Mechanical properties of the human scalp in tension

On a potential morpho-mechanical link between the gluteus maximus muscle and pelvic floor tissues

Stress urinary incontinence presents a condition not only found in female elderlies, but also in young athletes participating in high-impact sports such as volleyball or trampolining. Repeated jumps appear to be a predisposing factor. Yet the pathophysiology remains incompletely elucidated to date; especially with regard to the influence of the surrounding buttock tissues including gluteus maximus. The present study assessed the morpho-mechanical link between gluteus maximus and the pelvic floor female bodies. 25 pelves obtained from Thiel embalmed females were studied in a supine position. Strands of tissues connecting gluteus maximus with the pelvic floor obtained from 20 sides were assessed mechanically. Plastinates were evaluated to verify the dissection findings. In total, 49 hemipelves were included for data acquisition. The fascia of gluteus maximus yielded connections to the subcutaneous tissues, the fascia of the external anal sphincter and that of obturator internus and to the fascia of the urogenital diaphragm. The connection between gluteus maximus and the urogenital diaphragm withstood an average force of 23.6 ± 17.3 N. Cramér φ analyses demonstrated that the connections of the fasciae connecting gluteus maximus with its surroundings were consistent in the horizontal and sagittal planes, respectively. In conclusion, gluteus maximus is morphologically densely linked to the pelvic floor via strands of connective tissues investing the adjacent muscles. Though gluteus maximus has also been reported to facilitate urinary continence, the here presented morpho-mechanical link suggests that it may also have the potential to contribute to urinary stress incontinence. Future research combining clinical imaging with in-situ testing may help substantiate the potential influence from a clinical perspective.

Sample size considerations in soft tissue biomechanics

Biomechanical experiments help link tissue morphology with load-deformation characteristics. A tissue-dependent minimum sample number is indispensable to obtain accurate material properties. Stress-strain properties were retrieved from human dura mater and scalp skin, exemplifying two distinct soft tissues. Minimum sample sizes necessary for a stable estimation of material properties were obtained in a simulation study. One-thousand random samples were sequentially drawn for calculating the point at which a majority of the estimators settled within a corridor of stability at given tolerance levels around a 'complete' reference for the mean, median and coefficient of variation. Stable estimations of means and medians can be achieved below sample sizes of 30 at a ± 20%-tolerance within 80%-conformity for scalp skin and dura. Lower tolerance levels or higher conformity dramatically increase the required sample size. Conformity was barely ever reached for the coefficient of variation. The parameter type appears decisive for achieving conformity. STATEMENT OF SIGNIFICANCE: Biomechanical trials utilizing human tissues are needed to obtain material properties for surgical repair, tissue engineering and modeling purposes. Linking tissue mechanics with morphology helps elucidate form-function relationships, the 'morpho-mechanical link'. For material properties to be accurate, it is vital to examine a minimum number of samples. This number may vary between tissues, and the effects of intrinsic tissue characteristics on data accuracy are unclear to date. This study used data obtained from human dura and skin to compute minimum sample sizes required for estimating material properties at a stable level. It was shown that stable estimations are possible at a ± 20%-tolerance within 80%-conformity below sample sizes of 30. Higher accuracy warrants much higher sample sizes for most material properties.

Scalp and Calvarium Reconstruction

Defects of the scalp and calvarium pose unique reconstructive challenges due to the importance of this area in protecting the brain and its distance from larger donor vessels for free flap transfer. The wide range and complexity of reconstructive options make this a broad topic because the simplest defects are often closed or managed in the outpatient setting and the most complex require multilayer closure in the operating room with a multidisciplinary team and intensive postoperative care. In hair-bearing individuals, the scalp is an esthetically important area due to the importance of hair to self-esteem and sexual attraction.

Development and characterisation of hybrid composite skin simulants based on short polyethylene fibre and bioactive glass particle-reinforced silicone

Silicone elastomers are widely recognised as artificial skins for medical prosthesis and cranial injury assessment. Since silicone is not an ideal skin simulant due to the lack of mechanical stiffness and a fibrous structure, the present study aimed to tailor the mechanical and structural characteristics of silicone by integrating biocompatible reinforcements (namely, short polyethylene fibres and bioglass particles) to develop suitable bio-integrative skin simulant candidates. The influences of short polyethylene fibres and bioglass particles in the selected platinum silicone on the mechanical properties of silicone-based composite skin simulants were investigated with various factors, including filler concentration, KMnO4 surface treatment of the polyethylene fibre, and particle size. A comprehensive assessment of the tensile, compressive, and hardness properties of the examined composites was conducted, and they were compared with the properties of human biological skin. The results exhibited that the elastic moduli and the hardness of all composites increased with the concentration of both reinforcements. While integrating only the bioglass particles had the advantage of an insignificant effect on the hardness change of the silicone matrix, the composite with polyethylene fibres possessed superior tensile elastic modulus and tensile strength compared to those of the bioglass reinforced composite. The composites with 5% untreated polyethylene fibres, KMnO4 surface-treated fibres, and bioglass reinforcements enhanced the tensile elastic moduli from the pure silicone up to 32%, 44%, and 22%, respectively. It reflected that the surface treatment of the fibres promotes better interfacial adhesion between the silicone matrix and the fibres. Moreover, the smaller bioglass particle had a greater mechanical contribution than the larger glass particle. Systematically characterised for the first time, the developed composite skin simulants demonstrated essential mechanical properties within the range of the human skin and constituted better skin alternatives than pure silicone for various biomedical applications.

Review of Mechanisms and Research Methods for Blunt Ballistic Head Injury

Head injuries account for 15% to 20% of all military injuries and pose a high risk of causing functional disability and fatality. Blunt ballistic impacts are one of the threats that can lead to severe head injuries. This review aims to examine the mechanisms and injury risk assessment associated with blunt ballistic head injury (BBHI). The review further discusses research methods and instrumentation used in BBHI studies, focusing on their limitations and challenges. Studies on the mechanisms of focal and diffuse brain injuries remain largely inconclusive and require further efforts. Some studies have attempted to associate BBHIs with head mechanics, but more research is required to establish correlations between head mechanics and injury severity. Limited access to experimental models and a lack of instrumentation capable of measuring the mechanics of brain tissue in-situ are potential reasons for the lack of understanding of injury mechanisms, injury correlations and injury tolerance levels specific to this loading regime. Targeted research for understanding and assessing head injuries in blunt ballistic impacts is a necessary step in improving our ability to design protection systems to mitigate these injuries.

Experimental characterisation of porcine subcutaneous adipose tissue under blunt impact up to irreversible deformation

A deeper understanding of the mechanical characteristics of adipose tissue under large deformation is important for the analysis of blunt force trauma, as adipose tissue alters the stresses and strains that are transferred to subjacent tissues. Hence, results from drop tower tests of subcutaneous adipose tissue are presented (i) to characterise adipose tissue behaviour up to irreversible deformation, (ii) to relate this to the microstructural configuration, (iii) to quantify this deformation and (iv) to provide an analytical basis for computational modelling of adipose tissue under blunt impact. The drop tower experiments are performed exemplarily on porcine subcutaneous adipose tissue specimens for three different impact velocities and two impactor geometries. An approach based on photogrammetry is used to derive 3D representations of the deformation patterns directly after the impact. Median values for maximum impactor acceleration for tests with a flat cylindrical impactor geometry at impact velocities of 886 mm/s, 1253 mm/s and 2426 mm/s amount to 61.1 g, 121.6 g and 264.2 g, respectively, whereas thickness reduction of the specimens after impact amount to 16.7%, 30.5% and 39.3%, respectively. The according values for tests with a spherically shaped impactor at an impact velocity of 1253 mm/s are 184.2 g and 78.7%. Based on these results, it is hypothesised that, in the initial phase of a blunt impact, adipose tissue behaviour is mainly governed by the behaviour of the lipid inside the adipocytes, whereas for further loading, contribution of the extracellular collagen fibre network becomes more dominant.

On the correlations of biomechanical properties of super-imposed temporal tissue layers and their age-, sex-, side- and post-mortem interval dependence

Obtaining biomechanical properties of biological tissues for simulation purposes or graft developments is time and resource consuming. The number of samples required for biomechanical tests could be reduced if the load-deformation properties of a given tissue layer could be estimated from adjacent layers or if the biomechanical parameters were unaffected by age, bodyside, sex or post-mortem interval. This study investigates for the first time potential correlations of multiple super-imposed tissue layers using the temporal region of the human head as an area of broad interest in biomechanical modelling. Spearman correlations between biomechanical properties of the scalp, muscle fascia, muscle, bone and dura mater from up to 83 chemically unfixed cadavers were investigated. The association with age, sex and post-mortem interval was assessed. The results revealed sporadic correlations between the corresponding layers, such as the maximum force (r = 0.43) and ultimate tensile strength (r = 0.33) between scalp and muscle. Side- and age-dependence of the biomechanical properties were different between the tissue types. Strain at maximum force of fascia (r = -0.37) and elastic modulus of temporal muscle (r = 0.26) weakly correlated with post-mortem interval. Only strain at maximum force of scalp differed significantly between sexes. Uniaxial biomechanical properties of individual head tissue layers can thus not be estimated solely based on adjacent layers. Therefore, correlations between the tissues' biomechanical properties, anthropometric data and post-mortem interval need to be established independently for each layer. Sex seems not to be a relevant influencing factor for the passive tissue mechanics of the here investigated temporal head tissue layers.

Evaluation of the Kinematic Biofidelity and Inter-Test Repeatability of Global Accelerations and Brain Parenchyma Pressure for a Head-Brain Physical Model

Head surrogates are widely used in biomechanical research and headgear assessment. They are designed to approximate the inertial and mechanical properties of the head and are instrumented to measure global head kinematics. Due to the recent interest in studying disruption to the brain, some head models include internal fluid layers and brain tissue, and instrumentation to measure head intracranial biomechanics. However, it is unknown whether such models exhibit realistic human responses. Therefore, this study aims to assess the biofidelity and repeatability of a head model, the Blast Injury Protection Evaluation Device (BIPED), that can measure both global head kinematics and intraparenchymal pressure (IPP) for application in blunt impact, a common loading scenario in civilian life. Drop tests were conducted with the BIPED and the widely used Hybrid III headform. BIPED measures were compared to the Hybrid III data and published cadaveric data, and the biofidelity level of the global linear acceleration was quantified using CORrelation and Analysis (CORA) ratings. The repeatability of the acceleration and IPP measurements in multiple impact scenarios was evaluated via the coefficient of variation (COV) of the magnitudes and pulse durations. BIPED acceleration peaks were generally not significantly different from cadaver and Hybrid III data. The CORA ratings for the BIPED and Hybrid III accelerations ranged from 0.50 to 0.61 and 0.51 to 0.77, respectively. The COVs of acceleration and IPP were generally below 10%. This study is an important step toward a biofidelic head surrogate measuring both global kinematics and IPP in blunt impact.

Load-deformation characteristics of acellular human scalp: assessing tissue grafts from a material testing perspective

Acellular matrices seem promising scaffold materials for soft tissue regeneration. Biomechanical properties of such scaffolds were shown to be closely linked to tissue regeneration and cellular ingrowth. This given study investigated uniaxial load-deformation properties of 34 human acellular scalp samples and compared these to age-matched native tissues as well as acellular dura mater and acellular temporal muscle fascia. As previously observed for human acellular dura mater and temporal muscle fascia, elastic modulus (p = 0.13) and ultimate tensile strength (p = 0.80) of human scalp samples were unaffected by the cell removal. Acellular scalp samples showed a higher strain at maximum force compared to native counterparts (p = 0.02). The direct comparison of acellular scalp to acellular dura mater and temporal muscle fascia revealed a higher elasticity (p < 0.01) and strain at maximum force (p = 0.02), but similar ultimate tensile strength (p = 0.47). Elastic modulus and ultimate tensile strength of acellular scalp decreased with increasing post-mortem interval. The elongation behavior formed the main biomechanical difference between native and acellular human scalp samples with elastic modulus and ultimate tensile strength being similar when comparing the two.

What is Considered a Variation of Biomechanical Parameters in Tensile Tests of Collagen-Rich Human Soft Tissues? - Critical Considerations Using the Human Cranial Dura Mater as a Representative Morpho-Mechanic Model

Background and Objectives: Profound knowledge on the load-dependent behavior of human soft tissues is required for the development of suitable replacements as well as for realistic computer simulations. Regarding the former, e.g., the anisotropy of a particular biological tissue has to be represented with site- and direction-dependent particular mechanical values. Contrary to this concept of consistent mechanical properties of a defined soft tissue, mechanical parameters of soft tissues scatter considerably when being determined in tensile tests. In spite of numerous measures taken to standardize the mechanical testing of soft tissues, several setup- and tissue-related factors remain to influence the mechanical parameters of human soft tissues to a yet unknown extent. It is to date unclear if measurement extremes should be considered a variation or whether these data have to be deemed incorrect measurement outliers. This given study aimed to determine mechanical parameters of the human cranial dura mater as a model for human soft tissues using a highly standardized protocol and based on this, critically evaluate the definition for the term mechanical "variation" of human soft tissue. Materials and Methods: A total of 124 human dura mater samples with an age range of 3 weeks to 94 years were uniformly retrieved, osmotically adapted and mechanically tested using customized 3D-printed equipment in a quasi-static tensile testing setup. Scanning electron microscopy of 14 samples was conducted to relate the mechanical parameters to morphological features of the dura mater. Results: The here obtained mechanical parameters were scattered (elastic modulus = 46.06 MPa, interquartile range = 33.78 MPa; ultimate tensile strength = 5.56 MPa, interquartile range = 4.09 MPa; strain at maximum force = 16.58%, interquartile range = 4.81%). Scanning electron microscopy revealed a multi-layered nature of the dura mater with varying fiber directions between its outer and inner surface. Conclusions: It is concluded that mechanical parameters of soft tissues such as human dura mater are highly variable even if a highly standardized testing setup is involved. The tissue structure and composition appeared to be the main contributor to the scatter of the mechanical parameters. In consequence, mechanical variation of soft tissues can be defined as the extremes of a biomechanical parameter due to an uncontrollable change in tissue structure and/or the respective testing setup.

Standardized tensile testing of soft tissue using a 3D printed clamping system

Biomechanical testing of soft tissues forms the backbone in the experimental validation of tissue engineering and for modelling purposes. The standardized testing of soft tissues requires different experimental protocols and fixtures compared to hard tissues or non-biological materials due to their characteristics. Some of the most commonly-used clamping methods for soft tissue testing affect the tissues' mechanical properties as chemicals are involved to decelerate degradation and autolysis. Moreover, they are unsuitable for standardized and high-throughput testing. Material slippage is also a recurrent unwanted influence on the testing routine with impact on measurement validity. Addressing these issues, this protocol presents a clamping system for simplified testing of biological soft tissues with all necessary components manufactured utilizing 3D printing technology. Templates allow trimming the samples into standardized shapes and sizes while preparation tables facilitate clamping in a fixed distance. The key parts of the system are clamps with a pyramid design, which allow the mounting of biological soft tissues before transferring it into the testing device and minimize material slippage during tensile testing. Flexible holder arms are used to transfer samples from preparation tables into the testing device and simplify positioning. Mechanical testing itself is performed with digital image correlation for precise strain measurements.

Maximum tensile stress and strain of skin of the domestic pig-differences concerning pigs from organic and non-organic farming

The purpose of this work has been to determine differences in biomechanical properties of porcine skin from organic and non-organic farming as porcine skin is widely used as a model for human skin. A test apparatus was used, using gravity to stretch and finally tear a dumbbell-shaped specimen of prepared abdominal skin with a testing surface area of 25 × 4 mm. A total of 32 specimens were taken from seven individual pigs, three from organic and four from non-organic farming, in different orientations with respect to the Langer's lines. The tests were performed at a dynamic speed of around 1.66 m/s (corresponding to a nominal strain rate of 67 s^-1). Engineering strain at rupture was higher in pig skin from non-organic farming with values up to 321% as opposed to 90% in organic pig skin. The maximum tensile stress found in non-organic pig skin was lower than in pig skin from organic farming with maximum values of 34 MPa as opposed to 58 MPa. The reason for the difference in biomechanical properties is unclear; the effect of sunlight is discussed as well as other factors like age and exercise. It seems that the biomechanical properties of porcine skin from organic farming are more similar to those of human skin.

In vivo stress relaxation of human scalp

OBJECTIVE

Conduct a first in vivo study on the large deformation stress relaxation behavior of the human scalp.

METHODS

This study was conducted during Mohs micrographic surgery of the scalp of 14 patients aged 59-90 with wounds initially ranging from 9 to 41 mm wide. The initial wound diameter was measured under zero applied force. Then, the force required to close each wound using a single size 1 nylon suture and a SUTUREGARD suture retention device was measured, after which the suture was then locked in the retention device at fixed displacement. At time points of 300 s, 600 s, and 1800 s, the suture retention device was released, and the wound opening was again recorded at zero force, and the force required to close the wound was recorded.

RESULTS

The average wound closure force relaxed by 44% and 65% after 300 s and 1800 s, respectively. Average wound width decreased 30% and 42%, after 300 s and 1800 s, respectively, due to creep deformation. Furthermore, all wounds relaxed to be below 15 N of closure force after 600 s, which is considered the maximum clinically acceptable force. A relaxation time of ∼270 s and a threshold force for creep of ∼5 N was found.

SIGNIFICANCE

Results of this study provide the first quantitative clinical guidance for efficient scalp closure of large wounds by creep deformation and stress relaxation. Furthermore, the methodology developed here can be used as a basis for future in vivo studies of the stress relaxation and creep deformation of human scalp, which in turn can provide data for the development and validation of constitutive models for scalp deformation.

Mechanical characterisation of human and porcine scalp tissue at dynamic strain rates

Several biomedical applications require knowledge of the behaviour of the scalp, including skin grafting, skin expansion and head impact biomechanics. Scalp tissue exhibits a non-linear stress-strain relationship, anisotropy and its mechanical properties depend on strain rate. When modelling the behaviour of the scalp, all these factors should be considered in order to perform realistic simulations. Here, tensile tests at strain rates between 0.005 and 100 s^-1 have been conducted on porcine and human scalp in order to investigate the non-linearity, anisotropy, and strain rate dependence of the scalp mechanical properties. The effect of the orientation of the sample with respect to the Skin Tension Lines (STLs) was considered during the test. The results showed that anisotropy is evident in the hyperelastic response at low strain rates (0.005 s^-1) but not at higher strain rates (15-100 s^-1). The mechanical properties of porcine scalp differ from human scalp. In particular, the elastic modulus and the Ultimate Tensile Strength (UTS) of the porcine scalp were found to be almost twice the values of the human scalp, whereas the stretch at failure was not found to be significantly different. An anisotropic hyperelastic model (Gasser-Ogden-Holzapfel) was used to model the quasi-static behaviour of the tissue, whereas three different isotropic hyperelastic models (Fung, Gent and Ogden) were used to model the behaviour of scalp tissue at higher strain rates. The experimental results outlined here have important implications for those wishing to model the mechanical behaviour of scalp tissue both under quasi-static and dynamic loading conditions.

A Mathematical Model to Study the Mechanical Information Induced by Lifting-Thrusting Needle

Focusing on the mechanical effect of traditional Chinese acupuncture, this study builds a mathematical model that simulates the mechanical process of lifting-thrusting needle. Analytic and numerical solutions are obtained to explore the mechanical information (displacement, strain, stress, and energy) in the skin tissue. Our results show that (1) needle manipulation leads to tissue displacement and mechanical stress field, but the needle should be inserted into the right position (about π/ω cm around the acupoint, where ω is the angular frequency) and enough depth (about 2 cm in lower limbs) to achieve effective mechanical stimuli; (2) the tissue displacement decays with an increase of distance from the stimulus position, more rapidly at higher frequencies; (3) there is an inverse relationship between the area of the 'effective influence region' (where shear strain > 0.2) and the stimulus frequency, which means larger needle movement is needed at higher frequencies to achieve a better curative effect; (4) more energy is required to maintain high frequency manipulation. This study proposes a preliminary comprehension of the mechanical response around the needle during the acupuncture process.