3D extruded composite thermoelectric threads for flexible energy harvesting.

Whereas the rigid nature of standard thermoelectrics limits their use, flexible thermoelectric platforms can find much broader applications, for example, in low-power, wearable energy harvesting for internet-of-things applications. Here we realize continuous, flexible thermoelectric threads via a rapid extrusion of 3D-printable composite inks (Bi2Te3 n- or p-type micrograins within a non-conducting polymer as a binder) followed by compression through a roller-pair, and we demonstrate their applications in flexible, low-power energy harvesting. The thermoelectric power factors of these threads are enhanced up to 7 orders-of-magnitude after lateral compression, principally due to improved conductivity resulting from reduced void volume fraction and partial alignment of thermoelectric micrograins. This dependence is quantified using a conductivity/Seebeck vise for pressure-controlled studies. The resulting grain-to-grain conductivity is well explained with a modified percolation theory to model a pressure-dependent conductivity. Flexible thermoelectric modules are demonstrated to utilize thermal gradients either parallel or transverse to the thread direction.

Band engineering of thermoelectric materials.

Lead chalcogenides have long been used for space-based and thermoelectric remote power generation applications, but recent discoveries have revealed a much greater potential for these materials. This renaissance of interest combined with the need for increased energy efficiency has led to active consideration of thermoelectrics for practical waste heat recovery systems-such as the conversion of car exhaust heat into electricity. The simple high symmetry NaCl-type cubic structure, leads to several properties desirable for thermoelectricity, such as high valley degeneracy for high electrical conductivity and phonon anharmonicity for low thermal conductivity. The rich capabilities for both band structure and microstructure engineering enable a variety of approaches for achieving high thermoelectric performance in lead chalcogenides. This Review focuses on manipulation of the electronic and atomic structural features which makes up the thermoelectric quality factor. While these strategies are well demonstrated in lead chalcogenides, the principles used are equally applicable to most good thermoelectric materials that could enable improvement of thermoelectric devices from niche applications into the mainstream of energy technologies.

In vivo transluminal microvascular endothelial cell seeding on balloon injured rabbit arteries.

AIM: Seeding venous endothelial cells (EC) onto damaged vascular surfaces attenuates the development of intimal hyperplasia. Unlike venous EC, fat derived microvascular endothelial cells (MVEC) do not require a culture step to increase the yield. The authors investigated whether fat derived MVEC are suitable to reduce intimal hyperplasia after PTA. METHODS: Five rabbits were subjected to percutaneous transluminal angioplasty (PTA) of both iliac arteries. One side was seeded transluminally with autologous perirenal fat derived MVEC, using a double balloon catheter. The contralateral side was sham seeded, and served as a control. Follow-up was 4 weeks. Another rabbit was used for a feasibility experiment. This rabbit was subjected to a 1-sided seeding procedure and was sacrificed after 1 week. In a 7th rabbit, a 1-sided PTA was transformed, and autologous labelled cells were injected in the distal aorta instead of seeded, follow-up was 1 week. Histological investigation was per-formed. RESULTS: The MVEC seeded artery of the pilot experiment was patent. All sham seeded arteries (5) except for 1 were patent. The patent ones showed moderate intimal hyperplasia. MVEC seeding (5) resulted in occlusion twice. In the patent MVEC seeded arteries intimal hyperplasia was present in more extended form than in the sham seeded arteries. Both the patent MVEC- and sham-seeded arteries were covered with an EC layer. Injected labelled MVEC were not found again on the de-endothelialized artery. CONCLUSION: In this study seeding of fat derived MVEC on damaged native arteries results in an increased development of intimal hyperplasia and a decreased patency. One of the reasons may be the presence of non-EC in the seeded cell population.

Application of a clinical grade CD34-mediated method for the enrichment of microvascular endothelial cells from fat tissue.

BACKGROUND: Microvascular endothelial cells (MVEC) derived from s.c. fat are seeded on vascular grafts to prevent early occlusion. We have demonstrated the presence of contaminating cells contributing to MVEC seeding-related intimal hyperplasia in MVEC isolates from fat tissue. We found that cell isolates additionally purified after the isolation process, were associated with a reduced thrombogenicity and development of intimal hyperplasia in vitro. A combination of 11Fibrau (F11)- and CD14-coated Dynabeads was used to deplete the contaminating cells, fibroblasts, and monocytes/macrophages. Unfortunately, clinical-grade F11 is not available, and thus cannot be used for clinical practice. CD34 selection with clinical-grade products is widely used for the isolation of hematopoietic progenitors, and endothelial cells (EC) express CD34 on their surfaces. The aims of this study were to test the effectiveness of two different CD34-selection techniques for purification of MVEC, and to compare the results with those of the F11/CD14-method. METHODS: Liposuction fat was enzymatically digested and centrifuged twice to remove adipocytes and collagenase. CD34 selection was performed using the commercially available methods from Nexell or Miltenyi. Both techniques were modified for our use. The purity after isolation and culture, and recovery were determined by flow-cytometry (CD31-expression) and compared with that of cells purified with the F11/CD14-method. RESULTS: Besides MVEC, the contaminating fibroblasts and macrophages/monocytes weakly expressed the CD34 Ag. Enrichment of MVEC was not successful with the Miltenyi method. Variations in neither the dose of Ab nor the use of direct selection and different separation programs improved the results. With the Nexell method, MVEC were enriched to 86%, a comparable purity to that obtained with the F11/CD14-method. However, a lower recovery was achieved with the Nexell method. CONCLUSION: Enrichment of MVEC could be achieved with a modified protocol of the clinical grade CD34(+) selection method from Nexell, but not with the CD34 method from Miltenyi.

Reduction of non-endothelial cell contamination of microvascular endothelial cell seeded grafts decreases thrombogenicity and intimal hyperplasia.

INTRODUCTION: fat derived microvascular endothelial cells (MVEC) seeded on prosthetic vascular grafts, improve patency in animals. Results in humans were disappointing, due to thrombogenicity and progressive intimal hyperplasia. Also in animals intimal hyperplasia was found. We postulate that contaminating cells present in the transplant are involved in the intimal hyperplasia. We developed a method to further purify human MVEC from 40-90%. Here we tested the effects of enrichment upon thrombogenicity and seeding-related intimal hyperplasia. METHODS: liposuction fat was enzymatically digested and centrifuged. To enrich MVEC, contaminating macrophages and fibroblasts were removed with dynabeads coated with macrophage- and fibroblast-specific antibodies. Thrombogenicity was assessed by measuring tissue factor and thrombomodulin activity, presence of endothelial nitric oxide synthase and via perfusion of the cells with whole blood. To investigate seeding-related intimal hyperplasia, PTFE grafts were seeded with the cells and cultured for 3 weeks. RESULTS: tissue factor activity of purified cells was reduced compared to nonpurified cells. Purified cells showed thrombomodulin activity and eNOS expression. Fragment 1+2 and Fibrinopeptide A generation after perfusion of purified cells were significantly lower than after perfusion of nonpurified cells, and only nonpurified cells were covered with platelets and fibrin. Prostheses seeded with nonpurified cells showed an EC monolayer above a multilayer of myofibroblasts, prostheses seeded with purified cells only showed a single EC monolayer. Mixing experiments with human umbilical cord EC (HUVEC) and fibroblasts showed that when more than 25% HUVEC were present a confluent EC layer was formed. When the amount of fibroblasts was 25% or less, no development of a subendothelial multilayer of myofibroblasts was found within 3 weeks. CONCLUSION: reduction of non-endothelial cell contamination of microvascular endothelial cell seeded grafts decreases thrombogenicity and might prevent seeding-related intimal hyperplasia.

Contaminants from the transplant contribute to intimal hyperplasia associated with microvascular endothelial cell seeding.

OBJECTIVES: seeding prosthetic grafts with fat-derived microvascular endothelial cells (MVEC) results not only in a non-thrombogenic EC layer, but also in intimal hyperplasia. Here we investigated incidence, composition, progression, and cause of this intimal hyperplasia. DESIGN: EPTFE grafts with MVEC were implanted as carotid interpositions in six dogs with 1 month, and in three dogs with 4, 8 and 12 months follow-up. Grafts seeded without cells, implanted in the contralateral carotid, served as a control. In another three dogs labelled cells were seeded to investigate the contribution of the seeded cells (2-3 weeks). MATERIALS AND METHODS: MVEC were isolated from the falciform ligament. Cells were pressure seeded on ePTFE grafts. Labelling was performed using retroviral gene transduction. The grafts were analysed with immunohistochemical techniques. RESULTS: after 1 month, all patent non-seeded grafts (5/6) showed fibrin and platelet deposition, and all patent seeded grafts (5/6) were covered with a confluent endothelial monolayer on top of a multilayer of myofibroblasts, elastin and collagen. After long term follow-up, all non-seeded grafts were occluded, all patent seeded grafts (4 and 12 months) were covered with an EC-layer with intimal hyperplasia underneath. The thickness of the intima did not progress after 1 month. Transduced cells were found in the endothelial monolayer, hyperplastic intima and luminal part of the prosthesis. CONCLUSIONS: MVEC seeding in dogs results in intimal hyperplasia in all patent grafts, which contains myofibroblasts. Contaminants from the transplant contribute to this intimal hyperplasia.

Adhesive domains in the collagen III fragment alpha1(III)CB4 that support alpha2beta1- and von Willebrand factor-mediated platelet adhesion under flow conditions.

Seven overlapping peptides derived from the bovine alpha1(III)CB4 fragment of collagen III support static platelet adhesion, and an integrin alpha2beta1-recognition site has been assigned within this fragment to residues 522-528 of the collagen alpha1(III) chain; (25). In this study we found that two of the peptides, CB4(III)-6 and -7, were able to support platelet adhesion under flow conditions, whereas the other peptides showed either very little (CB4(III)-1 and -4) or no platelet adhesion at all (CB4(III)-2, -3 and -5). Using the recombinant leech anti-platelet protein (rLAPP), known to prevent both alpha2beta1 integrin- and von Willebrand factor (vWF)-binding to collagen, we observed almost complete inhibition of platelet adhesion to peptides CB4(III)-6 and -7. In solid-phase binding assays rLAPP bound to CB4(III)-6 and -7 and to CB4(III)-6/7, containing the peptide 6/7 overlap sequence, and not to any other peptide. Our results suggest that the overlap sequence GPP*GPRGGAGPP*GPEGGK (single-letter amino acid code, P* = hydroxyproline), corresponding to residues 523-540 of the alpha1(III) collagen chain, contains a binding site for rLAPP. Monoclonal antibodies (MoAbs) directed against the alpha2 subunit of integrin alpha2beta1 inhibited platelet adhesion to both CB4(III)-6 and -7 by about 50%, showing that the alpha2beta1-recognition site in this locality in alpha1(III)CB4 detected under static conditions is of sufficient affinity to withstand shear forces. Solid-phase binding studies indicated that vWF binds to CB4(III)-7 and to a lesser extent to CB4(III)-4. Furthermore, rLAPP competed with vWF in binding to CB4(III)-7. Our results indicate that residues 541-558 of the alpha1(III)-chain may contain one of the critical vWF-binding sites involved in the initial phase of platelet adhesion to collagen III. MoAbs against vWF (A1 and A3 domain) and glycoprotein (GP)Ib confirmed that vWF is involved in adhesion to CB4(III)-7 and showed that vWF is also involved in adhesion to CB4(III)-6 despite the absence of direct binding of vWF to the peptide. The existence of alpha2beta1-, vWF- and rLAPP-binding sites all in close proximity in alpha1(III)CB4 testifies to the importance of this locus in collagen III for its platelet reactivity.

Thrombomodulin activity of fat-derived microvascular endothelial cells seeded on expanded polytetrafluorethylene.

Lining the luminal surface of prosthetic vascular grafts with endothelial cells (cell seeding) will lower its thrombogenicity. Commonly used macrovascular human adult endothelial cells (HAEC) require in vitro cultivation before large enough numbers are obtained to cover grafts confluently. Fat-derived microvascular endothelial cells (MVEC) prove to be a good alternative as they can be harvested in much larger numbers while showing similar antithrombotic and fibrinolytic characteristics. An important anticoagulant function of macrovascular endothelial cells is due to the activity of thrombomodulin (TM) on their surface. In this study, the presence and functional activity of TM on fat-derived microvascular cells used in cell seeding was investigated. The expression and localization of TM on MVEC was studied using immunohistochemistry. Functional activity of TM on MVEC was measured by the generation of activated protein C (APC) and was compared to human umbilical vein endothelial cells (HUVEC). TM activity was studied in MVEC seeded on expanded polytetrafluorethylene (ePTFE) vascular prostheses and compared to blank prostheses. We found that TM was expressed on the surface of MVEC, both in vivo and vitro. TM-dependent generation of APC differed significantly between MVEC and HUVEC (3.98 +/- 1.2 vs. 3.0 +/- 0.7 nM, respectively). After seeding MVEC on vascular prostheses, TM activity did not change. APC generation was significantly higher on MVEC-seeded vascular grafts compared to blank grafts (4.0 +/- 0.7 vs. 1.7 +/- 0.5 nM, respectively). We conclude that TM is present and highly active on cultured MVEC. When seeded on ePTFE, MVEC retain the possibility to inhibit thrombin coagulant activity and to activate protein C. Therefore, since MVEC are readily available, the anticoagulant properties demonstrated here indicate that this cell type is suitable for cell seeding of vascular prostheses.

Thrombogenesis of different cell types seeded on vascular grafts and studied under blood-flow conditions.

BACKGROUND: Small-diameter vascular grafts tend to have an early and high occlusion rate. Cell seeding on the luminal surfaces of small-diameter vascular prostheses may provide an antithrombotic lining and improve both the short-term and the long-term patency rates. We studied the net results of procoagulant and anticoagulant properties of seeded grafts under blood-flow conditions, and we compared the different available types of donor cells. METHODS: Monolayers of liposuction-derived cultured human microvascular endothelial cells (MVECs), human adult endothelial cells (HAECs), human umbilical vein endothelial cells (HUVECs), and human mesothelial cells (MCs) that had been seeded on expanded polytetrafluoroethylene (ePTFE) grafts were perfused with marginally anticoagulated blood (20 U/mL low molecular weight heparin; shear rate, 400/s, 10 minutes) or with non-anticoagulated blood (shear rate, 100/s, 5 minutes). The thrombin and fibrin generation in time was studied with the measurement of the plasma levels of prothrombin fragment 1 and 2 (F 1+2) and of fibrinopeptide A (FPA). The plain ePTFE graft was taken as a control. RESULTS: When the seeded MCs were perfused with recirculating anticoagulated blood, a linear generation of F 1+2 in time was seen, with high levels of F 1+2 and FPA after 10 minutes (4.38 nmol/L and 362 ng/mL, respectively). Allopurinol was added, and the MCs generated less F 1+2 than the HAECs (0.7 nmol/L vs 1.86 nmol/L; P <.05). No fibrin formation was seen. The MVECs generated low amounts of F 1+2 (0.7 nmol/L; 10 minutes), and the HUVECs and the plain ePTFE graft generated the lowest amounts of F 1+2 (0.26 and 0.25 nmol/L, respectively). When the MCs were perfused with non-anticoagulated blood, high amounts of thrombin and fibrin were generated immediately and constantly and could not be decreased with allopurinol. The perfusion of the plain ePTFE graft showed a dramatic increase in F 1+2 and FPA levels towards the end of the experiments. The seeded HAECs, HUVECs, and MVECs inhibited this increase. These results were confirmed by means of scanning electron microscopy. CONCLUSION: Vascular prostheses that are seeded with cultured MCs are highly procoagulant. Standard ePTFE graft prostheses also initiate coagulation, which supports the idea of cell seeding. The endothelial cells, of which the MVECs are the most readily available, seem to preserve their anticoagulant properties after being seeded on vascular grafts.

In vivo experiments with mesothelial cell seeded ePTFE vascular grafts.

OBJECTIVES: To investigate the influence of mesothelial cell (MC) seeding on patency and neointimal formation of small diameter ePTFE grafts in a canine model. MATERIALS AND METHODS: MC were isolated from the omentum, cultured, seeded on fibronectin-coated ePTFE grafts (4 cm, 4 mm ID), and implanted in the carotid artery of five Beagle dogs. Each dog also received a non-seeded control graft. Patency was assessed by palpation immediately after implantation, and non-invasively by magnetic resonance angiography (MRA) after 1 week and just prior to sacrifice (4 weeks). Intimal thickness was quantified on histological sections by use of computer-aided morphometry. RESULTS: All grafts were patent after implantation. After 1 week, MRA showed the loss of lumen diameter in two seeded grafts. After 4 weeks, two seeded grafts were occluded, one seeded graft was severely stenosed, and all others were without angiographic lumen reduction. Histology and morphometry confirmed that two seeded grafts were occluded, and demonstrated that the other three seeded grafts showed significantly more intima formation (0.22-1.34 mm) than the control grafts (< 0.08 mm; p < 0.01). CONCLUSIONS: The MC seeding process decreases patency and increases neointimal formation of small diameter ePTFE grafts in dogs and does not seem to be useful for reduction of graft thrombogenicity.

Thrombomodulin activity on mesothelial cells: perspectives for mesothelial cells as an alternative for endothelial cells for cell seeding on vascular grafts.

Lining the luminal surface of prosthetic small diameter bypasses with endothelial cells (EC) will lower its thrombogenicity. Unfortunately, human EC are scarce. Mesothelial cells (MC) may be a valuable alternative for EC, since they are abundantly available and have antithrombotic and fibrinolytic properties. An important anticoagulant function of EC is due to thrombomodulin (TM) on the surface. The presence of TM on omentally derived human MC is not known but would increase the chance of successful use of MC for cell seeding procedures. The expression and localization of TM on human MC was studied using monoclonal antibodies. TM activity on cultured MC, and the influence of cytokines, was measured by the generation of activated protein C (APC), and was compared to EC. TM is expressed on the surface of MC as well as intracellularly, both in vivo and in vitro. The TM-dependent generation of APC was significantly higher on cultured MC than on cultured EC (817 +/- 141 PM v 262 +/- 38 PM; P < 0.001); their reaction to cytokines was almost identical. Seeding the MC onto vascular prostheses did not change the TM activity. Thrombomodulin is present and highly active on cultured and seeded MC. This may have major implications for MC as a source for cell seeding on vascular prostheses.

Tissue factor expression on mesothelial cells is induced during in vitro culture--manipulation of culture conditions creates perspectives for mesothelial cells as a source for cell seeding procedures on vascular grafts.

Lining the luminal surface of prosthetic small diameter bypasses with endothelial cells (EC) will lower its thrombogenicity. Unfortunately, human EC are only scarcely available. Mesothelial cells (MC) have antithrombotic properties in vivo and can be harvested in large numbers, from the omentum. Recent work demonstrated that the expression of tissue factor (TF) is induced in MC after isolation and culture. Different culture conditions were studied to suppress TF-expression. MC grown in pooled human serum (HS) are procoagulant (717 +/- 119 pM factor Xa/min.10(5) cells). Replacing HS for fetal calf serum, precoating the surface with extracellular matrix and the addition of the xanthine-oxidase inhibitor allopurinol, inhibited TF expression by 90% (p < 0.001). Allopurinol clearly reduced TF-mRNA levels. TF expression on cultured MC is an in-vitro effect due to culture conditions and the formation of oxygen free radicals. By reducing TF expression by 90%, we have established conditions in which MC are a good alternative for EC for seeding on synthetic grafts.

Time-resolved photoacoustic calorimetry: probing the energetics and dynamics of fast chemical and biochemical reactions.

Time-resolved photoacoustic calorimetry is a new experimental technique that measures the dynamics of enthalpy changes on the time scale of nanoseconds to microseconds for reactions initiated by absorption of light. When the reaction is carried out in water, it is also possible to obtain the dynamics of the corresponding volume changes. This method has been applied to a variety of biochemical, organic, and organometallic reactions.

The predictive value of myocardial perfusion scintigraphy after stress in patients without previous myocardial infarction.

Seventy-five patients who had chest pain but no history or ECG evidence of myocardial infarction (MI) underwent myocardial-stress perfusion scintigraphy (MSPS) with thallium-201, treadmill-stress testing (TST), and coronary cineangiography (CA). The sensitivities of MSPS and TST for coronary stenosis greater than or equal to 75% were 68% and 71%, respectively; their specificities were 97% and 79%, respectively (0.1 greater than p greater than 0.05). When the character of a patient's chest pain is considered, Bayesian analysis leads to the following conclusions: (a) MSPS can be useful in pre-CA screening of patients with chest pain but no MI if their pain is thought to be of uncertain or nonischemic origin: (b) the sensitivity of Tl-201 MSPS is not sufficient for pre-CA screening of patients without MI who have typical or atypical angina pectoris; (c) the sensitivity of MSPS would have to be approximately 95% in order for the test to be useful in pre-CA screening of patients who have atypical angina pectoris; (d) MSPS may be superior to TST in these applications; and (e) it is not clear that there is any advantage in combining MSPS and TST into a single screening test rather than using MSPS alone.