Nonlinear photoelasticity of rubber-like soft materials: comparison between theory and experiment

Revealing physical interactions of ultrasound waves with the body through photoelasticity imaging

Ultrasound is a ubiquitous technology in medicine for screening, diagnosis, and treatment of disease. The functionality and efficacy of different ultrasound modes relies strongly on our understanding of the physical interactions between ultrasound waves and biological tissue structures. This article reviews the use of photoelasticity imaging for investigating ultrasound fields and interactions. Physical interactions are described for different ultrasound technologies, including those using linear and nonlinear ultrasound waves, as well as shock waves. The use of optical modulation of light by ultrasound is presented for shadowgraphic and photoelastic techniques. Investigations into shock wave and burst wave lithotripsy using photoelastic methods are summarized, along with other endoscopic forms of lithotripsy. Photoelasticity in soft tissue surrogate materials is reviewed, and its deployment in investigating tissue-bubble interactions, generated ultrasound waves, and traumatic brain injury, are discussed. With the continued growth of medical ultrasound, photoelasticity imaging can play a role in elucidating the physical mechanisms leading to useful bioeffects of ultrasound for imaging and therapy.

Fatigue-Resistant Mechanoresponsive Color-Changing Hydrogels for Vision-Based Tactile Robots

Mechanoresponsive color-changing materials that can reversibly and resiliently change color in response to mechanical deformation are highly desirable for diverse modern technologies in optics, sensors, and robots; however, such materials are rarely achieved. Here, a fatigue-resistant mechanoresponsive color-changing hydrogel (FMCH) is reported that exhibits reversible, resilient, and predictable color changes under mechanical stress. At its undeformed state, the FMCH remains dark under a circular polariscope; upon uniaxial stretching of up to six times its initial length, it gradually shifts its color from black, to gray, yellow, and purple. Unlike traditional mechanoresponsive color-changing materials, FMCH maintains its performance across various strain rates for up to 10 000 cycles. Moreover, FMCH demonstrates superior mechanical properties with fracture toughness of 3000 J m-2, stretchability of 6, and fatigue threshold up to 400 J m-2. These exceptional mechanical and optical features are attributed to FMCH's substantial molecular entanglements and desirable hygroscopic salts, which synergistically enhance its mechanical toughness while preserving its color-changing performance. One application of this FMCH as a tactile sensoris then demonstrated for vision-based tactile robots, enabling them to discern material stiffness, object shape, spatial location, and applied pressure by translating stress distribution on the contact surface into discernible images.

Optimizing non-Newtonian fluids for impact protection of laminates

Non-Newtonian fluids can be used for the protection of flexible laminates. Understanding the coupling between the flow of the protecting fluid and the deformation of the protected solids is necessary in order to optimize this functionality. We present a scaling analysis of the problem based on a single coupling variable, the effective width of a squeeze flow between flat rigid plates, and predict that impact protection for laminates is optimized by using shear-thinning, and not shear-thickening, fluids. The prediction is verified experimentally by measuring the velocity and pressure in impact experiments. Our scaling analysis should be generically applicable for non-Newtonian fluid-solid interactions in diverse applications.

Algorithm and its application for automatic measurement of the full-field isoclinic parameter by digital phase-shifting photoelasticity

The photoelastic method is an experimental technique that combines optics and mechanics for a stress analysis. The photoelastic phase-shifting technique is different from the moiré, holography, and speckle phase-shifting techniques, which only need to measure one parameter. The photoelastic phase-shifting technique needs to assess isoclinic and isochromatic parameters, which affect each other, seriously hindering the development of the phase-shifting photoelasticity method. First, the interaction between the isoclinic and isochromatic parameters is analyzed in detail. Secondly, an algorithm is proposed to adjust the mutation and obtain the correct isoclinic parameter affected by the isochromatic parameter. This method can effectively eliminate the influence of the isochromatic parameter. The isoclinic parameter is consistent with the theoretical value, which verifies the effectiveness of this method. Finally, the photoelastic method uses the proposed algorithm to test the stress at different positions of the turbine blade root. Moreover, the bearing capacity of the turbine blade root is analyzed to provide support for the safe use and optimization design of the turbine.

Enhance Fracture Toughness and Fatigue Resistance of Hydrogels by Reversible Alignment of Nanofibers

Biological tissues, such as heart valve, tendon, etc., possess excellent mechanical properties, which arises from their inherent anisotropic arrangement of soft and hard phases. Inspired by the anisotropic structures, many methods have been developed to synthesize hydrogels that can achieve mechanical properties comparable to biological tissues. Here, we describe a new method to enhance fracture toughness and fatigue resistance of hydrogels by introducing nanofibers which can reversibly align with elastic deformation to form an anisotropic structure. As a demonstration, we introduce stiff, rod-like cellulose nanocrystals (CNCs) into a polyacrylamide (PAAm) network. CNCs aggregate into clusters to form hard phases and entangle with the PAAm network. The CNC/PAAm composite hydrogel is initially isotropic, becomes anisotropic upon loading, and recovers to be isotropic upon unloading. During the deformation, the aligned CNC clusters at the crack tip can transmit the stress over the size of the cluster, effectively resisting crack growth. We use photoelasticity and small-angle X-ray scattering (SAXS) tests to observe the change of microstructures associated with deformation. The fracture toughness of CNC/PAAm hydrogels with different sizes of CNCs can reach 1000 J/m². The fatigue threshold is about 100 J/m², an order of magnitude higher than that of PAAm hydrogel. This work provides a simple and general method to strengthen hydrogels under both monotonic and cyclic loads.

Photoelastic Organogel with Multiple Stimuli Responses

The photoelastic effect has many uses in mechanics today, but it is usually disregarded in flexible materials. Using 2-phenoxyethyl acrylate as a monomer and 4-cyano-4'-pentylbiphenyl (5CB) as a solvent, a multiple responsive photoelastic organogel (PO) with strong birefringence but low modulus is created. 5CB is a liquid crystal molecule that does not participate in the polymerization process and is always present as tiny molecules in the polymer. It endows the PO low modulus and high birefringence, as well as the ability to drive the birefringence using an electric field. This PO not only has high sensitivity and fast response as a photoelastic strain sensor, but also has a very sensitive response to heat, especially in the range of human body temperature. It also has a high dielectric constant and a strong correlation between the interference color and the applied electric field, allowing for easy writing and erasure of encrypted data. This unique multisignal response feature and low modulus that mimics human skin bring up new opportunities in the potential applications such as multiple information encryption, anticounterfeiting, and multifunctional wearable sensors.

Birefringent tissue-mimicking phantom for polarization-sensitive optical coherence tomography imaging

SIGNIFICANCE

Tissue birefringence is an important parameter to consider when designing realistic, tissue-mimicking phantoms. Options for suitable birefringent materials that can be used to accurately represent tissue scattering are limited.

AIM

To introduce a method of fabricating birefringent tissue phantoms with a commonly used material-polydimethylsiloxane (PDMS)-for imaging with polarization-sensitive optical coherence tomography (PS-OCT).

APPROACH

Stretch-induced birefringence was characterized in PDMS phantoms made with varying curing ratios, and the resulting phantom birefringence values were compared with those of biological tissues.

RESULTS

We showed that, with induced birefringence levels up to 2.1  ×  10  -  4, PDMS can be used to resemble the birefringence levels in weakly birefringent tissues. We demonstrated the use of PDMS in the development of phantoms to mimic the normal and diseased bladder wall layers, which can be differentiated by their birefringence levels.

CONCLUSIONS

PDMS allows accurate control of tissue scattering and thickness, and it exhibits controllable birefringent properties. The use of PDMS as a birefringent phantom material can be extended to other birefringence imaging systems beyond PS-OCT and to mimic other organs.