Ranking the fracture toughness of thin mammalian soft tissues using the scissors cutting test

Experimental Study of Needle Insertion into Gerbil Tympanic Membrane

The perforation characteristics and fracture-related mechanical properties of the tympanic membrane (TM) greatly affect surgical procedures like myringotomy and tympanostomy performed on the middle ear. We analyzed the most important features of the gerbil TM perforation using an experimental approach that was based on force measurement during a 2-cycle needle insertion/extraction process. Fracture energy, friction energy, strain energy, and hysteresis loss were taken into consideration for the analysis of the different stages of needle insertion and extraction. The results demonstrated that (1) although the TM shows viscoelastic behavior, the contribution of hysteresis loss was negligible compared to other irreversible dissipated energy components (i.e., fracture energy and friction energy). (2) The TM puncture force did not substantially change during the first hours after animal death, but interestingly, it increased after 1 week due to the drying effects of soft tissue. (3) The needle geometry affected the crack length and the most important features of the force-displacement plot for the needle insertion process (puncture force, puncture displacement, and jump-in force) increased with increasing needle diameter, whereas the insertion velocity only changed the puncture and jump-in forces (both increased with increasing insertion velocity) and did not have a noticeable effect on the puncture displacement. (4) The fracture toughness of the gerbil TM was almost independent of the needle geometry and was found to be around 0.33 ± 0.10 kJ/m2.

Composite Hydrogel Sealants for Annulus Fibrosus Repair

Intervertebral disc (IVD) herniation is a leading cause of disability and lower back pain, causing enormous socioeconomic burdens. The standard of care for disc herniation is nucleotomy, which alleviates pain but does not repair the annulus fibrosus (AF) defect nor recover the biomechanical function of the disc. Existing bioadhesives for AF repair are limited by insufficient adhesion and significant mechanical and geometrical mismatch with the AF tissue, resulting in the recurrence of protrusion or detachment of bioadhesives. Here, we report a composite hydrogel sealant constructed from a composite of a three-dimensional (3D)-printed thermoplastic polyurethane (TPU) mesh and tough hydrogel. We tailored the fiber angle and volume fraction of the TPU mesh design to match the angle-ply structure and mechanical properties of native AF. Also, we proposed and tested three types of geometrical design of the composite hydrogel sealant to match the defect shape and size. Our results show that the sealant could mimic native AF in terms of the elastic modulus, flexural modulus, and fracture toughness and form strong adhesion with the human AF tissue. The bovine IVD tests show the effectiveness of the composite hydrogel sealant for AF repair and biomechanics recovery and for preventing herniation with its heightened stiffness and superior adhesion. By harnessing the combined capabilities of 3D printing and bioadhesives, these composite hydrogel sealants demonstrate promising potential for diverse applications in tissue repair and regeneration.

A Low Mean Closing Load and a Decrease in Load Change at the Tip Increase the Comfort of Scissors

Introduction During surgery, surgeons intuitively recognize when they are using dull scissors and find them difficult to use. The purpose of this study was to objectively evaluate the physical characteristics of scissors and the comfort reported by surgeons to develop objective quality control standards for scissors used in surgery. Methods Sensory and measurement tests were conducted to evaluate the comfort and physical characteristics of ten pairs of Cooper scissors. As a sensory test, thirty-one volunteer surgeons opened and closed the scissors and selected three that felt comfortable and three that were uncomfortable. The results were scored. For measurement, a load was applied to the handle of the scissors. The load pressure and displacement of the width between each handle when the scissors were closed were measured. Results A strong negative correlation was found between the total comfort score and the mean load value between sensory and measurement tests (r=-0.717, p=0.0195). The correlation between the total score and the change in load at the tip showed a moderate negative correlation (r=-0.687, p=0.0282). Multiple regression analysis showed that the change in load at the tip was an independent factor affecting the total score. Conclusions Surgeons consider scissors with a low mean load required to close the scissors and a small change in load at the tip to be comfortable. The mean load on scissors and the change in load at the tip should be considered in the development of quality control standards for scissors used in surgery.

Impact of performance-based budgeting on quality outcomes in U.S. military health care facilities

BACKGROUND

Performance-based budgeting (PBB) is a variation of pay for performance that has been used in government hospitals but could be applicable to any integrated system. It works by increasing or decreasing funding based on preestablished performance thresholds, which incentivizes organizations to improve performance. In late 2006, the U.S. Army implemented a PBB program that tied hospital-level funding decisions to performance on key cost and quality-related metrics.

PURPOSE

The aim of this study was to estimate the impact of PBB on quality improvement in U.S. Army health care facilities.

APPROACH

This study used a retrospective difference-in-differences analysis of data from two Defense Health Agency data repositories. The merged data set encompassed administrative, demographic, and performance information about 428 military health care facilities. Facility-level performance data on quality indicators were compared between 187 Army PBB facilities and a comparison group of 241 non-PBB Navy and Air Force facilities before and after program implementation.

RESULTS

The Army's PBB programs had a positive impact on quality performance. Relative to comparison facilities, facilities that participated in PBB programs increased performance for over half of the indicators under investigation. Furthermore, performance was either sustained or continued to improve over 5 years for five of the six performance indicators examined long term.

CONCLUSION

Study findings indicate that PBB may be an effective policy mechanism for improving facility-level performance on quality indicators.

PRACTICE IMPLICATIONS

This study adds to the extant literature on pay for performance by examining the specific case of PBB. It demonstrates that quality performance can be influenced internally through centralized budgeting processes. Though specific to military hospitals, the findings might have applicability to other public and private sector hospitals who wish to incentivize performance internally in their organizational subunits through centralized budgeting processes.

Optimizing autoinjector devices using physics-based simulations and Gaussian processes

Autoinjectors are becoming a primary drug delivery option to the subcutaneous space. These devices need to work robustly and autonomously to maximize drug bio-availability. However, current designs ignore the coupling between autoinjector dynamics and tissue biomechanics. Here we present a Bayesian framework for optimization of autoinjector devices that can account for the coupled autoinjector-tissue biomechanics and uncertainty in tissue mechanical behavior. The framework relies on replacing the high fidelity model of tissue insertion with a Gaussian process (GP). The GP model is accurate yet computationally affordable, enabling a thorough sensitivity analysis that identified tissue properties, which are not part of the autoinjector design space, as important variables for the injection process. Higher fracture toughness decreases the crack depth, while tissue shear modulus has the opposite effect. The sensitivity analysis also shows that drug viscosity and spring force, which are part of the design space, affect the location and timing of drug delivery. Low viscosity could lead to premature delivery, but can be prevented with smaller spring forces, while higher viscosity could prevent premature delivery while demanding larger spring forces and increasing the time of injection. Increasing the spring force guarantees penetration to the desired depth, but it can result in undesirably high accelerations. The Bayesian optimization framework tackles the challenge of designing devices with performance metrics coupled to uncertain tissue properties. This work is important for the design of other medical devices for which optimization in the presence of material behavior uncertainty is needed.

Flaw-insensitive fatigue resistance of chemically fixed collagenous soft tissues

Bovine pericardium (BP) has been used as leaflets of prosthetic heart valves. The leaflets are sutured on metallic stents and can survive 400 million flaps (~10-year life span), unaffected by the suture holes. This flaw-insensitive fatigue resistance is unmatched by synthetic leaflets. We show that the endurance strength of BP under cyclic stretch is insensitive to cuts as long as 1 centimeter, about two orders of magnitude longer than that of a thermoplastic polyurethane (TPU). The flaw-insensitive fatigue resistance of BP results from the high strength of collagen fibers and soft matrix between them. When BP is stretched, the soft matrix enables a collagen fiber to transmit tension over a long length. The energy in the long length dissipates when the fiber breaks. We demonstrate that a BP leaflet greatly outperforms a TPU leaflet. It is hoped that these findings will aid the development of soft materials for flaw-insensitive fatigue resistance.

Dynamic interaction of injected liquid jet with skin layer interfaces revealed by microsecond imaging of optically cleared ex vivo skin tissue model

BACKGROUND

Needle-free jet injection (NFJI) systems enable a controlled and targeted delivery of drugs into skin tissue. However, a scarce understanding of their underlying mechanisms has been a major deterrent to the development of an efficient system. Primarily, the lack of a suitable visualization technique that could capture the dynamics of the injected fluid-tissue interaction with a microsecond range temporal resolution has emerged as a main limitation. A conventional needle-free injection system may inject the fluids within a few milliseconds and may need a temporal resolution in the microsecond range for obtaining the required images. However, the presently available imaging techniques for skin tissue visualization fail to achieve these required spatial and temporal resolutions. Previous studies on injected fluid-tissue interaction dynamics were conducted using in vitro media with a stiffness similar to that of skin tissue. However, these media are poor substitutes for real skin tissue, and the need for an imaging technique having ex vivo or in vivo imaging capability has been echoed in the previous reports.

METHODS

A near-infrared imaging technique that utilizes the optical absorption and fluorescence emission of indocyanine green dye, coupled with a tissue clearing technique, was developed for visualizing a NFJI in an ex vivo porcine skin tissue.

RESULTS

The optimal imaging conditions obtained by considering the optical properties of the developed system and mechanical properties of the cleared ex vivo samples are presented. Crucial information on the dynamic interaction of the injected liquid jet with the ex vivo skin tissue layers and their interfaces could be obtained.

CONCLUSIONS

The reported technique can be instrumental for understanding the injection mechanism and for the development of an efficient transdermal NFJI system as well.

Making Highly Elastic and Tough Hydrogels from Doughs

A hydrogel is often fabricated from preexisting polymer chains by covalently crosslinking them into a polymer network. The crosslinks make the hydrogel swell-resistant but brittle. This conflict is resolved here by making a hydrogel from a dough. The dough is formed by mixing long polymer chains with a small amount of water and photoinitiator. The dough is then homogenized by kneading and annealing at elevated temperatures, during which the crowded polymer chains densely entangle. The polymer chains are then sparsely crosslinked into a polymer network under an ultraviolet lamp, and submerged in water to swell to equilibrium. The resulting hydrogel is both swell-resistant and tough. The hydrogel also has near-perfect elasticity, high strength, high fatigue resistance, and low friction. The method is demonstrated with two widely used polymers, poly(ethylene glycol) and cellulose. These hydrogels have never been made swell-resistant, elastic, and tough before. The method is generally applicable to synthetic and natural polymers, and is compatible with industrial processing technologies, opening doors to the development of sustainable, high-performance hydrogels.

Fatigue-free artificial ionic skin toughened by self-healable elastic nanomesh

Robust ionic sensing materials that are both fatigue-resistant and self-healable like human skin are essential for soft electronics and robotics with extended service life. However, most existing self-healable artificial ionic skins produced on the basis of network reconfiguration suffer from a low fatigue threshold due to the easy fracture of low-energy amorphous polymer chains with susceptible crack propagation. Here we engineer a fatigue-free yet fully healable hybrid ionic skin toughened by a high-energy, self-healable elastic nanomesh, resembling the repairable nanofibrous interwoven structure of human skin. Such a design affords a superhigh fatigue threshold of 2950 J m⁻² while maintaining skin-like compliance, stretchability, and strain-adaptive stiffening response. Moreover, nanofiber tension-induced moisture breathing of ionic matrix leads to a record-high strain-sensing gauge factor of 66.8, far exceeding previous intrinsically stretchable ionic conductors. This concept creates opportunities for designing durable ion-conducting materials that replicate the unparalleled combinatory properties of natural skins more precisely.

Developing robust skin-like sensing materials is essential for soft electronics and robotics with extended service life. Here, inspired by the repairable nanofibrous structure of human skin, the authors engineer a fatigue-resistant artificial ionic skin toughened by self-healable elastic nanomesh.

Needle-free Mental Incisive Nerve Block:In vitro, Cadaveric, and Pilot Clinical Studies

The present study aimed to optimize Needle-Free Liquid Jet Injection (NFLJI) for Mental Incisive Nerve Blocks (MINB) and evaluate its clinical safety and feasibility. A MINB protocol was developed and optimized by series of NFLJI experiments in soft tissue phantoms and cadavers, then validated in two pilot Randomized Controlled Trials (RCT). The NFLJI penetration depth was found to be directly proportional to the supply pressure and volume. High-pressure NFLJIs (620 kPa or above) created maximum force and total work significantly greater than needle injections. Low-pressure NFLJIs (413 kPa), however, produced results similar to those of needle injections. Additionally, high-pressure NFLJIs created jet impingement pressure and maximum jet penetration pressure higher than low-pressure NFLJIs. Pilot RCTs revealed that high-pressure NFLJI caused a high risk of discomfort (60%) and paresthesia (20%); meanwhile, low-pressure NFLJI was less likely to cause complications (0%). The preliminary success rates of MINB from cadavers using NFLJIs and needles were 83.3% and 87.5%. In comparison, those from RCTs are 60% and 70%, respectively. To conclude, NFLJI supply pressure can be adjusted to achieve effective MINB with minimal complications. Furthermore, the cadaver study and pilot RCTs confirmed the feasibility for further non-inferiority RCT.

Heat Emitting Damage in Skin: A Thermal Pathway for Mechanical Algesia

Mechanical pain (or mechanical algesia) can both be a vital mechanism warning us for dangers or an undesired medical symptom important to mitigate. Thus, a comprehensive understanding of the different mechanisms responsible for this type of pain is paramount. In this work, we study the tearing of porcine skin in front of an infrared camera, and show that mechanical injuries in biological tissues can generate enough heat to stimulate the neural network. In particular, we report local temperature elevations of up to 24°C around fast cutaneous ruptures, which shall exceed the threshold of the neural nociceptors usually involved in thermal pain. Slower fractures exhibit lower temperature elevations, and we characterise such dependency to the damaging rate. Overall, we bring experimental evidence of a novel—thermal—pathway for direct mechanical algesia. In addition, the implications of this pathway are discussed for mechanical hyperalgesia, in which a role of the cutaneous thermal sensors has priorly been suspected. We also show that thermal dissipation shall actually account for a significant portion of the total skin's fracture energy, making temperature monitoring an efficient way to detect biological damages.

Fracture behaviour of human skin in deep needle insertion can be captured using validated cohesive zone finite-element method

Medical needles have shown an appreciable contribution to the development of novel medical devices and surgical technologies. A better understanding of needle-skin interactions can advance the design of medical needles, modern surgical robots, and haptic devices. This study employed finite element (FE) modelling to explore the effect of different mechanical and geometrical parameters on the needle's force-displacement relationship, the required force for the skin puncture, and generated mechanical stress around the cutting zone. To this end, we established a cohesive FE model, and identified its parameters by a three-stage parameter identification algorithm to closely replicate the experimental data of needle insertion into the human skin available in the literature. We showed that a bilinear cohesive model with initial stiffness of 5000 MPa/mm, failure traction of 2 MPa, and separation length of 1.6 mm can lead to a model that can closely replicate experimental results. The FE results indicated that while the coefficient of friction between the needle and skin substantially changes the needle reaction force, the insertion velocity does not have a noticeable effect on the reaction force. Regarding the geometrical parameters, needle cutting angle is the prominent factor in terms of stress fields generated in the skin tissue. However, the needle diameter is more influential on the needle reaction force. We also presented an energy study on the frictional dissipation, damage dissipation, and strain energy throughout the insertion process.

Multifaceted Design and Emerging Applications of Tissue Adhesives

Tissue adhesives can form appreciable adhesion with tissues and have found clinical use in a variety of medical settings such as wound closure, surgical sealants, regenerative medicine, and device attachment. The advantages of tissue adhesives include ease of implementation, rapid application, mitigation of tissue damage, and compatibility with minimally invasive procedures. The field of tissue adhesives is rapidly evolving, leading to tissue adhesives with superior mechanical properties and advanced functionality. Such adhesives enable new applications ranging from mobile health to cancer treatment. To provide guidelines for the rational design of tissue adhesives, here, existing strategies for tissue adhesives are synthesized into a multifaceted design, which comprises three design elements: the tissue, the adhesive surface, and the adhesive matrix. The mechanical, chemical, and biological considerations associated with each design element are reviewed. Throughout the report, the limitations of existing tissue adhesives and immediate opportunities for improvement are discussed. The recent progress of tissue adhesives in topical and implantable applications is highlighted, and then future directions toward next-generation tissue adhesives are outlined. The development of tissue adhesives will fuse disciplines and make broad impacts in engineering and medicine.

A highly robust approach to fabricate the mass-customizable mold of sharp-tipped biodegradable polymer microneedles for drug delivery

A skin-perforable dissolving microneedle is a promising mediator for painlessly delivering active pharmaceutical compounds across the skin. All the microneedle manufacturing processes so far, however, are much sensitive to input variation and unfavorable for make-to-order approach. Here, a robust method for fabricating mass-customizable master molds is developed to prepare sharp-tipped biodegradable polymer microneedles. Our approach combines the predrying and chip casting (PCC) of an ultrathick photoresist layer with a substrateless, inclined, and rotational exposure (SIR exposure). The PCC achieves the uniform reduction of solvent across the photoresist thickness which is critically required for the formation of a sharp tip; the SIR exposure creates master molds whose geometry is easily customizable and virtually insensitive to a variation in ultraviolet (UV) exposure dose. A theoretical model for the spatiotemporal distribution of UV dose under SIR exposure is established to show the technological superiority of our method. Next, our method's applicability is proven by fabricating a set of poly(lactic-co-glycolic) acid (PLGA) microneedles and performing both porcine skin penetration test and their in vitro degradation test. Our approach is verified to be robust in manufacturing mass-customizable molds for skin-perforable dissolving microneedles and to have high compatibility with almost all existing biodegradable polymers. The findings of this study lead to both a significant growth of dissolving microneedle-mediated drug delivery and better understanding of drug release kinetics.

Testing the influence of crushing surface variation on seed-cracking performance among beak morphs of the African seedcracker Pyrenestes ostrinus

Extreme phenotypic polymorphism is an oft-cited example of evolutionary theory in practice. Although these morphological variations are assumed to be adaptive, few studies have biomechanically tested such hypotheses. Pyrenestes ostrinus (the African seedcracker finch) shows an intraspecific polymorphism in beak size and shape that is entirely diet driven and allelically determined. Three distinct morphs feed upon soft sedge seeds during times of abundance, but during lean times switch to specializing on three different species of sedge seeds that differ significantly in hardness. Here, we test the hypothesis that beak morphology is directly related to consuming seeds of different hardness. We used a novel experimental analysis to test how beak morphology affects the efficiency of cracking sedge seeds of variable hardness, observing that neither mandibular ramus width nor crushing surface morphology had significant effects on the ability to crack different seed types. It is likely that feeding performance is correlated with other aspects of beak size and shape, such as beak depth and strength, muscle force or gape. Our results highlight how even seemingly straightforward examples of adaptive selection in nature can be complex in practice.

Controlled manual loading of body tissues: towards the next generation of pressure algometer

Assessing the responses of body tissue subjected to mechanical load is a fundamental component of the clinical examination, psychophysical assessments and bioengineering research. The forces applied during such assessments are usually generated manually, via the hands of the tester, and aimed at discreet tissue sites. It is therefore desirable to objectively quantify and optimise the control of manually applied force. However, current laboratory-grade manual devices and commercial software packages, in particular pressure algometer systems, are generally inflexible and expensive. This paper introduces and discusses several principles that should be implemented as design goals within a flexible, generic software application, given currently available force measurement hardware. We also discuss pitfalls that clinicians and researchers might face when using current pressure algometer systems and provide examples of these. Finally, we present our implementation of a pressure algometer system that achieves these goals in an efficient and affordable way for researchers and clinicians. As part of this effort, we will be sharing our configurable software application via a software repository.

Extracting the elasticity of the human skin in microscale and in-vivo from atomic force microscopy experiments using viscoelastic models

Detecting mechanical properties of the intact skin in-vivo leads to a novel quantitative method to diagnose skin diseases and to monitor skin conditions in clinical settings. Current research and clinical methods that detect skin mechanics have major limitations. The in-vitro experiments are done in non-physiological conditions and in-vivo clinical methods measurer unwanted mechanics of underneath fat and muscle tissues but report the measurement as skin mechanics. An ideal skin mechanics should be captured at skin scale (i.e., micron-scale) and in-vivo. However, extreme challenges of capturing the in-vivo skin mechanics in micron-scale including skin motion due to heart beep, breathing and movement of the subject, has hindered measurement of skin mechanics in-vivo.This study for the first time captures micro-scale mechanics (elasticity and viscoelasticity) of top layers of skin (i.e., the stratum corneum (SC) and stratum granulosum (SG)) in-vivo. In this study, the relevant literature is reviewed and Atomic Force Microscopy (AFM) was used to capture force-indentation curves on the fingertip skin of four human subjects at a high indentation speed of 40 μm/s. The skin of the same subject were tested in-vitro at 10 different indentation speeds ranging from 0.125 to 40 μm/s by AFM. This study extracts the in-vivo elasticity of SC and SG by detecting time-dependency of tested tissue using a fractional viscoelastic standard linear model developed for indentation. The in-vivo elasticity of SC and SG were smaller in females and in-vitro elasticity were higher than that of in-vivo results. The results were consistent with previous observations.

Self-Healing Hydrogels: The Next Paradigm Shift in Tissue Engineering?

Given their durability and long-term stability, self-healable hydrogels have, in the past few years, emerged as promising replacements for the many brittle hydrogels currently being used in preclinical or clinical trials. To this end, the incompatibility between hydrogel toughness and rapid self-healing remains unaddressed, and therefore most of the self-healable hydrogels still face serious challenges within the dynamic and mechanically demanding environment of human organs/tissues. Furthermore, depending on the target tissue, the self-healing hydrogels must comply with a wide range of properties including electrical, biological, and mechanical. Notably, the incorporation of nanomaterials into double-network hydrogels is showing great promise as a feasible way to generate self-healable hydrogels with the above-mentioned attributes. Here, the recent progress in the development of multifunctional and self-healable hydrogels for various tissue engineering applications is discussed in detail. Their potential applications within the rapidly expanding areas of bioelectronic hydrogels, cyborganics, and soft robotics are further highlighted.

Using Artificial Skin Devices as Skin Replacements: Insights into Superficial Treatment

Artificial skin devices are able to mimic the flexibility and sensory perception abilities of the skin. They have thus garnered attention in the biomedical field as potential skin replacements. This Review delves into issues pertaining to these skin-deep devices. It first elaborates on the roles that these devices have to fulfill as skin replacements, and identify strategies that are used to achieve such functionality. Following which, a comparison is done between the current state of these skin-deep devices and that of natural skin. Finally, an outlook on artificial skin devices is presented, which discusses how complementary technologies can create skin enhancements, and what challenges face such devices.

Effects of Tendon Degeneration on Predictions of Supraspinatus Tear Propagation

Rotator cuff tendons undergo degeneration with age, which could have an impact on tear propagation. The objective of this study was to predict tear propagation for different levels of tissue degeneration using an experimentally validated finite element model of a supraspinatus tendon. It was hypothesized that greater amounts of degeneration will result in tear propagation at lower loads than tendons with less degeneration. Using a previously-validated computational model of supraspinatus tendon, 1-cm tears were introduced in the anterior, middle, and posterior thirds of the tendon. Cohesive elements were assigned subject-specific failure properties to model tear propagation, and tendon degeneration ranging from "minimal" to "severe" was modeled by modifying its mechanical properties. Tears in tendons with severe degeneration required the smallest loads to propagate (122-207 N). Posterior tears required greater loads compared to middle and anterior tears at all levels of degeneration. Stress and strain required for tear propagation decreased substantially with degeneration, ranging from 8.5 MPa and 32.6% strain for minimal degeneration and 0.6 MPa and 4.5% strain for severe degeneration. Overall, this work indicates that greater amounts of tendon degeneration lead to greater risk of tear propagation, supporting the need for early detection and treatment of rotator cuff tears.

Fatigue exhaustion of the mitral valve tissue

Sudden failure and rupture of the tissue is a rare but serious short-term complication after the mitral valve surgical repair. Excessive cyclic loading on the suture line of the repair can progressively damage the surrounding tissue and finally cause tissue rupture. Moreover, mechanical over-tension, which occurs in a diseased mitral valve, gradually leads to tissue floppiness, mitral annular dilation, and leaflet rupture. In this work, the rupture mechanics of mitral valve is studied by characterizing the fracture toughness exhaustion of healthy tissue. Results of this study show that fracture toughness of the posterior mitral valve is lower than its anterior counterpart, indicating that posterior tissue is more prone to failure. Moreover, the decrease in fracture toughness by increasing the number of fatigue cycles shows that excessive mechanical loading leads to progressive failure and rupture of mitral valve tissue within a damage accumulative process.

Topological Adhesion of Wet Materials

Achieving strong adhesion between wet materials (i.e., tissues and hydrogels) is challenging. Existing adhesives are weak, toxic, incompatible with wet and soft surfaces, or restricted to specific functional groups from the wet materials. The approach reported here uses biocompatible polymer chains to achieve strong adhesion and retain softness, but requires no functional groups from the wet materials. In response to a trigger, the polymer chains form a network, in topological entanglement with the two polymer networks of the wet materials, stitching them together like a suture at the molecular scale. To illustrate topological adhesion, pH is used as a trigger. The stitching polymers are soluble in water in one pH range but form a polymer network in another pH range. Several stitching polymers are selected to create strong adhesion between hydrogels in full range of pH, as well as between hydrogels and various porcine tissues (liver, heart, artery, skin, and stomach). The adhesion energy above 1000 J m^-2 is achieved when the stitching polymer network elicits the hysteresis in the wet materials. The molecular suture can be designed to be permanent, transient, or removable on-demand. The topological adhesion may open many opportunities in complex and diverse environments.

Effects of tear size and location on predictions of supraspinatus tear propagation

Rotator cuff tears remain a significant clinical problem with a high incidence rate and severe clinical burden. Previous computational models developed to study rotator cuff tears have not modeled tissue damage and tear propagation. The objective of this study was to predict tear propagation for various combinations of tear size and location using an experimentally validated finite element model of supraspinatus tendon. It was hypothesized that larger rotator cuff tears propagate at lower loads than smaller tears, and that posterior tears require higher loads to propagate than anterior tears. Using a previously validated computational model of supraspinatus tendon, tears of size 0.5-1.5 cm were introduced to the tendon geometry in the anterior, middle, and posterior tendon thirds. Cohesive elements were assigned subject-specific failure properties and used to model tissue damage and tear propagation. A displacement of 5 mm was applied to the medial tendon edge to induce tear propagation. Model outputs included critical load required to propagate the tear, and principal stress and maximum principal strain at the anterior and posterior tear tips. For all tear sizes, posterior tears required the highest loads to propagate (247-567 N). Anterior tears generally required the least load to propagate (171-280 N). Stress and strain were larger on the articular side (maximum 33.9% articular strain vs 27.8% bursal strain). Overall, larger tears located in the anterior supraspinatus tendon that interrupt the rotator cable are most at risk for tear propagation, and should be carefully followed by clinicians when considering treatment options.

The hyperelastic and failure behaviors of skin in relation to the dynamic application of microscopic penetrators in a murine model

In-depth understanding of skin elastic and rupture behavior is fundamental to enable next-generation biomedical devices to directly access areas rich in cells and biomolecules. However, the paucity of skin mechanical characterization and lack of established fracture models limits their rational design. We present an experimental and numerical study of skin mechanics during dynamic interaction with individual and arrays of micro-penetrators. Initially, micro-indentation of individual skin strata revealed hyperelastic moduli were dramatically rate-dependent, enabling extrapolation of stiffness properties at high velocity regimes (>1ms^-1). A layered finite-element model satisfactorily predicted the penetration of micro-penetrators using characteristic fracture energies (∼10pJμm^-2) significantly lower than previously reported (≫100pJμm^-2). Interestingly, with our standard application conditions (∼2ms^-1, 35gpistonmass), ∼95% of the application kinetic energy was transferred to the backing support rather than the skin ∼5% (murine ear model). At higher velocities (∼10ms^-1) strain energy accumulated in the top skin layers, initiating fracture before stress waves transmitted deformation to the backing material, increasing energy transfer efficiency to 55%. Thus, the tools developed provide guidelines to rationally engineer skin penetrators to increase depth targeting consistency and payload delivery across patients whilst minimizing penetration energy to control skin inflammation, tolerability and acceptability.

STATEMENT OF SIGNIFICANCE

The mechanics of skin penetration by dynamically-applied microscopic tips is investigated using a combined experimental-computational approach. A FE model of skin is parameterized using indentation tests and a ductile-failure implementation validated against penetration assays. The simulations shed light on skin elastic and fracture properties, and elucidate the interaction with microprojection arrays for vaccine delivery allowing rational design of next-generation devices.

Microneedle-based minimally-invasive measurement of puncture resistance and fracture toughness of sclera

The sclera provides the structural support of the eye and protects the intraocular contents. Since it covers a large portion of the eye surface and has relatively high permeability for most drugs, the sclera has been used as a major pathway for drug administration. Recently, microneedle (MN) technology has shown the possibility of highly local and minimally-invasive drug delivery to the eye by MN insertion through the sclera or the suprachoroidal space. Although ocular MN needs to be inserted through the sclera, there has been no systematic study to understand the mechanical properties of the sclera, which are important to design ocular MNs. In this study, we investigated a MN-based method to measure the puncture resistance and fracture toughness of the sclera. To reflect the conditions of MN insertion into the sclera, force-displacement curves obtained from MN-insertion tests were used to estimate the puncture resistance and fracture toughness of sclera tissue. To understand the effect of the insertion conditions, dependency of the mechanical properties on insertion speeds, pre-strain of the sclera, and MN sizes were analyzed and discussed.

STATEMENT OF SIGNIFICANCE

Measurement of mechanical property of soft biological tissue is challenging due to variations between tissue samples or lack of well-defined measurement techniques. Although non-invasive measurement techniques such as nano/micro indentation were employed to locally measure the elastic modulus of soft biological materials, mechanical properties such as puncture resistance or fracture toughness, which requires "invasive" measurement and is important for the application of "microneedles or hypodermic needles", has not been well studied. In this work, we report minimally-invasive measurement of puncture resistance and fracture toughness of sclera using a double MN insertion method. Parametric studies showed that use of MN proved to be advantageous because of minimally-invasive insertion into tissue as well as higher sensitivity to sub-tissue architecture during the measurement.

The influence of composition and location on the toughness of human atherosclerotic femoral plaque tissue

The toughness of femoral atherosclerotic tissue is of pivotal importance to understanding the mechanism of luminal expansion during cutting balloon angioplasty (CBA) in the peripheral vessels. Furthermore, the ability to relate this parameter to plaque composition, pathological inclusions and location within the femoral vessels would allow for the improvement of existing CBA technology and for the stratification of patient treatment based on the predicted fracture response of the plaque tissue to CBA. Such information may lead to a reduction in clinically observed complications, an improvement in trial results and an increased adoption of the CBA technique to reduce vessel trauma and further endovascular treatment uptake. This study characterises the toughness of atherosclerotic plaque extracted from the femoral arteries of ten patients using a lubricated guillotine cutting test to determine the critical energy release rate. This information is related to the location that the plaque section was removed from within the femoral vessels and the composition of the plaque tissue, determined using Fourier Transform InfraRed spectroscopy, to establish the influence of location and composition on the toughness of the plaque tissue. Scanning electron microscopy (SEM) is employed to examine the fracture surfaces of the sections to determine the contribution of tissue morphology to toughness. Toughness results exhibit large inter and intra patient and location variance with values ranging far above and below the toughness of healthy porcine arterial tissue (Range: 1330-3035 for location and 140-4560J/m(2) for patients). No significant difference in mean toughness is observed between patients or location. However, the composition parameter representing the calcified tissue content of the plaque correlates significantly with sample toughness (r=0.949, p<0.001). SEM reveals the presence of large calcified regions in the toughest sections that are absent from the least tough sections. Regression analysis highlights the potential of employing the calcified tissue content of the plaque as a preoperative tool for predicting the fracture response of a target lesion to CBA (R(2)=0.885, p<0.001).

STATEMENT OF SIGNIFICANCE

This study addresses a gap in current knowledge regarding the influence of plaque location, composition and morphology on the toughness of human femoral plaque tissue. Such information is of great importance to the continued improvement of endovascular treatments, particularly cutting balloon angioplasty (CBA), which require experimentally derived data as a framework for assessing clinical cases and advancing medical devices. This study identifies that femoral plaque tissue exhibits large inter and intra patient and location variance regarding tissue toughness. Increasing calcified plaque content is demonstrated to correlate significantly with increasing toughness. This highlights the potential for predicting target lesion toughness which may lead to an increased adoption of the CBA technique and also further the uptake of endovascular treatment.

Fracture Toughness of Vocal Fold Tissue: A Preliminary Study

A customized mechanical tester that slices thin, soft samples was used to measure the fracture toughness of vocal fold tissue. Porcine vocal fold lamina propria was subjected to quasi-static, guillotine-like tests at three equally distanced regions along the anterior-posterior direction. The central one-third where high-velocity collisions between vocal folds occur was found to have the maximum fracture toughness. In contrast, the anterior one-third featured a lower toughness. Fracture toughness can be indicative of the risk of benign and malignant lesions in vocal fold tissue.

Mechanics of fragmentation of crocodile skin and other thin films

Fragmentation of thin layers of materials is mediated by a network of cracks on its surface. It is commonly seen in dehydrated paintings or asphalt pavements and even in graphene or other two-dimensional materials, but is also observed in the characteristic polygonal pattern on a crocodile's head. Here, we build a simple mechanical model of a thin film and investigate the generation and development of fragmentation patterns as the material is exposed to various modes of deformation. We find that the characteristic size of fragmentation, defined by the mean diameter of polygons, is strictly governed by mechanical properties of the film material. Our result demonstrates that skin fragmentation on the head of crocodiles is dominated by that it features a small ratio between the fracture energy and Young's modulus, and the patterns agree well with experimental observations. Understanding this mechanics-driven process could be applied to improve the lifetime and reliability of thin film coatings by mimicking crocodile skin.

Characterization of aortic tissue cutting process: experimental investigation using porcine ascending aorta

Understanding biomechanical responses during soft tissue cutting is important for developing surgical simulators and robot-assisted surgery with haptic feedback. The biomechanics involved in the aortic tissue cutting process is largely unknown. In this study, porcine ascending aorta was selected as a representative aortic tissue, and tissue cutting experiments were performed using a novel tissue cutting apparatus. The tissue cutting responses under various cutting conditions were investigated, including differing initial tissue lateral holding force and distance, cutting speed, cutter inclination angle, tissue anatomical orientation and thickness. The results from this study suggest that a "break-in" cutting force of about 4-12 N, a cutter "break-in" distance of 5-15 mm, and a continuous cutting force of 2-4 N were needed to cut through the porcine ascending aorta tissue. For all testing conditions investigated in this study, the cutting force vs. the cutter displacement curves exhibited similar characteristics. More importantly, this study demonstrated that tissue cutting involving one or more of the following conditions: a larger lateral holding force, a smaller lateral hold distance, a higher cutting speed or a larger inclination angle, could result in a smaller "break in" cutting force and a smaller "break-in" distance. In addition, it was found that the cutting force in the vessel longitudinal direction was larger than that in the circumferential direction. There was a strong correlation between the tissue thickness and the cutting force. The experimental results reported in this study could provide a basis for understanding the characteristic response of aortic tissue to scalpel cutting, and offer insight into the development of surgical simulators.

Characterization of fracture toughness exhaustion in pig aorta

BACKGROUND

Spontaneous rupture of the aorta (SRA) without aneurysm, dissection, inflammation or infection of the aortic wall can be of two types: traumatic and non-traumatic. SRA is most of the time a fatal event. Consequently, it is important to understand the conditions which lead to the aortic rupture, and, in the case of non-traumatic SRA, to predict the temporal likelihood of rupture.

METHOD OF APPROACH

The present work incorporates the temporal aspect by examining the effects of fatigue on aortic wall properties, and adopts an energy approach, based on fracture toughness, to evaluate the aorta's resistance to rupture. Fracture toughness characterization is a destructive testing process and as a consequence cannot be implemented as a clinical tool. However, using concepts in damage mechanics, in theory, it should be possible to indirectly assess fracture toughness from other mechanical properties, such as aortic wall stiffness. Tissue samples from non-aneurysmal porcine aortas were fatigued and were subjected to both biaxial and guillotine tests to assess wall stiffness variations and fracture toughness exhaustion, respectively.

RESULTS

The experiments reveal that aortic wall stiffness variations and fracture toughness exhaustion decreased as a function of loading cycles and can be modeled with exponential functions. After one million loading cycles, the stiffness ratio between the non-fatigued sample and the fatigued sample, dropped to about 0.85, while the fracture toughness ratio counterpart fell to about 0.80.

CONCLUSION

Consequently, the changes in both stiffness and fracture toughness as a function of applied fatigue cycles can be measured in aortic tissues. Moreover, these results suggest the possibility to use fracture toughness exhaustion curves as a fatigue criterion.

Towards a soft-tissue cutting simulator using the cohesive zone approach

Advancements in computational techniques have provided the ability to utilize detailed models in surgical training systems. The development of more complicated procedures coupled with reduced training time for medical residents has driven the need for accurate reality-based medical simulators. Extensive research has been conducted in the area of modeling general deformation of biological tissue; however few studies have focused on the physical properties of specific tool tissue interactions such as cutting. This paper presents a fracture mechanics based method to model the scalpel cutting of porcine liver by implementing a cohesive zone approach. Using in vivo cutting data, the parameters of the cohesive zone model are defined for the scalpel cutting process of soft biological tissues.

Estimation of fracture toughness of liver tissue: experiments and validation

The mechanical interaction between the surgical tools and the target soft tissue is mainly dictated by the fracture toughness of the tissue in several medical procedures, such as catheter insertion, robotic-guided needle placement, suturing, cutting or tearing, and biopsy. Despite the numerous experimental works on the fracture toughness of hard biomaterials, such as bone and dentin, only a very limited number of studies have focused on soft tissues, where the results do not show any consistency mainly due to the negligence of the puncturing/cutting tool geometry. In order to address this issue, we performed needle insertion experiments on 3 bovine livers with 4 custom-made needles having different diameters. A unique value for fracture toughness (J=164±6 J/m(2)) was obtained for the bovine liver by fitting a line to the toughness values estimated from the set of insertion experiments. In order to validate the experimental results, a finite element model of the bovine liver was developed and its hyper-viscoelastic material properties were estimated through an inverse solution based on static indentation and ramp-and-hold experiments. Then, needle insertion into the model was simulated utilizing an energy-based fracture mechanics approach. The insertion forces estimated from the FE simulations show an excellent agreement with those acquired from the physical experiments for all needle geometries.

Infusion pressure and pain during microneedle injection into skin of human subjects

Infusion into skin using hollow microneedles offers an attractive alternative to hypodermic needle injections. However, the fluid mechanics and pain associated with injection into skin using a microneedle have not been studied in detail before. Here, we report on the effect of microneedle insertion depth into skin, partial needle retraction, fluid infusion flow rate and the co-administration of hyaluronidase on infusion pressure during microneedle-based saline infusion, as well as on associated pain in human subjects. Infusion of up to a few hundred microliters of fluid required pressures of a few hundred mmHg, caused little to no pain, and showed weak dependence on infusion parameters. Infusion of larger volumes up to 1 mL required pressures up to a few thousand mmHg, but still usually caused little pain. In general, injection of larger volumes of fluid required larger pressures and application of larger pressures caused more pain, although other experimental parameters also played a significant role. Among the intradermal microneedle groups, microneedle length had little effect; microneedle retraction lowered infusion pressure but increased pain; lower flow rate reduced infusion pressure and kept pain low; and use of hyaluronidase also lowered infusion pressure and kept pain low. We conclude that microneedles offer a simple method to infuse fluid into the skin that can be carried out with little to no pain.

The effects of trapping and blade angle of notched dentitions on fracture of biological tissues

The material properties of food can exert a significant influence on tooth morphology. Although the stiffness or toughness of a material is usually of prime concern, other aspects of material properties (such as extensibility) can be of equal importance. Previous experimental work on the effect blade shape has on fracturing biological materials indicated a notched blade greatly reduced the work required to cut tough tissue. As a notched blade both traps materials and cuts at an angle, it is not clear which of these features leads to increased cutting efficiency. This paper tests whether the ability to cut at an angle or trap the material has the greater effect on the work to fracture required to cut tough tissues with different levels of extensibility (asparagus and fish muscle). Results show that the work to fracture required to cut more extensible materials is reduced by up to 50% when a trapping mechanism alone is used in comparison with an angled blade alone. For less extensible materials, the trapping ability of a notch seems to have no effect, whereas the angled blade reduces work to fracture by up to 25% relative to a straight blade. The aspects of blade shape most important to the breaking down of foods depend upon the relative stiffness or toughness, as well as other material properties.

Functional consequences of tooth design: effects of blade shape on energetics of cutting

Dental structures capture, retain and fragment food for ingestion. Gnathostome dentition should be viewed in the context of the prey's material properties. Animal muscle and skin are mechanically tough materials that resist fragmentation unless energy is continually supplied directly to the tip of the fracture by some device such as a blade edge. Despite the variety of bladed tooth morphologies in gnathostomes, few studies have experimentally examined the effects of different blade designs on cutting efficiency. We tested the effects of blades with and without contained notches and in a 'fang' configuration on the force and energy required to fracture raw, unprocessed biological tissues (fish and shrimp) using a double guillotine device. Blade design strongly affects the work required to fragment biological tissues. A notched blade reduced the work to fracture of tissues tested by up to 600 J m(-2) (50% reduction). The specific angle of the notch had a significant effect, with acute angles more effectively reducing work to fracture. A bladed triangle matched to a notch reduced work to fracture more than a notch-straight blade pair. Strain patterns seen while cutting photoelastic gelatin indicate that the reduction in work to fracture with triangular and notched blades arises from a combination of 'trapping ability' and blade approach angle causing the material to fracture at lower overall strain levels. These results show that the notched blade designs found in a wide variety of vertebrate dentitions reduce the energy expenditure (and presumably handling time) when cutting tough prey materials like animal flesh.

The effect of humidity on the fracture properties of human fingernails

Fingernails are a characteristic anatomical feature of primates and their function is dictated by the environment in which they are utilised. The present study examined the mechanical properties of human fingernails as a function of relative humidity (RH) and the subsequent moisture content of the nail material. Nail clippings were stored at a range of RH values and then weighed in order to determine their moisture content. There was a non-linear relationship between the moisture content of nails and the RH of their local environment. The in vivo moisture content of nails, measured from 55%to 80% RH, was between 14% and 30%, similar to other keratinous materials such as claws, hooves and feathers. Cutting tests on the nail samples showed that the work of fracture was between 11 and 22 kJ m–2, rising to a peak at 55% RH and falling at higher and lower humidities. At all RH values there was anisotropy within the nail between the proximal and lateral directions, the work of fracture being greater proximally. This anisotropy was greatest at 55% RH, at which point the proximal work of fracture was double the lateral value. These results suggest that the mechanical behaviour of human fingernails is optimised at in vivo conditions; they resist tearing most strongly under these conditions and particularly resist tearing into the nail bed. At more extreme humidity levels the fracture properties of the nail deteriorate; they are brittle when fully dry and fracture and split when wet.

Modeling the forces of cutting with scissors

Modeling forces applied to scissors during cutting of biological materials is useful for surgical simulation. Previous approaches to haptic display of scissor cutting are based on recording and replaying measured data. This paper presents an analytical model based on the concepts of contact mechanics and fracture mechanics to calculate forces applied to scissors during cutting of a slab of material. The model considers the process of cutting as a sequence of deformation and fracture phases. During deformation phases, forces applied to the scissors are calculated from a torque-angle response model synthesized from measurement data multiplied by a ratio that depends on the position of the cutting crack edge and the curve of the blades. Using the principle of conservation of energy, the forces of fracture are related to the fracture toughness of the material and the geometry of the blades of the scissors. The forces applied to scissors generally include high-frequency fluctuations. We show that the analytical model accurately predicts the average applied force. The cutting model is computationally efficient, so it can be used for real-time computations such as haptic rendering. Experimental results from cutting samples of paper, plastic, cloth, and chicken skin confirm the model, and the model is rendered in a haptic virtual environment.

Alterations in the mechanical properties and composition of skin in human growth hormone transgenic mice

Growth hormone is known to stimulate connective tissue, but the degree to which it influences skin biomechanical properties is unclear. This study tested the hypothesis that human growth hormone transgene expression changes the material properties and structural composition of adult mouse skin. Fracture toughness and elastic modulus were measured on freshly dissected dorsal skin and fixed samples were analyzed histologically. Transgenics had higher elastic moduli than their sex-matched non transgenic littermates, and male transgenics demonstrated increased fracture toughness. Male transgenics also had thicker skin than controls with a selectively increased dermis. In contrast, female transgenics had thinner skin than controls due to a reduced hypodermis. Biomechanical and histological variables were strongly correlated. Significant sex differences were present in nearly all comparisons indicating a dimorphic response to growth hormone in the skin. These data demonstrate that constant low-level growth hormone expression in marrow differentially affects skin layer thickness and concomitantly alters its biomechanical properties.

The penetration of a soft solid by a liquid jet, with application to the administration of a needle-free injection

Liquid jet injections have been performed on human skin in vivo and silicone rubber using Intraject needle-free injectors. The discharge characteristics of the liquid jet were measured using a custom-built test instrument. The experiments reveal that a high-speed liquid jet penetrates a soft solid by the formation and opening of a planar crack. The fluid stagnation pressure required for skin penetration decreases with increasing diameter of the liquid jet. These findings are consistent with the slow-speed penetration of a soft solid by a sharp-tipped punch. It is demonstrated that the Shergold-Fleck sharp-tipped punch penetration model [Shergold, O.A., Fleck, N.A., 2004. Mechanisms of deep penetration of soft solids. Proc. Roy. Soc. Lond. A 460, 3037-3058.] gives adequate predictions for the pressure required to penetrate a soft solid by a high-speed liquid jet.

Jet-induced skin puncture and its impact on needle-free jet injections: experimental studies and a predictive model

Needle-free jet injections constitute an important method of drug delivery, especially for insulin and vaccines. This report addresses the mechanisms of interactions of liquid jets with skin. Liquid jets first puncture the skin to form a hole through which the fluid is deposited into skin. Experimental studies showed that the depth of the hole significantly affects drug delivery by jet injections. At a constant jet exit velocity and nozzle diameter, the hole depth increased with increasing jet volume up to an asymptotic value and decreased with increasing values of skin's uniaxial Young's modulus. A theoretical model was developed to predict the hole depth as a function of jet and skin properties. A simplified model was first verified with polyacrylamide gels, a soft material in which the fluid mechanics during hole formation is well understood. Prediction of the hole depth in the skin is a first step in quantitatively predicting drug delivery by jet injection.

Experimental Investigation Into the Deep Penetration of Soft Solids by Sharp and Blunt Punches, With Application to the Piercing of Skin

An experimental study has been conducted on the penetration of silicone rubbers and human skin in vivo by sharp-tipped and flat-bottomed cylindrical punches. A penetrometer was developed to measure the penetration of human skin in vivo, while a conventional screw-driven testing machine was used to penetrate the silicone rubbers. The experiments reveal that the penetration mechanism of a soft solid depends upon the punch tip geometry: a sharp tipped punch penetrates by the formation and wedging open of a mode I planar crack, while a flat-bottomed punch penetrates by the growth of a mode II ring crack. The planar crack advances with the punch, and friction along the flanks of the punch leads to a rising load versus displacement response. In contrast, the flat-bottomed punch penetrates by jerky crack advance and the load on the punch is unsteady. The average penetration pressure on the shank cross section of a flat-bottomed punch exceeds that for a sharp-tipped punch of the same diameter. In addition, the penetration pressure decreases as the diameter of the sharp-tipped punch increases. These findings are in broad agreement with the predictions of Shergold and Fleck [Proc. R. Soc. London, Ser. A (in press)] who proposed models for the penetration of a soft solid by a sharp-tipped and flat-bottomed punch.

A Novel Application of the Principles of Linear Elastic Fracture Mechanics (LEFM) to the Fatigue Behavior of Tendon Tissue

Background: Experiments on the fatigue of tendons have shown that cyclic loading induces failure at stresses lower than the ultimate tensile strength (UTS) of the tendons. The number of cycles to failure Nf has been shown to be dependent upon the magnitude of the applied cyclic stress. Method of approach: Utilizing data collected by Schechtman (1995), we demonstrate that the principles of Linear Elastic Fracture Mechanics (LEFM) can be used to predict the fatigue behavior of tendons under cyclic loading for maximum stress levels that are higher than 10% of the ultimate tensile strength (UTS) of the tendon (the experimental results at 10% UTS did not fit with our equations). Conclusions: LEFM and other FM approaches may prove to be very valuable in advancing our understanding of damage accumulation in soft connective tissues.