The Mechanical Properties in the Vicinity of Grain Boundaries in Ultrafine-Grained and Polycrystalline Materials Studied by Nanoindentations

The strength of structural materials strongly depends on the structure and properties of grain boundaries. Interfaces usually act as barriers to dislocation motion and therefore strengthen materials with decreasing grain size, quantitatively described by the well-known Hall-Petch relation. However, interfaces in nanocrystalline materials are often covered with impurities or second phases, which may influence the mechanical properties. With nanoindentation testing it is now possible to probe the strength of interfaces like grain boundaries directly on a nanometer scale. Therefore this method was used to investigate the properties in the vicinity of grain boundaries in polycrystalline materials with conventional grain size and in ultrafine-grained metals prepared by equal channel angular pressing (ECAP), where no impurities are introduced during processing. Measurements on an austenitic steel clearly show a decreasing hardness close to the interface opposite to the general expected behavior of strengthening. In this case segregation effects strongly influence the mechanical properties near the boundaries. The nanoindentation investigations on ultrafine-grained Al and Cu show a strong strain rate sensitivity. Inelastic effects are also found between unloading-loading segments during indentations.

Mechanical properties of hyaline and repair cartilage studied by nanoindentation

Articular cartilage is a highly organized tissue that is well adapted to the functional demands in joints but difficult to replicate via tissue engineering or regeneration. Its viscoelastic properties allow cartilage to adapt to both slow and rapid mechanical loading. Several cartilage repair strategies that aim to restore tissue and protect it from further degeneration have been introduced. The key to their success is the quality of the newly formed tissue. In this study, periosteal cells loaded on a scaffold were used to repair large partial-thickness cartilage defects in the knee joint of miniature pigs. The repair cartilage was analyzed 26 weeks after surgery and compared both morphologically and mechanically with healthy hyaline cartilage. Contact stiffness, reduced modulus and hardness as key mechanical properties were examined in vitro by nanoindentation in phosphate-buffered saline at room temperature. In addition, the influence of tissue fixation with paraformaldehyde on the biomechanical properties was investigated. Although the repair process resulted in the formation of a stable fibrocartilaginous tissue, its contact stiffness was lower than that of hyaline cartilage by a factor of 10. Fixation with paraformaldehyde significantly increased the stiffness of cartilaginous tissue by one order of magnitude, and therefore, should not be used when studying biomechanical properties of cartilage. Our study suggests a sensitive method for measuring the contact stiffness of articular cartilage and demonstrates the importance of mechanical analysis for proper evaluation of the success of cartilage repair strategies.

Mechanisms of transcriptional repression by the t(8;21)-, t(12;21)-, and inv(16)-encoded fusion proteins

AML-1 is one of the most frequently translocated genes in human leukemia. AML-1 binds DNA and activates or represses transcription, while the chromosomal translocation fusion proteins in acute myeloid leukemia subvert these functions. The t(8;21) is the second most frequent translocation in acute myeloid leukemia and creates a fusion between the DNA binding domain of AML-1 and the ETO (also known as MTG8) corepressor. The t(12;21) is found in up to 25% of pediatric B cell acute lymphoblastic leukemias and fuses the ETS family transcription factor TEL to the amino terminus of AML-1. In addition, the inv(16), the most frequent translocation in acute myeloid leukemia, fuses the AML-1 cofactor CBFbeta to the smooth muscle myosin heavy chain MYH11. Both the t(8;21) and t(12;21) create transcriptional repressors that impair AML-1 target gene expression. We demonstrated that the fusion proteins encoded by these translocations contact the nuclear hormone corepressors (N-CoR/SMRT), mSin3A, and histone deacetylases. We have also found that both TEL and AML-1 interact with mSin3A. TEL also binds N-CoR and histone deacetylase-3, indicating that wild-type TEL is a transcriptional repressor. The t(12;21) fuses the mSin3A interaction domain of TEL to AML-1 to transform AML-1 from a regulated to an unregulated transcriptional repressor. The recognition that AML-1 interacts with mSin3A to repress transcription suggested that the inv(16) fusion protein might also repress the transcription of AML-1-target genes. In fact, the inv(16) encodes a protein that cooperates with AML-1 to repress transcription. The inv(16) fusion protein was found in a ternary complex with AML-1 and mSin3A, suggesting that the inv(16) also acts by recruiting transcriptional corepressors and histone deacetylases.

Pulmonary function and health-related quality of life in a sample of long-term survivors of the acute respiratory distress syndrome

OBJECTIVE

We performed a follow-up cohort analysis in order to delineate the correlation between pulmonary function (PF) and health-related quality of life (HRQL) in patients after ARDS.

DESIGN

Follow-up cohort study.

SETTING

A 20-bed ICU of a university teaching hospital.

PATIENTS

A cohort of 50 long-term survivors of ARDS.

MEASUREMENTS AND RESULTS

Measurements of PF (FVC, FEV1, TLC, D(LCO)) and HRQL (SF-36 Health Status Questionnaire) were made 5.5 years (median value) after discharge from the ICU. Impairments in PF (defined as PF results below 80% of the predicted value) were frequent but generally mild. Twenty patients had a single PF impairment (with limitations in FEV1/FVC ratio in 12 patients being the most common), four patients had two (with D(LCO) and FEV1/FVC ratio impairment the most common) and three patients had pathologic results in three PF tests (FEV1/FVC ratio, TLC and capillary pO2 during exercise in one case, FVC, TLC and capillary pO2 during exercise in the second patient and FVC, TLC and D(LCO) in the third). Compared to normal controls, survivors of ARDS showed impairments in all SF-36 health dimensions (p < 0.001). Patients with multiple (> 1) PF impairments described the lowest HRQL with major limitations in all SF-36 categories (p < 0.037) including physical and mental summary scores (36.5 vs 46.9, p = 0.037 and 31.3 vs 51.4, p = 0.003) when compared to patients with no or only one PF impairment.

CONCLUSIONS

Long-term survivors of ARDS have a significant reduction in HRQL and the presence of multiple PF impairments is associated with maximal decrements in HRQL.

The effect of stress doses of hydrocortisone during septic shock on posttraumatic stress disorder and health-related quality of life in survivors

OBJECTIVES

The exposure to intense physical and psychological stress during intensive care can result in posttraumatic stress disorder (PTSD) in survivors. Cortisol is a biological stress mediator that can have a protective effect during severe stress. The administration of stress doses of hydrocortisone during treatment in the intensive care unit could theoretically result in a lower incidence of PTSD. We tested this hypothesis in survivors of septic shock.

DESIGN

A retrospective case-controlled analysis.

SETTING

A 20-bed multidisciplinary intensive care unit of a tertiary-care university hospital.

PATIENTS

We identified 27 patients who received standard therapy for septic shock. These patients served as controls and were compared with an equal number of patients who received hydrocortisone in addition to standard treatment. These patients were selected from our database with regard to age (+/-4 yrs), gender, and cause of septic shock to be as similar as possible with control patients.

INTERVENTIONS

Patients from the hydrocortisone group had received stress doses of hydrocortisone (100 mg bolus, followed by 0.18 mg/kg/hr) in addition to standard treatment. Patients from the control group received standard protocol-driven treatment only. PTSD was diagnosed with the Posttraumatic Stress Syndrome-10 inventory, a self-report scale for diagnosis of PTSD. Health-related quality of life was measured using the Medical Outcomes Study Short-Form Survey (Medical Outcomes Trust, Boston, MA), which consists of 36 questions.

MEASUREMENTS AND MAIN RESULTS

Patients who received hydrocortisone during septic shock had a significantly lower incidence of PTSD than patients who received standard treatment only (5 of 27 vs. 16 of 27; p = .01) and had significantly higher scores on the mental health index of the Medical Outcomes Study Short-Form health-related quality-of-life questionnaire (68 vs. 44 points; p = .009).

CONCLUSIONS

Data from this study support the hypothesis that the administration of stress doses of hydrocortisone in doses equivalent to the maximal endocrine secretion rate during septic shock reduces the incidence of PTSD and improves emotional well-being in survivors. This hypothesis should be tested in a prospective randomized trial.

Sensitivity and specificity of a screening test to document traumatic experiences and to diagnose post-traumatic stress disorder in ARDS patients after intensive care treatment

OBJECTIVE

Many survivors of critical illness and intensive care unit (ICU) treatment have traumatic memories such as nightmares, panic or pain which can be associated with the development of posttraumatic stress disorder (PTSD). In order to simplify the rapid and early detection of PTSD in such patients, we modified an existing questionnaire for diagnosis of PTSD and validated the instrument in a cohort of ARDS patients after long-term ICU therapy.

DESIGN

Follow-up cohort study.

SETTING

The 20-bed ICU of a university teaching hospital.

PATIENTS

A cohort of 52 long-term survivors of the acute respiratory distress syndrome (ARDS).

INTERVENTIONS AND MEASUREMENTS

The questionnaire was administered to the study cohort at two time points 2 years apart. At the second evaluation, the patients underwent a structured interview with two trained psychiatrists to diagnose PTSD according to Diagnostic and Statistical Manual of Mental Disorders, 4th edition (DSM-IV) criteria. The reliability and validity of the questionnaire was then estimated and its specificity, sensitivity and optimal decision threshold determined using receiver operating characteristic (ROC) curve analyses.

RESULTS

The questionnaire showed a high internal consistency (Crohnbach's alpha = 0.93) and a high test-retest reliability (intraclass correlation coefficient alpha = 0.89). There was evidence of construct validity by a linear relationship between scores and the number of traumatic memories from the ICU the patients described (Spearman's rho = 0.48, p < 0.01). Criterion validity was demonstrated by ROC curve analyses resulting in a sensitivity of 77.0% and a specificity of 97.5% for the diagnosis of PTSD.

CONCLUSIONS

The questionnaire was found to be a responsive, valid and reliable instrument to screen survivors of intensive care for PTSD.

A closed rebreathing system for dose maintenance during Partial Liquid Ventilation

Partial Liquid Ventilation (PLV), a treatment for acute respiratory failure in which the lungs are filled, either partially or to functional residual capacity (FRC), with perfluorochemical (PFC) liquid while the patient is on mechanical gas ventilation, has progressed to clinical trials using the PFC perflubron (PFB). Because gas expired during PLV is laden with PFB vapor, PFB is lost via evaporation, which increases dose consumption and necessitates periodic redosing. A device has been developed to minimize evaporative loss by confining PFC vapor to a gas volume breathed by the patient, which is isolated from the ventilator. This closed rebreathing system works with the ventilator such that after the lung is filled with PFB, the patient is connected to the rebreathing system, with breathing still "driven" by the ventilator. The rebreathing system consists of two gas circuits, or compartments, separated by a flexible bag (in a box) partition. One compartment is in gas communication with the lung, while the second communicates with the ventilator. The O2 level on the patient side is matched to that on the ventilator side by sensing gas concentrations and by feedback control of O2 introduction. Similarly, air is introduced into the patient side under pressure-based feedback control to maintain a constant gas volume. On inspiration, the ventilator delivers the tidal volume (breath) into the box surrounding the bag, which, in turn, is transmitted through the bag to the lung. On expiration, the process is reversed. Unidirectional circulation of gas in the rebreathing circuit is achieved via check valves, and expired CO2 is removed by a barium hydroxide lime cartridge. Airway humidification is maintained by captive water vapor in the system and water vapor from the CO2 absorber. It is recommended that flow, pressure, O2, and CO2 levels be monitored at the patient "Y," i.e., the proximal end of the endotracheal tube. Performance data from both in-vitro experiments and in-vivo PLV experiments in pigs are presented. The authors conclude that with the closed rebreathing system, the dose can be safely maintained with fewer redosing procedures, and an approximately 90% savings in dose is achieved.