An in vitro Model System to Study the Damaging Effects of Prolonged Mechanical Loading of the Epidermis

Effects of Heat and Pressure Loading on Erythema and Skin Microclimate at Pressure-Prone Areas of Semirecumbent Healthy Adults

ABSTRACT

BACKGROUND

Altered localized microclimate conditions are known to increase the risk of pressure injuries in immobile patients. Limited in vivo studies have examined how skin properties change over time and under different skin microclimate conditions.

OBJECTIVE

To examine changes in erythema, stratum corneum hydration, and skin temperature in response to an altered microclimate, simulated by the introduction of heat, at the skin-support surface interface of semirecumbent healthy adults.

METHODS

In this quasi-experimental study, researchers obtained noninvasive biophysical skin measures at the elbows, heels, and sacrum of healthy adults positioned semirecumbently. Participants advanced through a controlled condition and heat intervention. Repeated measures were gathered at the anatomic sites every 10 minutes for 60 minutes per condition. Linear mixed models were used to compare skin measures over time and between conditions.

RESULTS

Large regional variation in skin measures emerged by condition and among participants. The introduction of heat increased skin temperature at the heels (P < .001), elbows (P = .005), and sacrum (P < .001). Erythema increased at the sacrum (P = .012) but at no other anatomic testing site. There was no impact on stratum corneum hydration as a result of increased heat at any anatomic testing site.

CONCLUSIONS

More frequent monitoring cycles for erythematous skin discoloration and thermal changes may be required at the sacral skin in immobile patients. Distinctive protocols in relation to skin-care regimens may be warranted for individual patients. Skin cooling, enabled by intermittent off-loading of pressure-prone areas, may assist optimization of the skin microclimate.

Sub-epidermal moisture measurement: an evidence-based approach to the assessment for early evidence of pressure ulcer presence

This paper aims to discuss the literature pertaining to early pressure-shear induced tissue damage detection, with emphasis on sub-epidermal moisture measurement (SEM). The current method for pressure detection is visual skin assessment (VSA); however, this method is fraught with challenges. Advances in early detection of pressure ulcers are reported in the literature and mainly involve measuring inflammation markers on weight-bearing anatomical areas in order to capture the first signs of tissue damage. One novel technique currently in use is SEM measurement. This biophysical marker is the product of plasma that leaks as a response to local inflammation arising due to pressure-shear induced damage over bony prominences. The early detection of tissue damage is beneficial in two different ways. First, it enables early intervention when the damage is still microscopic and reversible and, therefore, has the potential to prevent further aggravation of healthy surrounding tissue. This arises by avoiding the causation of the problem and stopping the knock-on effect of inflammation, especially when the rapid pressure ulceration pathway of deformation is in place. Second, when the slow ischaemic-reperfusion related mechanism is undergoing, cell death can be avoided when the problem is identified before the cell reaches the "death threshold," completely averting a pressure ulcer.

A New Craniothoracic Mattress for Immobilization of the Cervical Spine in Critical Care Patients

Current immobilization techniques of the cervical spine are associated with complications including pressure ulcers, discomfort, and elevated intracranial pressures with limited access to the thorax and airway. In this study, a newly developed craniothoracic immobilizer (Pharaoh mattress) for critical care patients with cervical injury was tested for its restriction of cervical movement, peak interface pressures, comfort, and radiolucency, and compared with headblocks strapped to a spineboard. Cervical movement was measured by roentgen stereophotogrammetric analysis in 5 fresh frozen cadavers. Peak interface and discomfort pressures were measured in 10 healthy volunteers. Radiographic absorption was calculated by measuring the total emission radiation with and without immobilizer. The Pharaoh mattress caused a mean restriction of 59% (SD: 15) flexion-extension, 77% (SD: 14) lateral bending, and 93% (SD: 3) rotation, compared with the unrestricted situation. No significant differences in restriction of cervical movement were found between headblocks strapped to a spineboard and the Pharaoh mattress. The mean peak pressures on the Pharaoh mattress were significantly lower than on the spineboard. Healthy volunteers gave significantly lower numeric discomfort scores on the Pharaoh mattress than on the spineboard. The Pharaoh mattress absorbed more x-rays than the spineboard. The Pharaoh mattress provides similar restriction of cervical movement compared with headblocks strapped to a spineboard but with lower interface pressures and increased comfort. This new mattress could be useful for immobilization of the cervical spine in critical care patients with mechanically instable spinal fractures.

An ex vivo porcine skin model to evaluate pressure-reducing devices of different mechanical properties used for pressure ulcer prevention

Pressure ulcers are complex wounds caused by pressure- and shear-induced trauma to skin and underlying tissues. Pressure-reducing devices, such as dressings, have been shown to successfully reduce pressure ulcer incidence, when used in adjunct to pressure ulcer preventative care. While pressure-reducing devices are available in a range of materials, with differing mechanical properties, understanding of how a material's mechanical properties will influence clinical efficacy remains limited. The aim of this study was to establish a standardized ex vivo model to allow comparison of the cell protection potential of two gel-like pressure-reducing devices with differing mechanical properties (elastic moduli of 77 vs. 35 kPa). The devices also displayed differing energy dissipation under compressive loading, and resisted strain differently under constant load in compressive creep tests. To evaluate biological efficacy we employed a new ex vivo porcine skin model, with a confirmed elastic moduli closely matching that of human skin (113 vs. 119 kPa, respectively). Static loads up to 20 kPa were applied to porcine skin ex vivo with subsequent evaluation of pressure-induced cell death and cytokine release. Pressure application alone increased the percentage of epidermal apoptotic cells from less than 2% to over 40%, and increased cellular secretion of the pro-inflammatory cytokine TNF-alpha. Co-application of a pressure-reducing device significantly reduced both cellular apoptosis and cytokine production, protecting against cellular damage. These data reveal new insight into the relationship between mechanical properties of pressure-reducing devices and their biological effects. After appropriate validation of these results in clinical pressure ulcer prevention with all tissue layers present between the bony prominence and external surface, this ex vivo porcine skin model could be widely employed to optimize design and evaluation of devices aimed at reducing pressure-induced skin damage.

In vitro models for evaluating safety and efficacy of novel technologies for skin drug delivery

For preclinical testing of novel therapeutics, predictive in vitro models of the human skin are required to assess efficacy, absorption and safety. Simple as well as more sophisticated three-dimensional organotypic models of the human skin emerged as versatile and powerful tools simulating healthy as well as diseased skin states. Besides addressing the demands of research and industry, such models serve as valid alternative to animal testing. Recently, the acceptance of several models by regulatory authorities corroborates their role as important building block for preclinical development. However, valid assessment of readout parameters derived from these models requires suitable analytical techniques. Standard analytical methods are mostly destructive and limited regarding in-depth investigation on molecular level. The combination of adequate in vitro models with modern non-invasive analytical modalities bears a great potential to address important skin drug delivery related questions. Topics of interest are for instance the assessment of repeated dosing effects and xenobiotic biotransformation, which cannot be analyzed by destructive techniques. This review provides a comprehensive overview of current in vitro skin models differing in functional complexity and mimicking healthy as well as diseased skin states. Further, benefits and limitations regarding analytical evaluation of efficacy, absorption and safety of novel drug carrier systems applied to such models are discussed along with a prospective view of anticipated future directions. In addition, emerging non-invasive imaging modalities are introduced and their significance and potential to advance current knowledge in the field of skin drug delivery is explored.

Early Detection of Pressure Ulcer Development Following Traumatic Spinal Cord Injury Using Inflammatory Mediators

OBJECTIVE

To identify changes in concentrations of inflammatory mediators in plasma and urine after traumatic spinal cord injury (SCI) and before the occurrence of a first pressure ulcer.

DESIGN

Retrospective; secondary analysis of existing data.

SETTING

Acute hospitalization and inpatient rehabilitation sites at a university medical center.

PARTICIPANTS

Individuals with a pressure ulcer and plasma samples (n=17) and individuals with a pressure ulcer and urine samples (n=15) were matched by age and plasma/urine sample days to individuals with SCI and no pressure ulcer (N=35).

INTERVENTIONS

Not applicable.

MAIN OUTCOME MEASURES

Plasma and urine samples were assayed in patients with SCI, capturing samples within 4 days after the SCI to a week before the formation of the first pressure ulcer. The Wilcoxon signed-rank test was performed to identify changes in the inflammatory mediators between the 2 time points.

RESULTS

An increase in concentration of the chemokine interferon-γ-induced protein of 10kd/CXCL10 in plasma (P<.01) and a decrease in concentration of the cytokine interferon-α in urine (P=.01) were observed before occurrence of a first pressure ulcer (∼4d) compared with matched controls.

CONCLUSIONS

Altered levels of inflammatory mediators in plasma and urine may be associated with pressure ulcer development after traumatic SCI. These inflammatory mediators should be explored as possible biomarkers for identifying individuals at risk for pressure ulcer formation.

Fibroblast heterogeneity and its implications for engineering organotypic skin models in vitro

Advances in cell culture methods, multidisciplinary research, clinical need to replace lost skin tissues and regulatory need to replace animal models with alternative test methods has led to development of three dimensional models of human skin. In general, these in vitro models of skin consist of keratinocytes cultured over fibroblast-populated dermal matrices. Accumulating evidences indicate that mesenchyme-derived signals are essential for epidermal morphogenesis, homeostasis and differentiation. Various studies show that fibroblasts isolated from different tissues in the body are dynamic in nature and are morphologically and functionally heterogeneous subpopulations. Further, these differences seem to be dictated by the local biological and physical microenvironment the fibroblasts reside resulting in "positional identity or memory". Furthermore, the heterogeneity among the fibroblasts play a critical role in scarless wound healing and complete restoration of native tissue architecture in fetus and oral mucosa; and excessive scar formation in diseased states like keloids and hypertrophic scars. In this review, we summarize current concepts about the heterogeneity among fibroblasts and their role in various wound healing environments. Further, we contemplate how the insights on fibroblast heterogeneity could be applied for the development of next generation organotypic skin models.

Analysis of Gene Expression in Experimental Pressure Ulcers in the Rat with Special Reference to Inflammatory Cytokines

Pressure ulcers have been investigated in a few animal models, but the molecular mechanisms of pressure ulcers are not well understood. We hypothesized that pressure results in up-regulation of inflammatory cytokines and those cytokines contribute to the formation of pressure ulcers. We measured genome-wide changes in transcript levels after compression, and focused especially on inflammatory cytokines. The abdominal wall of rats was compressed at 100 mmHg for 4 hours by two magnets. Specimens were obtained 12 hours, 1, or 3 days after compression, and analyzed by light microscopy, microarray, Real-Time PCR, and ELISA. The skin and subcutaneous tissue in the compressed area were markedly thickened. The microarray showed that numerous genes were up-regulated after the compression. Up-regulated genes were involved in apoptosis, inflammation, oxidative stress, proteolysis, hypoxia, and so on. Real-Time PCR showed the up-regulation of granulocyte-macrophage colony stimulating factor (GM-CSF), interferon γ (IFN-γ), interleukin 1β (IL-1β), interleukin 1 receptor antagonist gene (IL1Ra), interleukin 6 (IL-6), interleukin 10 (IL-10), matrix metalloproteinase 3 (MMP-3), tissue inhibitor of metalloproteinase 1 (TIMP-1), and tumor necrosis factor α (TNF-α) at 12 hours, IFN-γ, IL-6, IL-10, MMP-3, and TIMP-1 at 1 day, and IFN-γ, IL-6, and MMP-3 at 3 days. Some genes from subcutaneous tissue were up-regulated temporarily, and others were kept at high levels of expression. ELISA data showed that the concentrations of IL-1β and IL-6 proteins were most notably increased following compression. Prolonged up-regulation of IL-1β, and IL-6 might enhance local inflammation, and continuous local inflammation may contribute to the pressure ulcer formation. In addition, GM-CSF, IFN-γ, MMP-3, and TIMP-1 were not reported previously in the wound healing process, and those genes may have a role in development of the pressure ulcers. Expression data from Real-Time PCR were generally in good agreement with those of the microarray. Our microarray data were useful for identifying genes involved in pressure ulcer formation. However, the expression levels of the genes didn’t necessarily correspond with protein production. As such, the functions of these cytokines need to be further investigated.

The use of skin models in drug development

Three dimensional (3D) tissue models of the human skin are probably the most developed and understood in vitro engineered constructs. The motivation to accomplish organotypic structures was driven by the clinics to enable transplantation of in vitro grown tissue substitutes and by the cosmetics industry as alternative test substrates in order to replace animal models. Today a huge variety of 3D human skin models exist, covering a multitude of scientific and/or technical demands. This review summarizes and discusses different approaches of skin model development and sets them into the context of drug development. Although human skin models have become indispensable for the cosmetics industry, they have not yet started their triumphal procession in pharmaceutical research and development. For drug development these tissue models may be of particular interest for a) systemically acting drugs applied on the skin, and b) drugs acting at the site of application in the case of skin diseases or disorders. Although quite a broad spectrum of models covering different aspects of the skin as a biologically acting surface exists, these are most often single stand-alone approaches. In order to enable the comprehensive application into drug development processes, the approaches have to be synchronized to allow a cross-over comparison. Besides the development of biological relevant models, other issues are not less important in the context of drug development: standardized production procedures, process automation, establishment of significant analytical methods, and data correlation. For the successful routine use of engineered human skin models in drug development, major requirements were defined. If these requirements can be accomplished in the next few years, human organotypic skin models will become indispensable for drug development, too.

Backrest position in prevention of pressure ulcers and ventilator-associated pneumonia: conflicting recommendations

Pressure ulcers and ventilator-associated pneumonia (VAP) are both common in acute and critical care settings and are considerable sources of morbidity, mortality, and health care costs. To prevent pressure ulcers, guidelines limit bed backrest elevation to less than 30 degrees, whereas recommendations to reduce VAP include use of backrest elevations of 30 degrees or more. Although a variety of risk factors beyond patient position have been identified for both pressure ulcers and VAP, this article will focus on summarizing the major evidence for each of these apparently conflicting positioning strategies and discuss implications for practice in managing mechanically ventilated patients with risk factors for both pressure ulcers and VAP.

Cytokine release in tissue-engineered epidermal equivalents after prolonged mechanical loading

Prolonged mechanical loading of soft tissues may result in degeneration of these tissues, resulting in formation of pressure ulcers. The risk assessment of individuals might be improved by including measurements of the tissue response to mechanical loading. Cytokines, which are released by the top layer of the skin upon chemical irritation, might be used to determine the epidermal response to mechanical loading. This chapter describes methods to measure the release of cytokines IL-1 alpha, IL-1RA, and IL-8 from tissue-engineered epidermal equivalents in response to sustained mechanical loading. A custom-built loading device was used to apply load to the epidermal equivalents and the cytokines were measured using an enzyme-linked immunosorbent assay.

The transport profile of cytokines in epidermal equivalents subjected to mechanical loading

Pressure ulcer risk assessment might be optimized by incorporating the soft tissue reaction to mechanical loading in the currently used risk assessment scales. Cytokines, like IL-1alpha, IL-1RA, IL-8, and TNF-alpha, might be used to determine this tissue reaction, since they are released after 24 h of mechanical loading of epidermal equivalents. In the current study, the release and transport of these cytokines with time was evaluated. Epidermal equivalents were subjected to 20 kPa for different time periods (1, 2, 4, 6, 8, 16, and 24 h). Compared to the unloaded control group, a significant increase was found for IL-1alpha (4.7-fold), IL-1RA (4.8-fold), and IL-8 (3.6-fold) release after 1 h loading. For TNF-alpha, the release was significantly increased after 4 h of loading (5.1-fold compared to the unloaded situation), coinciding with the first signs of gross structural tissue damage. These cytokine values were determined in the surrounding medium and a transport model was developed to evaluate the distribution of cytokines inside the culture. These simulations revealed that all IL-8 and TNF-alpha was released from the keratinocytes, whereas most of the IL-1alpha and IL-1RA remained inside the keratinocytes during the 24 h loading period. In conclusion, IL-1alpha, IL-1RA, and IL-8 appear promising biochemical markers for pressure ulcer risk assessment, since their release is increased after 1 h of epidermal loading and before the onset of structural tissue damage.

Study of skin-fabric interactions of relevance to decubitus: friction and contact-pressure measurements

BACKGROUND/PURPOSE

Prolonged pressure as well as friction and shear forces at the skin-textile interface are decisive physical parameters in the development of decubitus. The present article describes the contact phenomena at the skin-textile interface and the development of a purpose-built textile friction analyser (TFA) for the tribological assessment of skin-fabric interactions, in connection with decubitus prevention.

METHODS

Interface pressure distributions were recorded in the pelvic and femoral regions between supine persons and a foam mattress. Fabrics made of various natural and synthetic yarns were investigated using the TFA. A vertical load of 7.7 kPa was applied to the swatches, simulating high interface pressures at the skin-fabric interface and clinical conditions of bedridden persons. Fabrics were rubbed in reciprocating motions against a validated skin-simulating material to determine static as well as dynamic friction coefficients (COFs).

RESULTS

Maximum contact pressures ranged from 5.2 to 7.7 kPa (39-58 mmHg) and exceeded the capillary closure pressure (32 mmHg) in all investigated bedding positions. For both COFs, a factor of 2.5 was found between the samples with the lowest and highest values. Our results were in a similar range to COFs found in measurements on human skin in vivo. The results showed that our test method can detect differences of 0.01 in friction coefficients.

CONCLUSION

TFA measurements allow the objective and reliable study of the tribology of the skin-textile biointerface and will be used to develop medical textiles with improved performance and greater efficacy for decubitus prevention.

Cytokine and chemokine release upon prolonged mechanical loading of the epidermis

At this moment, pressure ulcer risk assessment is dominated by subjective measures and does not predict pressure ulcer development satisfactorily. Objective measures are, therefore, needed for an early detection of these ulcers. The current in vitro study evaluates cytokines and chemokines [interleukin 1alpha (IL-1alpha), interleukin 1 receptor antagonist (IL-1RA), tumor necrosis factor alpha (TNF-alpha) and interleukin 8 (CXCL8/IL-8)] as early markers for mechanically-induced epidermal damage. Various degrees of epidermal damage were induced by subjecting commercially available epidermal equivalents (EpiDerm) to increasing pressures (0, 50, 75, 100, 150, and 200 mmHg) for 24 h, using a loading device. At the end of the loading experiment, tissue damage was assessed by histological examination and by evaluation of the cell membrane integrity. Cytokines and chemokines were determined in the culture supernatant. Sustained epidermal loading resulted in an increased release of IL-1alpha, IL-1RA, TNF-alpha and CXCL8/IL-8. This was first observed at 75 mmHg, when the tissue was only slightly damaged. Swollen cells, vacuoles, necrosis and affected cell membranes were observed at pressures higher than 75 mmHg. Furthermore, at 150 and 200 mmHg, the cells in the lower part of the epidermis were severely compressed. In conclusion, IL-1alpha, IL-1RA, TNF-alpha and CXCL8/IL-8 are released in vitro as a result of sustained mechanical loading of the epidermis. The first increase in cytokines and chemokines was observed when the epidermal tissue was only slightly damaged. Therefore, these cytokines and chemokines are potential markers for the objective, early detection of mechanically-induced skin damage, such as pressure ulcers.

The relative contributions of compression and hypoxia to development of muscle tissue damage: an in vitro study

Deep pressure ulcers develop in tissues subjected to sustained mechanical loading. Though it has been hypothesized that this damage mechanism results from local tissue ischemia, it has recently been shown with a cell model that sustained compression can cause cell deformation, leading to tissue breakdown. The present study focuses on the assessment of cell viability during compression and ischemia in an in vitro muscle model to determine their relative contributions to damage development. A model system was developed consisting of engineered skeletal muscle produced from the culture of murine muscle cells in a collagen gel. The tissue was subjected to 0, 20, or 40% compression under hypoxic or normoxic conditions. Experiments were performed on the stage of a microscope and cell viability was monitored using fluorescent markers for apoptotic and necrotic cell death. Hypoxia did not lead to significant cell death over a 22 h period. By contrast, compression led to immediate cell death that increased with time. No additional effect of hypoxia on cell death was observed. These data show that contrary to existing theories, compression can cause development of muscle damage and that hypoxia does not contribute to cell death development within 22 h in engineered muscle.