses the ability to store impact energy and transmit excess energy to the posterior segment of the eye through its serial connection to the limbus and sclera. In this manner, the viscoelastic properties of the cornea, in concert with that of the posterior segment of the pig eye, function to prevent mechanical failure of the primary focusing element of the eye. Results of resonant frequency studies suggest that the 100-120 Hz and 150-160 Hz resonant frequency peaks reside in the anterior segment of the cornea since the removal of the anterior segment of the cornea decreases the peak heights at these resonant frequencies. These results suggest that there is more than one collagen fibril network found in the anterior portion of the cornea that provides structural integrity to prevent corneal delamination and that VOCT may be useful clinically to diagnose corneal diseases.

Innovative antibacterial electrospun nanofibers mats depending on piezoelectric generation

This paper introduces a new approach of testing piezoelectric nanofibers as antibacterial mat. In this work, both Polyvinylidene fluoride (PVDF) and PVDF embedded with thermoplastic polyurethane nanofibers are synthesized as nanofibers mat via electrospinning technique. Then, such mat is analyzed as piezoelectric material to generate electric voltage under different mechanical excitations. Furthermore, morphological and chemical characteristics have been operated to prove the existence of beta sheets piezoelectricity of the synthesized nanofibers mats. Then, the synthesized nanofibers surfaces have been cyclically stretched and exposed to bacteria specimen. It has been noticed that the generated voltage and the corresponding localized electric field positively affect the growth of bacteria and reduces the formation of K. penomenue samples bacteria colonies. In addition, the effect of both stretching frequency and pulses numbers have been studied on the bacteria count, growth kinetics, and protein leakage. Our contribution here is to introduce an innovative way of the direct impact of the generated electric field from piezoelectric nanofibers on the reduction of bacteria growth, without depending on traditional anti-bacterial nanoparticles. This work can open a new trend of the usability of piezoelectric nanofibers through masks, filters, and wound curing mats within anti-bacterial biological applications.

Development and validation of subject-specific 3D human head models based on a nonlinear visco-hyperelastic constitutive framework

Computational head models are promising tools for understanding and predicting traumatic brain injuries. Most available head models are developed using inputs (i.e. head geometry, material properties and boundary conditions) from experiments on cadavers or animals and employ hereditary integral-based constitutive models that assume linear viscoelasticity in part of the rate-sensitive material response. This leads to high uncertainty and poor accuracy in capturing the nonlinear brain tissue response. To resolve these issues, a framework for the development of subject-specific three-dimensional head models is proposed, in which all inputs are derived in vivo from the same living human subject: head geometry via magnetic resonance imaging (MRI), brain tissue properties via magnetic resonance elastography (MRE), and full-field strain-response of the brain under rapid head rotation via tagged MRI. A nonlinear, viscous dissipation-based visco-hyperelastic constitutive model is employed to capture brain tissue response. Head models are validated using quantitative metrics that compare spatial strain distribution, temporal strain evolution, and the magnitude of strain maxima, with the corresponding experimental observations from tagged MRI. Results show that our head models accurately capture the strain-response of the brain. Further, employment of the nonlinear visco-hyperelastic constitutive framework provides improvements in the prediction of peak strains and temporal strain evolution over hereditary integral-based models.

Role of stiffness and physico-chemical properties of tumour microenvironment on breast cancer cell stemness

Several physico-chemical properties of the tumour microenvironment (TME) are dysregulated during tumour progression, such as tissue stiffness, extracellular pH and interstitial fluid flow. Traditional preclinical models, although useful to study biological processes, do not provide sufficient control over these physico-chemical properties, hence limiting the understanding of cause-effect relationships between the TME and cancer cells. Breast cancer stem cells (B-CSCs), a dynamic population within the tumour, are known to affect tumour progression, metastasis and therapeutic resistance. With their emerging importance in disease physiology, it is essential to study the interplay between above-mentioned TME physico-chemical variables and B-CSC marker expression. In this work, 3D in vitro models with controlled physico-chemical properties (hydrogel stiffness and composition, perfusion, pH) were used to mimic normal and tumour breast tissue to study changes in proliferation, morphology and B-CSC population in two separate breast cancer cell lines (MCF-7 and MDA-MB 231). Cells encapsulated in alginate-gelatin hydrogels varying in stiffness (2-10 kPa), density and adhesion ligand (gelatin) were perfused (500 µL/min) for up to 14 days. Physiological (pH 7.4) and tumorigenic (pH 6.5) media were used to mimic changes in extracellular pH within the TME. We found that both cell lines have distinct responses to changes in physico-chemical factors in terms of proliferation, cell aggregates size and morphology. Most importantly, stiff and dense hydrogels (10 kPa) and acidic pH (6.5) play a key role in B-CSCs dynamics, increasing both epithelial (E-CSCs) and mesenchymal cancer stem cell (M-CSCs) marker expression, supporting direct impact of the physico-chemical microenvironment on disease onset and progression. STATEMENT OF SIGNIFICANCE: Currently no studies evaluate the impact of physico-chemical properties of the tumour microenvironment on breast cancer stem cell (B-CSC) marker expression in a single in vitro model and at the same time. In this study, 3D in vitro models with varying stiffness, extracellular pH and fluid flow are used to recapitulate the breast tumour microenvironment to evaluate for the first time their direct effect on multiple breast cancer phenotypes: cell proliferation, cell aggregate size and shape, and B-CSC markers. Results suggest these models could open new ways of monitoring disease phenotypes, from the early-onset to progression, as well as being used as testing platforms for effective identification of specific phenotypes in the presence of relevant tumour physico-chemical microenvironment.

Constitutive Equations for Analyzing Stress Relaxation and Creep of Viscoelastic Materials Based on Standard Linear Solid Model Derived with Finite Loading Rate

The viscoelastic properties of materials such as polymers can be quantitatively evaluated by measuring and analyzing the viscoelastic behaviors such as stress relaxation and creep. The standard linear solid model is a classical and commonly used mathematical model for analyzing stress relaxation and creep behaviors. Traditionally, the constitutive equations for analyzing stress relaxation and creep behaviors based on the standard linear solid model are derived using the assumption that the loading is a step function, implying that the loading rate used in the loading process of stress relaxation and creep tests is infinite. Using such constitutive equations may cause significant errors in analyses since the loading rate must be finite (no matter how fast it is) in a real stress relaxation or creep experiment. The purpose of this paper is to introduce the constitutive equations for analyzing stress relaxation and creep behaviors based on the standard linear solid model derived with a finite loading rate. The finite element computational simulation results demonstrate that the constitutive equations derived with a finite loading rate can produce accurate results in the evaluation of all viscoelastic parameters regardless of the loading rate in most cases. It is recommended that the constitutive equations derived with a finite loading rate should replace the traditional ones derived with an infinite loading rate to analyze stress relaxation and creep behaviors for quantitatively evaluating the viscoelastic properties of materials.

Estimation of Tensile Modulus of a Thermoplastic Material from Dynamic Mechanical Analysis: Application to Polyamide 66

The mechanical properties of thermoplastic materials depend on temperature and strain rate. This study examined the development of a procedure to predict tensile moduli at different strain rates and temperatures, using experimental data from three-point-bending dynamic mechanical analysis (DMA). The method integrated different classical concepts of rheology to establish a closed formulation that will allow researchers save an important amount of time. Furthermore, it implied a significant decrease in the number of tests when compared to the commonly used procedure with a universal testing machine (UTM). The method was validated by means of a prediction of tensile moduli of polyamide PA66 in the linear elastic range, over a temperature range that included the glass-transition temperature. The method was applicable to thermo-rheologically simple materials under the hypotheses of isotropy, homogeneity, small deformations, and linear viscoelasticity. This method could be applicable to other thermoplastic materials, although it must be tested using these other materials to determine to what extent it can be applied reliably.

Decellularized liver ECM-based 3D scaffolds: Compositional, physical, chemical, rheological, thermal, mechanical, and in vitro biological evaluations

The extracellular matrix (ECM) is involved in many critical cellular interactions through its biological macromolecules. In this study, a macroporous 3D scaffold originating from decellularized bovine liver ECM (dL-ECM), with defined compositional, physical, chemical, rheological, thermal, mechanical, and in vitro biological properties was developed. First, protocols were determined that effectively remove cells and DNA while ECM retains biological macromolecules collagen, elastin, sGAGs in tissue. Rheological analysis revealed the elastic properties of pepsin-digested dL-ECM. Then, dL-ECM hydrogel was neutralized, molded, formed into macroporous (~100-200 μm) scaffolds in aqueous medium at 37 °C, and lyophilized. The scaffolds had water retention ability, and were mechanically stable for at least 14 days in the culture medium. The findings also showed that increasing the dL-ECM concentration from 10 mg/mL to 20 mg/mL resulted in a significant increase in the mechanical strength of the scaffolds. The hemolysis test revealed high in vitro hemocompatibility of the dL-ECM scaffolds. Studies investigating the viability and proliferation status of human adipose stem cells seeded over a 2-week culture period have demonstrated the suitability of dL-ECM scaffolds as a cell substrate. Prospective studies may reveal the extent to which 3D dL-ECM sponges have the potential to create a biomimetic environment for cells.

Mechanical Properties of Isolated Primary Cilia Measured by Micro-tensile Test and Atomic Force Microscopy

Mechanotransduction is a well-known mechanism by which cells sense their surrounding mechanical environment, convert mechanical stimuli into biochemical signals, and eventually change their morphology and functions. Primary cilia are believed to be mechanosensors existing on the surface of the cell membrane and support cells to sense surrounding mechanical signals. Knowing the mechanical properties of primary cilia is essential to understand their responses, such as sensitivity to mechanical stimuli. Previous studies have so far conducted flow experiments or optical trap techniques to measure the flexural rigidity EI (E: Young’s modulus, I: second moment of inertia) of primary cilia; however, the flexural rigidity is not a material property of materials and depends on mathematical models used in the determination, leading to a discrepancy between studies. For better characterization of primary cilia mechanics, Young’s modulus should be directly and precisely measured. In this study, the tensile Young’s modulus of isolated primary cilia is, for the first time, measured by using an in-house micro-tensile tester. The different strain rates of 0.01–0.3 s⁻¹ were applied to isolated primary cilia, which showed a strain rate–dependent Young’s modulus in the range of 69.5–240.0 kPa on average. Atomic force microscopy was also performed to measure the local Young’s modulus of primary cilia, showing the Young’s modulus within the order of tens to hundreds of kPa. This study could directly provide the global and local Young’s moduli, which will benefit better understanding of primary cilia mechanics.

Characterising and engineering biomimetic materials for viscoelastic mechanotransduction studies

The mechanical behavior of soft tissue extracellular matrix is time dependent. Moreover, it evolves over time due to physiological processes as well as aging and disease. Measuring and quantifying the time-dependent mechanical behavior of soft tissues and materials pose a challenge, not only because of their labile and hydrated nature but also because of the lack of a common definition of terms and understanding of models for characterizing viscoelasticity. Here, we review the most important measurement techniques and models used to determine the viscoelastic properties of soft hydrated materials—or hydrogels—underlining the difference between viscoelastic behavior and the properties and descriptors used to quantify viscoelasticity. We then discuss the principal factors, which determine tissue viscoelasticity in vivo and summarize what we currently know about cell response to time-dependent materials, outlining fundamental factors that have to be considered when interpreting results. Particular attention is given to the relationship between the different time scales involved (mechanical, cellular and observation time scales), as well as scaling principles, all of which must be considered when designing viscoelastic materials and performing experiments for biomechanics or mechanobiology applications. From this overview, key considerations and directions for furthering insights and applications in the emergent field of cell viscoelastic mechanotransduction are provided.

Biomechanics of pollen pellet removal by the honey bee

Honey bees (Apis mellifera) carry pollen back to their hive by mixing it with nectar and forming it into a pellet. The pellet must be firmly attached to their legs during flight, but also easily removable when deposited in the hive. How does the honey bee achieve these contrary aims? In this experimental study, we film honey bees removing pollen pellets and find they peel them off at speeds 2-10 times slower than their typical grooming speeds. Using a self-built pollen scraper, we find that slow removal speeds reduce the force and work required to remove the pellet under shear stress. Creep tests on individual pollen pellets revealed that pollen pellets are viscoelastic materials characterized by a Maxwell model with long relaxation times. The relaxation time enables the pellet to remain a solid during both transport and removal. We hope that this work inspires further research into viscoelastic materials in nature.

Soft Polymer-Based Technique for Cellular Force Sensing

Soft polymers have emerged as a vital type of material adopted in biomedical engineering to perform various biomechanical characterisations such as sensing cellular forces. Distinct advantages of these materials used in cellular force sensing include maintaining normal functions of cells, resembling in vivo mechanical characteristics, and adapting to the customised functionality demanded in individual applications. A wide range of techniques has been developed with various designs and fabrication processes for the desired soft polymeric structures, as well as measurement methodologies in sensing cellular forces. This review highlights the merits and demerits of these soft polymer-based techniques for measuring cellular contraction force with emphasis on their quantitativeness and cell-friendliness. Moreover, how the viscoelastic properties of soft polymers influence the force measurement is addressed. More importantly, the future trends and advancements of soft polymer-based techniques, such as new designs and fabrication processes for cellular force sensing, are also addressed in this review.

Effect of Changing Heart Rate on the Ocular Pulse and Dynamic Biomechanical Behavior of the Optic Nerve Head

Purpose

To study the effect of changing heart rate on the ocular pulse and the dynamic biomechanical behavior of the optic nerve head (ONH) using a comprehensive mathematical model.

Methods

In a finite element model of a healthy eye, a biphasic choroid consisted of a solid phase with connective tissues and a fluid phase with blood, and the lamina cribrosa (LC) was viscoelastic as characterized by a stress-relaxation test. We applied arterial pressures at 18 ocular entry sites (posterior ciliary arteries), and venous pressures at four exit sites (vortex veins). In the model, the heart rate was varied from 60 to 120 bpm (increment: 20 bpm). We assessed the ocular pulse amplitude (OPA), pulse volume, ONH deformations, and the dynamic modulus of the LC at different heart rates.

Results

With an increasing heart rate, the OPA decreased by 0.04 mm Hg for every 10 bpm increase in heart rate. The ocular pulse volume decreased linearly by 0.13 µL for every 10 bpm increase in heart rate. The storage modulus and the loss modulus of the LC increased by 0.014 and 0.04 MPa, respectively, for every 10 bpm increase in heart rate.

Conclusions

In our model, the OPA, pulse volume, and ONH deformations decreased with an increasing heart rate, whereas the LC became stiffer. The effects of blood pressure/heart rate changes on ONH stiffening may be of interest for glaucoma pathology.

Tuning Viscoelasticity with Minor Changes in Speed of Sound in an Ultrasound Phantom Material

The acoustic properties of ultrasound phantom materials have always been important, but with new applications interrogating tissue mechanical properties, viscoelasticity has also become an interesting feature to consider. Along with Young's modulus, the viscous component of tissue is affected by certain diseases and can therefore be used as a biomarker. Furthermore, viscoelasticity varies between tissue types and individuals, and therefore it would be useful with a phantom material that reflects this physiological range. Here we describe a gel for ultrasound imaging with a range of mechanical properties given by mixing different ratios of two oil-based gels, clear ballistic and styrene-ethylene/butylene-styrene (SEBS). The gels were mixed in five different proportions, ranging from 0-100% of either gel. For each of the gel compositions, we measured time of flight to determine speed of sound, narrowband ultrasound transmission for attenuation, stress-relaxation for viscoelasticity, mass and volume. Analysis of the stress-relaxation data using the generalized Maxwell model suggests that the material can be described by five parameters, E₀, E₁, E₂, η₁ and η₂, and that each of these parameters decreases as more SEBS is incorporated into the mixed material. Instantaneous Young's modulus (the sum of E₀, E₁ and E₂ in our model) ranges between 49 and 117 kPa for the different ratios, similar to values reported for cancerous tissue. Despite the large span of obtainable mechanical properties, speed of sound is relatively constant regardless of composition, with mean value estimates (± 95 % CI) between 1438 ± 9 and 1455 ± 3 m/s for pure and mixed gels. This was attributed to a variation in density and Poisson's ratio, following from the relation linking them to speed of sound and elasticity. Furthermore, both speed of sound and attenuation were within a suitable range for ultrasound phantoms. Combining this ballistic gel with SEBS copolymer in oil allows for control of mechanical properties, both elastic and viscous as evaluated by the material model. Furthermore, it does so without compromising ease of use, longevity and safety of the pre-made gel.

Functionalized Enzyme-Responsive Biomaterials to Model Tissue Stiffening in vitro

The mechanical properties of the cellular microenvironment play a crucial role in modulating cell function, and many pathophysiological processes are accompanied by variations in extracellular matrix (ECM) stiffness. Lysyl oxidase (LOx) is one of the enzymes involved in several ECM-stiffening processes. Here, we engineered poly(ethylene glycol) (PEG)-based hydrogels with controlled mechanical properties in the range typical of soft tissues. These hydrogels were functionalized featuring free primary amines, which allows an additional chemical LOx-responsive behavior with increase in crosslinks and hydrogel elastic modulus, mimicking biological ECM-stiffening mechanisms. Hydrogels with elastic moduli in the range of 0.5-4 kPa were obtained after a first photopolymerization step. The increase in elastic modulus of the functionalized and enzyme-responsive hydrogels was also characterized after the second-step enzymatic reaction, recording an increase in hydrogel stiffness up to 0.5 kPa after incubation with LOx. Finally, hydrogel precursors containing HepG2 (bioinks) were used to form three-dimensional (3D) in vitro models to mimic hepatic tissue and test PEG-based hydrogel biocompatibility. Hepatic functional markers were measured up to 7 days of culture, suggesting further use of such 3D models to study cell mechanobiology and response to dynamic variation of hydrogels stiffness. The results show that the functionalized hydrogels presented in this work match the mechanical properties of soft tissues, allow dynamic variations of hydrogel stiffness, and can be used to mimic changes in the microenvironment properties of soft tissues typical of inflammation and pathological changes at early stages (e.g., fibrosis, cancer).

Engineering Gels with Time-Evolving Viscoelasticity

From a mechanical point of view, a native extracellular matrix (ECM) is viscoelastic. It also possesses time-evolving or dynamic behaviour, since pathophysiological processes such as ageing alter their mechanical properties over time. On the other hand, biomaterial research on mechanobiology has focused mainly on the development of substrates with varying stiffness, with a few recent contributions on time- or space-dependent substrate mechanics. This work reports on a new method for engineering dynamic viscoelastic substrates, i.e., substrates in which viscoelastic parameters can change or evolve with time, providing a tool for investigating cell response to the mechanical microenvironment. In particular, a two-step (chemical and enzymatic) crosslinking strategy was implemented to modulate the viscoelastic properties of gelatin hydrogels. First, gels with different glutaraldehyde concentrations were developed to mimic a wide range of soft tissue viscoelastic behaviours. Then their mechanical behaviour was modulated over time using microbial transglutaminase. Typically, enzymatically induced mechanical alterations occurred within the first 24 h of reaction and then the characteristic time constant decreased although the elastic properties were maintained almost constant for up to seven days. Preliminary cell culture tests showed that cells adhered to the gels, and their viability was similar to that of controls. Thus, the strategy proposed in this work is suitable for studying cell response and adaptation to temporal variations of substrate mechanics during culture.

A multilayer micromechanical elastic modulus measuring method in ex vivo human aneurysmal abdominal aortas

Abdominal aortic aneurysms (AAA) are common and potentially life-threatening aortic dilatations, due to the effect of hemodynamic changes on the aortic wall. Previous research has shown a potential pathophysiological role for increased macroscopic aneurysmal wall stiffness; however, not investigating micromechanical stiffness. We aimed to compile a new protocol to examine micromechanical live aortic stiffness (elastic moduli), correlated to histological findings with quantitative immunofluorescence (QIF). Live AAA biopsies (n = 7) and non-dilated aortas (controls; n = 3) were sectioned. Local elastic moduli of aortic intima, media and adventitia were analysed in the direction towards the lumen and vice versa with nanoindentation. Smooth muscle cells (SMC), collagen and fibroblasts were examined using QIF. Nanoindentation of AAA vs. controls demonstrated a 4-fold decrease in elastic moduli (p = 0.022) for layers combined and a 26-fold decrease (p = 0.017) for media-to-intima direction. QIF of AAA vs. controls revealed a 4-, 3- and 6-fold decrease of SMC, collagen and fibroblasts, respectively (p = 0.036). Correlations were found between bidirectional intima and media measurements (ρ = 0.661, p = 0.038) and all QIF analyses (ρ = 0.857-0.905, p = 0.002-0.007). We present a novel protocol to analyse microscopic elastic moduli in live aortic tissues using nanoindentation. Hence, our preliminary findings of decreased elastic moduli and altered wall composition warrant further microscopic stiffness investigation to potentially clarify AAA pathophysiology and to explore potential treatment by wall strengthening. STATEMENT OF SIGNIFICANCE: Although extensive research on the pathophysiology of dilated abdominal aortas (aneurysms) has been performed, the exact underlying pathways are still largely unclear. Previously, the macroscopic stiffness of the pathologic and healthy aortic wall has been studied. This study however, for the first time, studied the microscopic stiffness changes in live tissue of dilated and non-dilated abdominal aortas. This new protocol provides a device to analyse the alterations on cellular level within their microenvironment, whereas previous studies studied the aorta as a whole. Outcomes of these measurements might help to better understand the underlying origin of the incidence and progression of aneurysms and other aortic diseases.

[The elimination method of preloading force for soft tissue based on the linear loading region]

Aiming at the problem of the influence of preloading force on its mechanical response in soft tissue compression experiments, an elimination method of preloading force based on linear loading region is proposed. Unconfined compression experiments under a variety of different preloading forces are performed. The influence of the preloading force on the parameters of constitutive model is analyzed. In the preload phase, the mechanical response of the soft tissue is taken as a linear model. The preloading force is eliminated by taking the preloading phase into account throughout the response process. According to five different preloading forces of the unconfined compression experiments, the elimination method is validated with two different constitutive models of soft tissue, and the error between the models obtained by the preloading force elimination method and the traditional method with the experimental results is compared. The results show that the error obtained by preloading force elimination method is significantly smaller than the traditional method. The preloading force elimination method can eliminate the influence of preloading force on mechanical response to a certain extent, and constitutive model parameters which are closer to the true properties of soft tissue can be obtained.

Viscoelastic properties of shock wave exposed brain tissue subjected to unconfined compression experiments

Traumatic brain injuries (TBI) affect millions of people each year. While research has been dedicated to determining the mechanical properties of the uninjured brain, there has been a lack of investigation on the mechanical properties of the brain after experiencing a primary blast-induced TBI. In this paper, whole porcine brains were exposed to a shock wave to simulate blast-induced TBI. First, ten (10) brains were subjected to unconfined compression experiments immediately following shock wave exposure. In addition, 22 brains exposed to a shock wave were placed in saline solution and refrigerated between 30 minutes and 6.0 hours before undergoing unconfined compression experiments. This study aimed to investigate the effect of a time delay on the viscoelastic properties in the event that an experiment cannot be completed immediately. Samples from both soaked and freshly extracted brains were subjected to compressive rates of 5, 50, and 500 mm/min during the unconfined compression experiments. The fractional Zener (FZ) viscoelastic model was applied to obtain the brain's material properties. The length of time in the solution statistically influenced three of the four FZ coefficients, E₀ (instantaneous elastic response), τ₀ (relaxation time), and α (fractional order). Further, the compressive rate statistically influenced τ₀ and α.

Influence of saline solution absorption and compressive rate on the material properties of brain tissue

Traumatic brain injuries (TBI) affect millions of people each year and can result in long-term difficulties in thinking or focusing. Due to the number of people affected by these injuries, significant research has been dedicated to determining the mechanical properties of the brain using postmortem tissue from animals harvested within 24 h. The postmortem brain tissue is often stored in a solution until a rheological experiment is ready to begin. However, the effect of storage duration on the mechanical behavior of brain tissue is not understood. In this paper, postmortem porcine brains were placed in normal saline solution (0.9% NaCl) and refrigerated between 30 min and 6.5 h to allow the brain to absorb the solution. Afterwards, samples from both soaked and freshly extracted brains were subjected to unconfined compression tests at compressive rates of 5, 50, and 500 mm/min. The fractional Zener viscoelastic model was applied to obtain the brain's mechanical properties. While the results did not show a significant relationship between absorption and the long-term stiffness (E_∞), both the relaxation time (τ₀) and fractional order (α) were statistically influenced by both the length of time in the solution and compressive rate. Further, the instantaneous stiffness (E₀) was statistically influenced by the length of time in solution, though not the compressive rate.

Characterization of perfused and sectioned liver tissue in a full indentation cycle using a visco-hyperelastic model

Realistic modeling of biologic material is required for optimizing fidelity in computer-aided surgical training and assistance systems. The modeling of liver tissue has remained challenging due to its nonlinear viscoelastic properties and high hysteresis of the stress-strain relation. While prior studies have described the behavior of liver tissue during the loading status (in elongation, compression, or indentation tests) or unloading status (in stress relaxation or creep tests), a hysteresis curve with both loading and unloading processes was incompletely defined. We seek to use a single material model to characterize the mechanical properties of liver tissue in a full indentation cycle ex vivo perfused and then sectioned. Based on measurements taken from ex-vivo perfused porcine livers, we converted force-displacement curves to stress-strain curves and developed a visco-hyperelastic constitutive model to characterize the liver's mechanical behavior at different locations under various rates of indentation (1, 2, 5, 10, and 20 mm/s). The proposed model is a mixed visco-hyperelastic model with up to 6 coefficients. The normalized root mean square standard deviations of fitted curves are less than 5% and 10% in low (<0.05) and high strain (>0.3) conditions respectively.

Comparison of frequency and strain-rate domain mechanical characterization

Indentation is becoming increasingly popular to test soft tissues and (bio)materials. Each material exhibits an unknown intrinsic “mechanical behaviour”. However, limited consensus on its “mechanical properties” (i.e. quantitative descriptors of mechanical behaviour) is generally present in the literature due to a number of factors, which include sample preparation, testing method and analysis model chosen. Viscoelastic characterisation – critical in applications subjected to dynamic loading conditions – can be performed in either the time- or frequency-domain. It is thus important to selectively investigate whether the testing domain affects the mechanical results or not. We recently presented an optomechanical indentation tool which enables both strain-rate (nano-\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\dot{\varepsilon }M$$\end{document}ε˙M) and frequency domain (DMA) measurements while keeping the sample under the same physical conditions and eliminating any other variability factor. In this study, a poly(dimethylsiloxane) sample was characterised with our system. The DMA data were inverted to the time-domain through integral transformations and then directly related to nano-\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\dot{\varepsilon }M$$\end{document}ε˙M strain-rate dependent results, showing that, even though the data do not perfectly overlap, there is an excellent correlation between them. This approach indicates that one can convert an oscillatory measurement into a strain-rate one and still capture the trend of the “mechanical behaviour” of the sample investigated.

Bioinspired liver scaffold design criteria

Maintaining hepatic functional characteristics in-vitro is considered one of the main challenges in engineering liver tissue. As hepatocytes cultured ex-vivo are deprived of their native extracellular matrix (ECM) milieu, developing scaffolds that mimic the biomechanical and physicochemical properties of the native ECM is thought to be a promising approach for successful tissue engineering and regenerative medicine applications. On the basis that the decellularized liver matrix represents the ideal design template for engineering bioinspired hepatic scaffolds, to derive quantitative descriptors of liver ECM architecture, we characterised decellularised liver matrices in terms of their biochemical, viscoelastic and structural features along with porosity, permeability and wettability. Together, these data provide a unique set of quantitative design criteria which can be used to generate guidelines for fabricating biomaterial scaffolds for liver tissue engineering. As proof-of-concept, we investigated hepatic cell response to substrate viscoelasticity. On collagen hydrogels mimicking decellularised liver mechanics, cells showed superior morphology, higher viability and albumin secretion than on stiffer and less viscous substrates. Although scaffold properties are generally inspired by those of native tissues, our results indicate significant differences between the mechano-structural characteristics of untreated and decellularised hepatic tissue. Therefore, we suggest that design rules - such as mechanical properties and swelling behaviour - for engineering biomimetic scaffolds be re-examined through further studies on substrates matching the features of decellularized liver matrices.

Impact of PEGylation on the mucolytic activity of recombinant human deoxyribonuclease I in cystic fibrosis sputum

Highly viscous mucus and its impaired clearance characterize the lungs of patients with cystic fibrosis (CF). Pulmonary secretions of patients with CF display increased concentrations of high molecular weight components such as DNA and actin. Recombinant human deoxyribonuclease I (rhDNase) delivered by inhalation cleaves DNA filaments contained in respiratory secretions and thins them. However, rapid clearance of rhDNase from the lungs implies a daily administration and thereby a high therapy burden and a reduced patient compliance. A PEGylated version of rhDNase could sustain the presence of the protein within the lungs and reduce its administration frequency. Here, we evaluated the enzymatic activity of rhDNase conjugated to a two-arm 40 kDa polyethylene glycol (PEG40) in CF sputa. Rheology data indicated that both rhDNase and PEG40-rhDNase presented similar mucolytic activity in CF sputa, independently of the purulence of the sputum samples as well as of their DNA, actin and ions contents. The macroscopic appearance of the samples correlated with the DNA content of the sputa: the more purulent the sample, the higher the DNA concentration. Finally, quantification of the enzymes in CF sputa following rheology measurement suggests that PEGylation largely increases the stability of rhDNase in CF respiratory secretions, since 24-fold more PEG40-rhDNase than rhDNase was recovered from the samples. The present results are considered positive and provide support to the continuation of the research on a long acting version of rhDNase to treat CF lung disease.

Adhesion and relaxation of a soft elastomer on surfaces with skin like roughness

For designing new skin adhesives, the complex mechanical interaction of soft elastomers with surfaces of various roughnesses needs to be better understood. We systematically studied the effects of a wide set of roughness characteristics, film thickness, hold time and material relaxation on the adhesive behaviour of the silicone elastomer SSA 7-9800 (Dow Corning). As model surfaces, we used epoxy replicas obtained from substrates with roughness ranging from very smooth to skin-like. Our results demonstrate that films of thin and intermediate thickness (60 and 160 µm) adhered best to a sub-micron rough surface, with a pull-off stress of about 50 kPa. Significant variations in pull-off stress and detachment mechanism with roughness and hold time were found. In contrast, 320 µm thick films adhered with lower pull-off stress of about 17 kPa, but were less sensitive to roughness and hold time. It is demonstrated that the adhesion performance of the silicone films to rough surfaces can be tuned by tailoring the film thickness and contact time.

Tissue perfusion rate estimation with compression-based photoacoustic-ultrasound imaging

Tissue perfusion is essential for transporting blood oxygen and nutrients. Measurement of tissue perfusion rate would have a significant impact in clinical and preclinical arenas. However, there are few techniques to image this important parameter and they typically require contrast agents. A label-free methodology based on tissue compression and imaging with a high-frequency photoacoustic-ultrasound system is introduced for estimating and visualizing tissue perfusion rates. Experiments demonstrate statistically significant differences in depth-resolved perfusion rates in a human subject with various temperature exposure conditions.

Manufacturing of hydrogel biomaterials with controlled mechanical properties for tissue engineering applications

Hydrogels have been recognized as crucial biomaterials in the field of tissue engineering, regenerative medicine, and drug delivery applications due to their specific characteristics. These biomaterials benefit from retaining a large amount of water, effective mass transfer, similarity to natural tissues and the ability to form different shapes. However, having relatively poor mechanical properties is a limiting factor associated with hydrogel biomaterials. Controlling the biomechanical properties of hydrogels is of paramount importance. In this work, firstly, mechanical characteristics of hydrogels and methods employed for characterizing these properties are explored. Subsequently, the most common approaches used for tuning mechanical properties of hydrogels including but are not limited to, interpenetrating polymer networks, nanocomposites, self-assembly techniques, and co-polymerization are discussed. The performance of different techniques used for tuning biomechanical properties of hydrogels is further compared. Such techniques involve lithography techniques for replication of tissues with complex mechanical profiles; microfluidic techniques applicable for generating gradients of mechanical properties in hydrogel biomaterials for engineering complex human tissues like intervertebral discs, osteochondral tissues, blood vessels and skin layers; and electrospinning techniques for synthesis of hybrid hydrogels and highly ordered fibers with tunable mechanical and biological properties. We finally discuss future perspectives and challenges for controlling biomimetic hydrogel materials possessing proper biomechanical properties.

STATEMENT OF SIGNIFICANCE

Hydrogels biomaterials are essential constituting components of engineered tissues with the applications in regenerative medicine and drug delivery. The mechanical properties of hydrogels play crucial roles in regulating the interactions between cells and extracellular matrix and directing the cells phenotype and genotype. Despite significant advances in developing methods and techniques with the ability of tuning the biomechanical properties of hydrogels, there are still challenges regarding the synthesis of hydrogels with complex mechanical profiles as well as limitations in vascularization and patterning of complex structures of natural tissues which barricade the production of sophisticated organs. Therefore, in addition to a review on advanced methods and techniques for measuring a variety of different biomechanical characteristics of hydrogels, the new techniques for enhancing the biomechanics of hydrogels are presented. It is expected that this review will profit future works for regulating the biomechanical properties of hydrogel biomaterials to satisfy the demands of a variety of different human tissues.

Micro-Mechanical Viscoelastic Properties of Crosslinked Hydrogels Using the Nano-Epsilon Dot Method

Engineering materials that recapitulate pathophysiological mechanical properties of native tissues in vitro is of interest for the development of biomimetic organ models. To date, the majority of studies have focused on designing hydrogels for cell cultures which mimic native tissue stiffness or quasi-static elastic moduli through a variety of crosslinking strategies, while their viscoelastic (time-dependent) behavior has been largely ignored. To provide a more complete description of the biomechanical environment felt by cells, we focused on characterizing the micro-mechanical viscoelastic properties of crosslinked hydrogels at typical cell length scales. In particular, gelatin hydrogels crosslinked with different glutaraldehyde (GTA) concentrations were analyzed via nano-indentation tests using the nano-epsilon dot method. The experimental data were fitted to a Maxwell Standard Linear Solid model, showing that increasing GTA concentration results in increased instantaneous and equilibrium elastic moduli and in a higher characteristic relaxation time. Therefore, not only do gelatin hydrogels become stiffer with increasing crosslinker concentration (as reported in the literature), but there is also a concomitant change in their viscoelastic behavior towards a more elastic one. As the degree of crosslinking alters both the elastic and viscous behavior of hydrogels, caution should be taken when attributing cell response merely to substrate stiffness, as the two effects cannot be decoupled.

On the adhesion-cohesion balance and oxygen consumption characteristics of liver organoids

Liver organoids (LOs) are of interest in tissue replacement, hepatotoxicity and pathophysiological studies. However, it is still unclear what triggers LO self-assembly and what the optimal environment is for their culture. Hypothesizing that LO formation occurs as a result of a fine balance between cell-substrate adhesion and cell-cell cohesion, we used 3 cell types (hepatocytes, liver sinusoidal endothelial cells and mesenchymal stem cells) to investigate LO self-assembly on different substrates keeping the culture parameters (e.g. culture media, cell types/number) and substrate stiffness constant. As cellular spheroids may suffer from oxygen depletion in the core, we also sought to identify the optimal culture conditions for LOs in order to guarantee an adequate supply of oxygen during proliferation and differentiation. The oxygen consumption characteristics of LOs were measured using an O₂ sensor and used to model the O₂ concentration gradient in the organoids. We show that no LO formation occurs on highly adhesive hepatic extra-cellular matrix-based substrates, suggesting that cellular aggregation requires an optimal trade-off between the adhesiveness of a substrate and the cohesive forces between cells and that this balance is modulated by substrate mechanics. Thus, in addition to substrate stiffness, physicochemical properties, which are also critical for cell adhesion, play a role in LO self-assembly.

Sample, testing and analysis variables affecting liver mechanical properties: A review

Given the critical role of liver mechanics in regulating cell response and directing the development of tissue fibrosis, accurately characterising its mechanical behaviour is of relevance for both diagnostic purposes as well as for tissue engineering and for the development of in-vitro models. Determining and quantifying the mechanical behaviour of soft biological tissues is, however, highly challenging due to their intrinsic labile nature. Indeed, a unique set of values of liver mechanical properties is still lacking to date; testing conditions can significantly affect sample status and hence the measured behaviour and reported results are strongly dependent on the adopted testing method and configuration as well as sample type and status. This review aims at summarising the bulk mechanical properties of liver described in the literature, discussing the possible sources of variation and their implications on the reported results. We distinguish between the intrinsic mechanical behaviour of hepatic tissue, which depends on sample variables, and the measured mechanical properties which also depend on the testing and analysis methods. Finally, the review provides guidelines on tissue preparation and testing conditions for generating reproducible data which can be meaningfully compared across laboratories.

STATEMENT OF SIGNIFICANCE

Soft tissue mechanics is widely investigated, but poorly understood. This review identifies and discusses sample and testing variables which can influence the mechanical behaviour of hepatic tissue and consequently the measured mechanical properties. To encourage the biomaterial community towards more standardized testing of soft tissues and enable comparisons between data from different laboratories, we have established new testing methods and experimental recommendations for sample preparation and testing. The review could be of wide interest to scientists involved in biomaterials research because it addresses and proposes guidelines for several issues related to the mechanical testing of soft tissues whose implications have not been considered together before.

The nano-epsilon dot method for strain rate viscoelastic characterisation of soft biomaterials by spherical nano-indentation

Nano-indentation is widely used for probing the micromechanical properties of materials. Based on the indentation of surfaces using probes with a well-defined geometry, the elastic and viscoelastic constants of materials can be determined by relating indenter geometry and measured load and displacement to parameters which represent stress and deformation. Here we describe a method to derive the viscoelastic properties of soft hydrated materials at the micro-scale using constant strain rates and stress-free initial conditions. Using a new self-consistent definition of indentation stress and strain and corresponding unique depth-independent expression for indentation strain rate, the epsilon dot method, which is suitable for bulk compression testing, is transformed to nano-indentation. We demonstrate how two materials can be tested with a displacement controlled commercial nano-indentor using the nano-espilon dot method (nano-ε̇M) to give values of instantaneous and equilibrium elastic moduli and time constants with high precision. As samples are tested in stress-free initial conditions, the nano-ε̇M could be useful for characterising the micro-mechanical behaviour of soft materials such as hydrogels and biological tissues at cell length scales.

Decoupling the role of stiffness from other hydroxyapatite signalling cues in periosteal derived stem cell differentiation

Bone extracellular matrix (ECM) is a natural composite made of collagen and mineral hydroxyapatite (HA). Dynamic cell-ECM interactions play a critical role in regulating cell differentiation and function. Understanding the principal ECM cues promoting osteogenic differentiation would be pivotal for both bone tissue engineering and regenerative medicine. Altering the mineral content generally modifies the stiffness as well as other physicochemical cues provided by composite materials, complicating the “cause-effect” analysis of resultant cell behaviour. To isolate the contribution of mechanical cues from other HA-derived signals, we developed and characterised composite HA/gelatin scaffolds with different mineral contents along with a set of stiffness-matched HA-free gelatin scaffolds. Samples were seeded with human periosteal derived progenitor cells (PDPCs) and cultured over 7 days, analysing their resultant morphology and gene expression. Our results show that both stiffness and HA contribute to directing PDPC osteogenic differentiation, highlighting the role of stiffness in triggering the expression of osteogenic genes and of HA in accelerating the process, particularly at high concentrations.