Use of Rigid Spherical Inclusions in Young’s Moduli Determination: Application to DNA-Crosslinked Gels

Multicomponent DNA Polymerization Motor Gels

Hydrogels with the ability to change shape in response to biochemical stimuli are important for biosensing, smart medicine, drug delivery, and soft robotics. Here, a family of multicomponent DNA polymerization motor gels with different polymer backbones is created, including acrylamide-co-bis-acrylamide (Am-BIS), poly(ethylene glycol) diacrylate (PEGDA), and gelatin-methacryloyl (GelMA) that swell extensively in response to specific DNA sequences. A common mechanism, a polymerization motor that induces swelling is driven by a cascade of DNA hairpin insertions into hydrogel crosslinks. These multicomponent hydrogels can be photopatterned into distinct shapes, have a broad range of mechanical properties, including tunable shear moduli between 297 and 3888 Pa and enhanced biocompatibility. Human cells adhere to the GelMA-DNA gels and remain viable during ≈70% volumetric swelling of the gel scaffold induced by DNA sequences. The results demonstrate the generality of sequential DNA hairpin insertion as a mechanism for inducing shape change in multicomponent hydrogels, suggesting widespread applicability of polymerization motor gels in biomaterials science and engineering.

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

BACKGROUND

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Translation of a Coated Rigid Spherical Inclusion in an Elastic Matrix: Exact Solution, and Implications for Mechanobiology

The displacement of relatively rigid beads within a relatively compliant, elastic matrix can be used to measure the mechanical properties of the matrix. For example, in mechanobiological studies, magnetic or reflective beads can be displaced with a known external force to estimate the matrix modulus. Although such beads are generally rigid compared to the matrix, the material surrounding the beads typically differs from the matrix in one or two ways. The first case, as is common in mechanobiological experimentation, is the situation in which the bead must be coated with materials such as protein ligands that enable adhesion to the matrix. These layers typically differ in stiffness relative to the matrix material. The second case, common for uncoated beads, is the situation in which the beads disrupt the structure of the hydrogel or polymer, leading to a region of enhanced or reduced stiffness in the neighborhood of the bead. To address both cases, we developed the first analytical solution of the problem of translation of a coated, rigid spherical inclusion displaced within an isotropic elastic matrix by a remotely applied force. The solution is applicable to cases of arbitrary coating stiffness and size of the coating. We conclude by discussing applications of the solution to mechanobiology.

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.

Preparation of DNA-crosslinked polyacrylamide hydrogels

Mechanobiology is an emerging scientific area that addresses the critical role of physical cues in directing cell morphology and function. For example, the effect of tissue elasticity on cell function is a major area of mechanobiology research because tissue stiffness modulates with disease, development, and injury. Static tissue-mimicking materials, or materials that cannot alter stiffness once cells are plated, are predominately used to investigate the effects of tissue stiffness on cell functions. While information gathered from static studies is valuable, these studies are not indicative of the dynamic nature of the cellular microenvironment in vivo. To better address the effects of dynamic stiffness on cell function, we developed a DNA-crosslinked polyacrylamide hydrogel system (DNA gels). Unlike other dynamic substrates, DNA gels have the ability to decrease or increase in stiffness after fabrication without stimuli. DNA gels consist of DNA crosslinks that are polymerized into a polyacrylamide backbone. Adding and removing crosslinks via delivery of single-stranded DNA allows temporal, spatial, and reversible control of gel elasticity. We have shown in previous reports that dynamic modulation of DNA gel elasticity influences fibroblast and neuron behavior. In this report and video, we provide a schematic that describes the DNA gel crosslinking mechanisms and step-by-step instructions on the preparation DNA gels.

Mechanical Properties of DNA-Crosslinked Polyacrylamide Hydrogels with Increasing Crosslinker Density

DNA-cross-linked polyacrylamide hydrogels (DNA gels) are dynamic mechanical substrates. The addition of DNA oligomers can either increase or decrease the crosslinker density to modulate mechanical properties. These DNA-responsive gels show promise as substrates for cell culture and tissue-engineering applications, since the gels allow time-dependent mechanical modulation. Previously, we reported that fibroblasts plated on DNA gels responded to modulation in elasticity via an increase or decrease in crosslinker density. To better characterize fibroblast mechanical signals, changes in stress and elastic modulus of DNA gels were measured over time as crosslinker density altered. In a previous study, we observed that as crosslinker density decreased, stress was generated, and elasticity changed over time; however, we had not evaluated stress and elastic modulus measurements of DNA gels as crosslinker density increased. Here, we completed this set of fibroblast studies by reporting stress and elastic modulus measurements over time as the crosslinker density increased. We found that the stress generated and the elastic modulus alterations were correlated. Hence, it seemed impossible to separate the effect of stress from the effect of modulus changes for fibroblasts plated on DNA gels. Yet, previous results and controls revealed that stress contributed to fibroblast behavior.

Measurement of the traction force of biological cells by digital holography

The traction force produced by biological cells has been visualized as distortions in flexible substrata. We have utilized quantitative phase microscopy by digital holography (DH-QPM) to study the wrinkling of a silicone rubber film by motile fibroblasts. Surface deformation and the cellular traction force have been measured from phase profiles in a direct and straightforward manner. DH-QPM is shown to provide highly efficient and versatile means for quantitatively analyzing cellular motility.

Self-healing biomaterials

The goal of this review is to introduce the biomaterials community to the emerging field of self-healing materials, and also to suggest how one could utilize and modify self-healing approaches to develop new classes of biomaterials. A brief discussion of the in vivo mechanical loading and resultant failures experienced by biomedical implants is followed by presentation of the self-healing methods for combating mechanical failure. If conventional composite materials that retard failure may be considered zeroth generation self-healing materials, then taxonomically speaking, first generation self-healing materials describe approaches that "halt" and "fill" damage, whereas second generation self-healing materials strive to "fully restore" the prefailed material structure. In spite of limited commercial use to date, primarily because the technical details have not been suitably optimized, it is likely from a practical standpoint that first generation approaches will be the first to be employed commercially, whereas second generation approaches may take longer to implement. For self-healing biomaterials the optimization of technical considerations is further compounded by the additional constraints of toxicity and biocompatibility, necessitating inclusion of separate discussions of design criteria for self-healing biomaterials.

Aptamer based reversible DNA induced hydrogel system for molecular recognition and separation

An aptamer-based DNA strand, captured with a specific molecule from a multi-species system, was used to initiate crosslinking of the hydrogel. The hydrogel can then be dissolved with a displacement of the DNA strand and the captured molecule can be released. Recognition and separation at the molecular level is thus achieved.

A Nonintrusive Method of Measuring the Local Mechanical Properties of Soft Hydrogels Using Magnetic Microneedles

Soft hydrogels serving as substrates for cell attachment are used to culture many types of cells. The mechanical properties of these gels influence cell morphology, growth, and differentiation. For studies of cell growth on inhomogeneous gels, techniques by which the mechanical properties of the substrate can be measured within the proximity of a given cell are of interest. We describe an apparatus that allows the determination of local gel elasticity by measuring the response of embedded micron-sized magnetic needles to applied magnetic fields. This microscope-based four-magnet apparatus can apply both force and torque on the microneedles. The force and the torque are manipulated by changing the values of the magnetic field at the four poles of the magnet using a feedback circuit driven by LABVIEW. Using Hall probes, we have mapped out the magnetic field and field gradients produced by each pole when all the other poles are held at zero magnetic field. We have verified that superposition of these field maps allows one to obtain field maps for the case when the poles are held at arbitrary field values. This allows one to apply known fields and field gradients to a given microneedle. An imaging system is employed to measure the displacement and rotation of the needles. Polyacrylamide hydrogels of known elasticity were used to determine the relationship between the field gradient at the location of the needles and the force acting on the needles. This relationship allows the force on the microneedle to be determined from a known field gradient. This together with a measurement of the displacement of the needle in a given gel allows one to determine the stiffness (F∕δ) of the gel and the elastic modulus, provided Poison’s ratio is known. Using this method, the stiffness and the modulus of elasticity of type-I collagen gels were found to be 2.64±0.05nN∕μm and 284.6±5.9Pa, respectively. This apparatus is presently being employed to track the mechanical stiffness of the DNA-cross-linked hydrogels, developed by our group, whose mechanical properties can be varied on demand by adding or removing cross-linker strands. Thus a system that can be utilized to track the local properties of soft media as a function of time with minimum mechanical disturbance in the presence of cells is presented.

Electric-field-induced displacement of charged spherical colloids in compressible hydrogels

This paper concerns the electric-field-induced displacement of a charged spherical colloid embedded in an uncharged compressible hydrogel. Previous theoretical calculations for incompressible polymer skeletons predict sub-nanometre particle displacements within the experimentally accessible parameter space (e.g., particle surface charge density, polymer shear modulus, and electric field strength). Accordingly, the prevailing expectation is that an experimental test of the theory would be extraordinarily difficult. In this work, however, we solved the electrokinetic model for compressible polymer skeletons with arbitrary Poisson's ratio. The most striking result, obtained from numerically exact solutions of the full model and an analytical boundary-layer approximation, is that polymer compressibility admits particle displacements that increase linearly with particle size when the radius is greater than the Debye length. This scaling is qualitatively different than previously obtained for incompressible skeletons, where the ratio of the particle displacement to the electric field approaches a particle-size-independent constant. The displacement is also much more sensitive to the hydrodynamic permeability of the polymer skeleton. Therefore, when compressible hydrogels are deformed at frequencies below their reciprocal draining time, our theory identifies the parameter space where displacements could be registered using optical microscopy. In turn, this will help to establish a quantitative connection between the electric-field-induced particle displacement and physicochemical characteristics of the particle-polymer interface.

Functional modulation of ES-derived hepatocyte lineage cells via substrate compliance alteration

Pluripotent embryonic stem cells represent a promising renewable cell source to generate a variety of differentiated cell types including hepatocyte lineage cells, and may ultimately be incorporated into extracorporeal bioartificial liver devices and cell replacement therapies. Recently, we and others have utilized sodium butyrate to directly differentiate hepatocyte-like cells from murine embryonic stem cells cultured in a monolayer configuration. However, to incorporate stem cell technology into clinical and pharmaceutical applications, and hopefully increase the therapeutic potential of these differentiated cells for liver disease treatment, a major challenge remains in sustaining differentiated functions for an extended period of time in their secondary culture environment. In the present work, we have investigated the use of polyacrylamide hydrogels with defined mechanical compliances as a cell culture platform for improving and/or stabilizing functions of these hepatocyte-like cells. Several functional assays, e.g., urea secretion, intracellular albumin content, and albumin secretion, were performed to characterize hepatic functions of cells on polyacrylamide gels with stiffnesses of 5, 46.6, and 230 kPa. In conjunction with the mechanical and cell morphological characterization, we showed that hepatic functions of sodium butyrate differentiated cells were sustained and further enhanced on compliant substrates. This study promises to offer insights into regulating stem cell differentiation via mechanical stimuli, and assist us with designing a variety of dynamic culture systems for applications in tissue and cellular engineering.