Experimental study on tissue phantoms to understand the effect of injury and suturing on human skin mechanical properties

Biomechanical Response of Head Surrogate With and Without the Helmet

Measurements of brain deformations under injurious loading scenarios are actively sought. In this work, we report experimentally measured head kinematics and corresponding dynamic, two-dimensional brain simulant deformations in head surrogates under a blunt impact, with and without a helmet. Head surrogates used in this work consisted of skin, skull, dura, falx, tentorium, and brain stimulants. The head surrogate geometry was based on the global human body models consortium's head model. A base head surrogate consisting of skin-skull-brain was considered. In addition, the response of two other head surrogates, skin-skull-dura-brain, and skin-skull-dura-brain-falx-tentorium, was investigated. Head surrogate response was studied for sagittal and coronal plane rotations for impactor velocities of 1 and 3 m/s. Response of head surrogates was compared against strain measurements in PMHS. The strain pattern in the brain simulant was heterogenous, and peak strains were established within ∼30 ms. The choice of head surrogate affect the spatiotemporal evolution of strain. For no helmet case, peak MPS of ∼50-60% and peak MSS of ∼35-50% were seen in brain simulant corresponding to peak rotational accelerations of ∼5000-7000 rad/s2. Peak head kinematics and peak MPS have been reduced by up to 75% and 45%, respectively, with the conventional helmet and by up to 90% and 85%, respectively, with the helmet with antirotational pads. Overall, these results provide important, new data on brain simulant strains under a variety of loading scenarios-with and without the helmets.

Evaluation of microMend wound closure device in repairing skin lacerations

BACKGROUND

microMend, a novel microstaple skin closure device, may be able to close simple lacerations. This study aimed to evaluate the feasibility and acceptability of using microMend to close these wounds in the ED.

METHODS

This was an open-label, single-arm clinical study conducted at two EDs within a large urban academic medical centre. Wounds closed with microMend underwent assessments performed at days 0, 7, 30 and 90. Photographs of treated wounds were rated by two plastic surgeons using a 100 mm visual analogue scale (VAS) and a wound evaluation scale (WES), which has a best possible score of 6. Participants rated pain during application and both participants and providers rated their satisfaction with the device.

RESULTS

Thirty-one participants were enrolled in the study: 48% were female and the mean age of participants was 45.6 (95% CI 39.1 to 52.1). The mean wound length was 2.35 cm (95% CI 1.77 to 2.92), with a range of 1-10 cm. Mean VAS and WES scores at day 90 as evaluated by two plastic surgeons were 84.1 mm (95% CI 80.2 to 87.9) and 4.91 (95% CI 4.54 to 5.29), respectively. The mean pain score with application of the devices was 7.28 mm (95% CI 2.88 to 11.68) on a scale of 0-100 mm using VAS. Local anaesthesia was used in 9 patients (29%, 95% CI 20.7 to 37.3) of participants (of whom 5 required deep sutures). Ninety per cent (90%) of participants rated their overall assessment of the device as excellent (74%) or good (16%) at day 90. There were no serious adverse events in any participants in the study.

CONCLUSION

microMend appears to be an acceptable alternative for closing skin lacerations in the ED, providing good cosmetic results, with high levels of satisfaction by patients and providers. Randomised trials are needed to compare microMend with other wound closure products.

TRIAL REGISTRATION NUMBER

NCT03830515.

Mechanical properties of whole-body soft human tissues: a review

The mechanical properties of soft tissues play a key role in studying human injuries and their mitigation strategies. While such properties are indispensable for computational modelling of biological systems, they serve as important references in loading and failure experiments, and also for the development of tissue simulants. To date, experimental studies have measured the mechanical properties of peripheral tissues (e.g. skin)in-vivoand limited internal tissuesex-vivoin cadavers (e.g. brain and the heart). The lack of knowledge on a majority of human tissues inhibit their study for applications ranging from surgical planning, ballistic testing, implantable medical device development, and the assessment of traumatic injuries. The purpose of this work is to overcome such challenges through an extensive review of the literature reporting the mechanical properties of whole-body soft tissues from head to toe. Specifically, the available linear mechanical properties of all human tissues were compiled. Non-linear biomechanical models were also introduced, and the soft human tissues characterized using such models were summarized. The literature gaps identified from this work will help future biomechanical studies on soft human tissue characterization and the development of accurate medical models for the study and mitigation of injuries.

Measurement of brain simulant strains in head surrogate under impact loading

Impact-induced traumatic brain injury (TBI) is a major source of disability and mortality. Knowledge of brain strains during impact (accelerative) loading is critical for the overall management of TBI, including the development of injury thresholds, personal protective equipment, and validation of computational models. Despite these needs, the current understanding of brain strains in humans or humanlike surrogates is limited, especially for injury causing loading magnitudes. Toward this end, we measured full-field, in-plane (2D) strains in a brain simulant using the hemispherical head surrogate. The hemispherical head was mounted on the Hybrid-III neck and subjected to impact loading using a linear impactor system. The resulting head kinematics was measured using a triaxial accelerometer and angular rate sensors. Dynamic, 2D strains in a brain simulant were obtained using high-speed imaging and digital image correlation. Concurrent finite element (FE) simulations of the experiment were also performed to gain additional insights. The role of stiff membranes of the head was also studied using experiments. Our results suggest that rotational modes dominate the response of the brain simulant. The wave propagation in the brain simulant as a result of impact has a timescale of ~100 ms. We obtain peak strains of ~20%, ~40%, ~60% for peak rotational accelerations of ~838, ~5170, ~11,860 rad/s², respectively. Further, peak strains in cortical regions are higher than subcortical regions by up to ~70%. The agreement between the experiments and FE simulations is reasonable in terms of spatiotemporal evolution of strain pattern and peak strain magnitudes. Experiments with the addition of falx and tentorium indicate significant strain concentration (up to 115%) in the brain simulant near the interface of falx or tentorium and brain simulant. Overall, this work provides important insights into the biomechanics of strain in the brain simulant during impact loading.

Human Adipose-Derived Mesenchymal Stem Cells-Derived Exosomal microRNA-19b Promotes the Healing of Skin Wounds Through Modulation of the CCL1/TGF-β Signaling Axis

Introduction

Human adipose-derived mesenchymal stem cells (ADMSCs) with their secretory factors are able to induce collagen synthesis and fibroblast migration in the wound healing process. This study is launched to figure out the effect of human ADMSCs-derived exosomes on skin wound healing.

Methods

ADMSCs were extracted and ADMSCs-derived exosomes were identified. Skin damage models were established by treating HaCaT cells and human skin fibroblasts with H₂O₂. Next, the roles of ADMSCs and their derived exosomes were investigated. The exosomal miRNA then was analyzed, and the function of miRNA on the H₂O₂-induced cells was studied by miRNA suppression. Bioinformatics analysis, luciferase activity and RIP assays were implemented to find the target genes ofthe miRNA and the modulated pathways. A mouse skin damage model was induced to elucidate the effects of exosomes in vivo by injecting exosomes.

Results

H₂O₂ treatment significantly reduced the viability of HaCaT cells and increased their apoptosis rate. Co-culture with ADMSCs or their derived exosomes could improve the cell damage caused by H₂O₂. Meanwhile, H₂O₂ treatment promoted the internalization of exosomes. ADMSCs and their derived exosomes significantly increased miR-19b expression in the recipient cells, while inhibiting miR-19b resulted in a reduction in the therapeutic effect of ADMSCs-derived exosomes. Besides, miR-19b regulated the TGF-β pathway by targeting CCL1. The therapeutic effect of exosomes was further confirmed by a mouse model of skin damage.

Conclusion

Our study indicates that exosomal miR-19b derived from ADMSCs regulates the TGF-β pathway by targeting CCL1, thereby promoting the healing of skin wounds.

Mechanical Modeling of Healthy and Diseased Calcaneal Fat Pad Surrogates

The calcaneal fat pad is a major load bearing component of the human foot due to daily gait activities such as standing, walking, and running. Heel and arch pain pathologies such as plantar fasciitis, which over one third of the world population suffers from, is a consequent effect of calcaneal fat pad damage. Also, fat pad stiffening and ulceration has been observed due to diabetes mellitus. To date, the biomechanics of fat pad damage is poorly understood due to the unavailability of live human models (because of ethical and biosafety issues) or biofidelic surrogates for testing. This also precludes the study of the effectiveness of preventive custom orthotics for foot pain pathologies caused due to fat pad damage. The current work addresses this key gap in the literature with the development of novel biofidelic surrogates， which simulate the in vivo and in vitro compressive mechanical properties of a healthy calcaneal fat pad. Also, surrogates were developed to simulate the in vivo mechanical behavior of the fat pad due to plantar fasciitis and diabetes. A four-part elastomeric material system was used to fabricate the surrogates, and their mechanical properties were characterized using dynamic and cyclic load testing. Different strain (or displacement) rates were tested to understand surrogate behavior due to high impact loads. These surrogates can be integrated with a prosthetic foot model and mechanically tested to characterize the shock absorption in different simulated gait activities, and due to varying fat pad material property in foot pain pathologies (i.e., plantar fasciitis, diabetes, and injury). Additionally, such a foot surrogate model, fitted with a custom orthotic and footwear, can be used for the experimental testing of shock absorption characteristics of preventive orthoses.

Biomechanical Modeling of Wounded Skin

Skin injury is the most common type of injury, which manifests itself in the form of wounds and cuts. A minor wound repairs itself within a short span of time. However, deep wounds require adequate care and sometime clinical interventions such as surgical suturing for their timely closure and healing. In literature, mechanical properties of skin and other tissues are well known. However, the anisotropic behavior of wounded skin has not been studied yet, specifically with respect to localized overstraining and possibilities of rupture. In the current work, the biomechanics of common skin wound geometries were studied with a biofidelic skin phantom, using uniaxial mechanical testing and Digital Image Correlation (DIC). Global and local mechanical properties were investigated, and possibilities of rupture due to localized overstraining were studied across different wound geometries and locations. Based on the experiments, a finite element (FE) model was developed for a common elliptical skin wound geometry. The fidelity of this FE model was evaluated with simulation of uniaxial tension tests. The induced strain distributions and stress-stretch responses of the FE model correlated very well with the experiments (R2 > 0.95). This model would be useful for prediction of the mechanical response of common wound geometries, especially with respect to their chances of rupture due to localized overstraining. This knowledge would be indispensable for pre-surgical planning, and also in robotic surgeries, for selection of appropriate wound closure techniques, which do not overstrain the skin tissue or initiate tearing.

Novel insole design for diabetic foot ulcer management

Around the world, over 400 million people suffer from diabetes. In a chronic diabetic condition, the skin underneath the foot often becomes extremely soft and brittle, resulting in the development of foot ulcers. In literature, a plethora of footwear designs have been developed to reduce the induced stresses on a diabetic foot and to consequently prevent the incidences of foot ulcers. However, to date, no insole design exists which can handle post-ulcer diabetic foot conditions without hindering the mobility of the patients. In the current work, a novel custom insole design with arch support and ulcer isolations was tested for effective stress reduction in a diabetic foot with ulcers using finite element modeling. A full-scale model of the foot was developed with ulcers of different geometries and sizes at the heel and metatarsal regions of the foot. The stresses at the ulcer locations were quantified for standing and walking with and without the novel custom insole model. The effect of material properties of the insole on the ulcer stress reduction was quantified extensively. Also, the effectivity of a novel synthetic skin material as the insole material was tested for stress offloading at the ulcers and the rest of the foot. From the analyses, peak stress reductions were observed at the ulcers up to 91.5% due to the ulcer isolation in the novel custom insole design and the skin-like material. Specifically, the ulcer isolation feature in the insole was found to be approximately 25% more effective in peak stress reduction for commonly occurring ulcers with irregular geometry, over the tested regular circular ulcer geometry. Also, a threshold material stiffness was found for the custom insole, below which the peak stresses at the ulcers did not decrease any further. Based on this information, a working prototype of the custom insole was developed with custom ulcer isolations, which will be subjected to further testing. The results of this study would inform better custom insole designing and material selection for post-ulcer diabetic conditions, with effective stress reduction at the ulcers, and the possibilities of preventing further ulceration.

Biomechanical Modeling of Prosthetic Mesh and Human Tissue Surrogate Interaction

Surgical repair of hernia and prolapse with prosthetic meshes are well-known to cause pain, infection, hernia recurrence, and mesh contraction and failures. In literature, mesh failure mechanics have been studied with uniaxial, biaxial, and cyclic load testing of dry and wet meshes. Also, extensive experimental studies have been conducted on surrogates, such as non-human primates and rodents, to understand the effect of mesh stiffness, pore size, and knitting patterns on mesh biocompatibility. However, the mechanical properties of such animal tissue surrogates are widely different from human tissues. Therefore, to date, mechanics of the interaction between mesh and human tissues is poorly understood. This work addresses this gap in literature by experimentally and computationally modeling the biomechanical behavior of mesh, sutured to human tissue phantom under tension. A commercially available mesh (Prolene®) was sutured to vaginal tissue phantom material and tested at different uniaxial strains and strain rates. Global and local stresses at the tissue phantom, suture, and mesh were analyzed. The results of this study provide important insights into the mechanics of prosthetic mesh failure and will be indispensable for better mesh design in the future.

Human Skin-Like Composite Materials for Blast Induced Injury Mitigation

Armors and military grade personal protection equipment (PPE) materials to date are bulky and are not designed to effectively mitigate blast impacts. In the current work, a human skin-like castable simulant material was developed and its blast mitigation characteristics (in terms of induced stress reduction at the bone and muscles) were characterized in the presence of composite reinforcements. The reinforcement employed was Kevlar 129 (commonly used in advanced combat helmets), which was embedded within the novel skin simulant material as the matrix and used to cover a representative extremity based human skin, muscle and bone section finite element (FE) model. The composite variations tested were continuous and short-fiber types, lay-ups (0/0, 90/0, and 45/45 orientations) and different fiber volume fractions. From the analyses, the 0/0 continuous fiber lay-up with a fiber volume fraction close to 0.1 (or 10%) was found to reduce the blast-induced dynamic stresses at the bone and muscle sections by 78% and 70% respectively. These findings indicate that this novel skin simulant material with Kevlar 129 reinforcement, with further experimental testing, may present future opportunities in blast resistant armor padding designing.

Biomechanical Modeling of Human Skin Tissue Surrogates

Surrogates, which precisely simulate nonlinear mechanical properties of the human skin at different body sites, would be indispensable for biomechanical testing applications, such as estimating the accurate load response of skin implants and prosthetics to study the biomechanics of static and dynamic loading conditions on the skin, dermatological and sports injuries, and estimating the dynamic load response of lethal and nonlethal ballistics. To date, human skin surrogates have been developed mainly with materials, such as gelatin and polydimethylsiloxane (PDMS), based on assumption of simplified mechanical properties, such as an average elastic modulus (estimated through indentation tests), and Poisson’s ratio. In addition, pigskin and cowhides, which have widely varying mechanical properties, have been used to simulate human skin. In the current work, a novel elastomer-based material system is developed, which precisely mimics the nonlinear stress–stretch behavior, elastic modulus at high and low strains, and fracture strengths of the natural human skin at different body sites. The manufacturing and fabrication process of these skin surrogates are discussed, and mechanical testing results are presented.

BIOMECHANICAL CHARACTERIZATION OF NORMAL AND PROLAPSED VAGINAL TISSUE SURROGATES

In the recent years, poorly evaluated gynecological surgeries and urogynecological mesh implantations have been affecting millions of women in the US and across the globe. These failed surgeries could be mainly attributed to the nonavailability of vaginal tissues (due to ethical and biosafety issues), which does not allow any experimental testing of operation and mesh implantation techniques before an actual surgery. A surrogate which behaves biomechanically like the human vaginal tissue would be indispensable for simulating surgical suture of vaginal tissues in prolapse surgery, hysterectomy or surgery during traumatic child births (such as Cesarean). Also, vaginal tissue surrogates simulating the various prolapse conditions (such as vaginal tissue stiffening) would be very useful to evaluate tissue modifications due to prolapse, and also mesh and vaginal tissue interactions. In the current work, a low cost four-part silicone-based material was developed, which precisely simulates the linear and nonlinear mechanical behavior of the normal human vaginal tissue. Additionally, a range of four-part silicone-based novel materials were developed which precisely mimics the mechanical behavior of stiffened vaginal tissues at different degrees of prolapse. The linear and nonlinear mechanical behavior of all such novel materials were characterized using elastic and hyperelastic formulations. Such precisely characterized normal and prolapsed vaginal tissue surrogates have not been developed anywhere to date as per the best of our knowledge and would be clinically helpful for gynecological surgical planning in the future.

Tissue Anisotropy Modeling Using Soft Composite Materials

Soft tissues in general exhibit anisotropic mechanical behavior, which varies in three dimensions based on the location of the tissue in the body. In the past, there have been few attempts to numerically model tissue anisotropy using composite-based formulations (involving fibers embedded within a matrix material). However, so far, tissue anisotropy has not been modeled experimentally. In the current work, novel elastomer-based soft composite materials were developed in the form of experimental test coupons, to model the macroscopic anisotropy in tissue mechanical properties. A soft elastomer matrix was fabricated, and fibers made of a stiffer elastomer material were embedded within the matrix material to generate the test coupons. The coupons were tested on a mechanical testing machine, and the resulting stress-versus-stretch responses were studied. The fiber volume fraction (FVF), fiber spacing, and orientations were varied to estimate the changes in the mechanical responses. The mechanical behavior of the soft composites was characterized using hyperelastic material models such as Mooney-Rivlin's, Humphrey's, and Veronda-Westmann's model and also compared with the anisotropic mechanical behavior of the human skin, pelvic tissues, and brain tissues. This work lays the foundation for the experimental modelling of tissue anisotropy, which combined with microscopic studies on tissues can lead to refinements in the simulation of localized fiber distribution and orientations, and enable the development of biofidelic anisotropic tissue phantom materials for various tissue engineering and testing applications.

Vaginal Changes Due to Varying Degrees of Rectocele Prolapse: A Computational Study

Pelvic organ prolapse (POP), downward descent of the pelvic organs resulting in a protrusion of the vagina, is a highly prevalent condition, responsible for 300,000 surgeries in the U.S. annually. Rectocele, a posterior vaginal wall (PVW) prolapse of the rectum, is the second most common type of POP after cystocele. A rectocele usually manifests itself along with other types of prolapse with multicompartment pelvic floor defects. To date, the specific mechanics of rectocele formation are poorly understood, which does not allow its early stage detection and progression prediction over time. Recently, with the advancement of imaging and computational modeling techniques, a plethora of finite element (FE) models have been developed to study vaginal prolapse from different perspectives and allow a better understanding of dynamic interactions of pelvic organs and their supporting structures. So far, most studies have focused on anterior vaginal prolapse (AVP) (or cystocele) and limited data exist on the role of pelvic muscles and ligaments on the development and progression of rectocele. In this work, a full-scale magnetic resonance imaging (MRI) based three-dimensional (3D) computational model of the female pelvic anatomy, comprising the vaginal canal, uterus, and rectum, was developed to study the effect of varying degrees (or sizes) of rectocele prolapse on the vaginal canal for the first time. Vaginal wall displacements and stresses generated due to the varying rectocele size and average abdominal pressures were estimated. Considering the direction pointing from anterior to posterior side of the pelvic system as the positive Y-direction, it was found that rectocele leads to negative Y-direction displacements, causing the vaginal cross section to shrink significantly at the lower half of the vaginal canal. Besides the negative Y displacements, the rectocele bulging was observed to push the PVW downward toward the vaginal hiatus, exhibiting the well-known "kneeling effect." Also, the stress field on the PVW was found to localize at the upper half of the vaginal canal and shift eventually to the lower half with increase in rectocele size. Additionally, clinical relevance and implications of the results were discussed.