Ultrafast Electric Field Pulse Control of Giant Temperature Change in Ferroelectrics.

There is a surge of interest in developing environmentally friendly solid-state-based cooling technology. Here, we point out that a fast cooling rate (≈10^{11} K/s) can be achieved by driving solid crystals to a high-temperature phase with a properly designed electric field pulse. Specifically, we predict that an ultrafast electric field pulse can cause a giant temperature decrease up to 32 K in PbTiO_{3} occurring on few picosecond time scales. We explain the underlying physics of this giant electric field pulse-induced temperature change with the concept of internal energy redistribution: the electric field does work on a ferroelectric crystal and redistributes its internal energy, and the way the kinetic energy is redistributed determines the temperature change and strongly depends on the electric field temporal profile. This concept is supported by our all-atom molecular dynamics simulations of PbTiO_{3} and BaTiO_{3}. Moreover, this internal energy redistribution concept can also be applied to understand electrocaloric effect. We further propose new strategies for inducing giant cooling effect with ultrafast electric field pulse. This Letter offers a general framework to understand electric-field-induced temperature change and highlights the opportunities of electric field engineering for controlled design of fast and efficient cooling technology.

Double Dirac Semimetals in Three Dimensions.

We study a class of Dirac semimetals that feature an eightfold-degenerate double Dirac point. We show that 7 of the 230 space groups can host such Dirac points and argue that they all generically display linear dispersion. We introduce an explicit tight-binding model for space groups 130 and 135. Space group 135 can host an intrinsic double Dirac semimetal with no additional states at the Fermi energy. This defines a symmetry-protected topological critical point, and we show that a uniaxial compressive strain applied in different directions leads to topologically distinct insulating phases. In addition, the double Dirac semimetal can accommodate topological line defects that bind helical modes. Connections are made to theories of strongly interacting filling-enforced semimetals, and potential materials realizations are discussed.

Dirac semimetal in three dimensions.

We show that the pseudorelativistic physics of graphene near the Fermi level can be extended to three dimensional (3D) materials. Unlike in phase transitions from inversion symmetric topological to normal insulators, we show that particular space groups also allow 3D Dirac points as symmetry protected degeneracies. We provide criteria necessary to identify these groups and, as an example, present ab initio calculations of ß-cristobalite BiO(2) which exhibits three Dirac points at the Fermi level. We find that ß-cristobalite BiO(2) is metastable, so it can be physically realized as a 3D analog to graphene.

Direct in situ determination of the polarization dependence of physisorption on ferroelectric surfaces.

The ability to manipulate dipole orientation in ferroelectric oxides holds promise as a method to tailor surface reactivity for specific applications. As ferroelectric domains can be patterned at the nanoscale, domain-specific surface chemistries may provide a method for fabrication of nanoscale devices. Although studies over the past 50 yr have suggested that ferroelectric domain orientation may affect the energetics of adsorption, definitive evidence is still lacking. Domain-dependent sticking coefficients are observed using temperature-programmed desorption and scanning surface potential microscopy, supported by first-principles calculations of the reaction coordinate. The first unambiguous observations of differences in the energetics of physisorption on ferroelectric domains are presented here for CH(3)OH and CO(2) on BaTiO(3) and Pb(Ti(0.52)Zr(0.48))O(3) surfaces.

Kinetic control of tunable multi-state switching in ferroelectric thin films.

Deterministic creation of multiple ferroelectric states with intermediate values of polarization remains challenging due to the inherent bi-stability of ferroelectric switching. Here we show the ability to select any desired intermediate polarization value via control of the switching pathway in (111)-oriented PbZr0.2Ti0.8O3 films. Such switching phenomena are driven by kinetic control of the volume fraction of two geometrically different domain structures which are generated by two distinct switching pathways: one direct, bipolar-like switching and another multi-step switching process with the formation of a thermodynamically-stable intermediate twinning structure. Such control of switching pathways is enabled by the competition between elastic and electrostatic energies which favors different types of ferroelastic switching that can occur. Overall, our work demonstrates an alternative approach that transcends the inherent bi-stability of ferroelectrics to create non-volatile, deterministic, and repeatedly obtainable multi-state polarization without compromising other important properties, and holds promise for non-volatile multi-state functional applications.

Natural history of the canine vein pouch aneurysm model.

BACKGROUND AND PURPOSE: The canine vein pouch aneurysm model is widely used for testing and development of devices directed at the endovascular treatment of aneurysms. Our purpose was to determine the incidence of spontaneous thrombosis and rupture of these aneurysms. MATERIALS AND METHODS: A retrospective review of laboratory records of canine vein pouch aneurysms made during a 6-year period was performed. The aneurysm and parent artery dimensions as well as incidences of spontaneous thrombosis and rupture were noted. RESULTS: During the interval studied, 326 vein patch aneurysms were made in 310 canines. Of these, 102 were sidewall (lateral) and 224 were bifurcation aneurysms. Spontaneous occlusion occurred in 9 of the sidewall aneurysms and in only 1 of the bifurcation aneurysms. None of the aneurysms ruptured. CONCLUSION: Spontaneous occlusion of the sidewall canine vein patch aneurysm occurred less than 10% of the time; in the bifurcation aneurysms, it almost never occurred. These characteristics enhance the value of this model for use in testing of devices intended for the endovascular treatment of aneurysms.

Origin and structure of polar domains in doped molecular crystals.

Doping is a primary tool for the modification of the properties of materials. Occlusion of guest molecules in crystals generally reduces their symmetry by the creation of polar domains, which engender polarization and pyroelectricity in the doped crystals. Here we describe a molecular-level determination of the structure of such polar domains, as created by low dopant concentrations (<0.5%). The approach comprises crystal engineering and pyroelectric measurements, together with dispersion-corrected density functional theory and classical molecular dynamics calculations of the doped crystals, using neutron diffraction data of the host at different temperatures. This approach is illustrated using centrosymmetric α-glycine crystals doped with minute amounts of different L-amino acids. The experimentally determined pyroelectric coefficients are explained by the structure and polarization calculations, thus providing strong support for the local and global understanding of how different dopants influence the properties of molecular crystals.

Evaluation of the TriSpan neck bridge device for the treatment of wide-necked aneurysms: an experimental study in canines.

BACKGROUND AND PURPOSE: Many wide-necked aneurysms are difficult or impossible to treat with the Guglielmi detachable coil (GDC). The purpose of this study was to evaluate the use of a neck bridging device, the TriSpan coil, in combination with standard GDCs for the treatment of wide-necked aneurysms in an experimental canine aneurysm model. METHODS: Of 24 experimental aneurysms in 12 animals, 19 (7 lateral and 12 terminal) were treated with the TriSpan coil in conjunction with standard GDCs. Digital subtraction angiography (DSA) was performed on all animals immediately after treatment. In 6 animals, follow-up DSA and histological evaluation were performed 4 weeks after treatment. In the remaining 6, DSA was done at both 90 and 180 days after treatment. Histological evaluation was done immediately after the 180-day angiographic evaluation. RESULTS: The TriSpan was easy to use in conjunction with the standard GDC. Because of their geometry, some lateral aneurysms were difficult or impossible to treat with this device. Greater than 90% aneurysm occlusion was obtained in all 19 aneurysms. In no instance was there evidence of coil migration, herniation, or aneurysm recanalization. Histological evaluation of the tissue surrounding the TriSpan coil showed tissue responses similar to that seen with standard GDCs. CONCLUSIONS: These results show that the TriSpan coil in conjunction with standard GDCs can be used safely and effectively for the treatment of wide-necked aneurysms in this canine model. Positioning and deployment of the neck bridge in aneurysms having an acute angle with the long axis of their parent artery are difficult or impossible. It is likely that this device, used in conjunction with the standard GDC, will allow treatment of some wide-necked aneurysms that are not treatable with the GDC alone.

Aneurysm hemodynamics: an experimental study.

PURPOSE: To study the flow of blood in aneurysms. METHODS: A canine model was used to study the hemodynamics of lateral, bifurcation, and terminal aneurysms with angiography and color Doppler techniques. FINDINGS: Flow within experimental aneurysms, although not laminar, is seldom if ever turbulent, but rather is highly predictable, varying primarily according to the relationship of the aneurysm to its parent artery. CONCLUSIONS: These studies support earlier in vitro work and provide further evidence that not all aneurysms share similar stresses. A more complete understanding of these hemodynamic features will be useful in the establishment of criteria that allow recognition of those aneurysms that are more or less likely to rupture, to grow, or to thrombose.

Intraarterial thrombolysis in a pig model: a preliminary note.

PURPOSE: To develop a pig model of arterial thrombosis suitable for assessing different methods of thrombolysis and to use this model to compare the efficacy of intraarterial thrombolysis performed by continuous proximal urokinase infusion versus mechanical clot disruption combined with intrathrombic urokinase injection. METHODS: In a control group of five pigs, a thrombus was made in a short segment of femoral artery and observed for 2 hours to assess its stability. In a treatment group of six pigs, intraarterial thrombolysis was performed immediately after thrombus formation. Thrombolysis was accomplished by continuously infusing urokinase into the proximal leading edge of the thrombus in three pigs and by mechanical clot disruption combined with intrathrombic urokinase injection in the remaining three pigs. RESULTS: There was no spontaneous reestablishment of flow in the control group during the 2-hour observation period. In the first treatment group, no flow was observed after a 1-hour treatment period when urokinase was infused continuously into the proximal edge of the thrombus. In the second treatment group, with mechanical clot disruption and intrathrombic urokinase injection, some degree of flow was observed in all three pigs. Reestablishment of flow was more sustained and of a greater degree with the addition of systemic heparinization. CONCLUSION: This animal model could provide a useful way to evaluate and compare different methods of thrombolysis. Our results suggest that mechanical clot disruption combined with intrathrombic urokinase injection is more effective in achieving reestablishment of flow than is continuous infusion of urokinase into the proximal edge of the thrombus.

Treatment of experimental canine carotid aneurysms with platinum coils.

PURPOSE: To evaluate and compare the deliverability, positioning, stability, and effectiveness of aneurysm occlusion and the incidence of parent-artery thrombosis of two different types of platinum coils, using a canine carotid aneurysm model. METHODS: 29 experimental canine carotid aneurysms (19 lateral, 6 bifurcation, and 4 terminal) were constructed and treated with complex-shaped fibered platinum coils and simple curved nonfibered platinum Guglielmi detachable coils (GDCs). RESULTS: Fibered complex coils were stable, producing 38% complete aneurysm occlusion and 61% average reduction in aneurysm lumen size but resulting in 19% parent artery occlusions. GDC coils were stable, producing 31% complete aneurysm occlusion and 95% average reduction in aneurysm lumen size with no parent-artery occlusions. CONCLUSIONS: GDC coils produced an average reduction in aneurysm lumen size of 95% without any associated parent-artery occlusions. There were no delayed migrations of GDC coils. The ability to remove, reposition, and detach a coil was the most significant feature of the GDC coil.

Canine model of terminal arterial aneurysm.

A canine model of terminal aneurysms using a venous pouch surgical technique is described. This model mimics the anatomy and hemodynamics of some types of carotid and basilar tip aneurysms. This technique produces aneurysms 15 x 21 mm in size. The aneurysms have been used to investigate the hemodynamics and treatment of terminal aneurysms.

Flow dynamics of lateral carotid artery aneurysms and their effects on coils and balloons: an experimental study in dogs.

PURPOSE: To investigate the hemodynamic characteristics of lateral carotid artery aneurysms in a canine model and to determine their influence on coils and balloons. MATERIALS AND METHODS: Forty aneurysms were created in fourteen dogs and their hemodynamic characteristics and influence on coils and balloons were evaluated with angiography and color Doppler pre- and postplacement. Twenty aneurysms were treated with coils, eight with balloons, and 12 aneurysms served as controls. RESULTS: The aneurysms demonstrated three distinct zones of flow: 1) an inflow zone entering at the distal aspect of the aneurysm ostium, 2) an outflow zone exiting at the proximal ostium, and 3) a central slow flow vortex. The inflow zone is a determining factor in the placement and stability of coils and balloons placed within the aneurysm and in the thrombosis of an aneurysm. The force of the inflow is considerable and can alter the shape of coils and displace both coils and balloons positioned within the aneurysm. CONCLUSIONS: Coils and balloons need to be of shapes and sizes that do not conform to the inflow and outflow zones. Filling the aneurysm and blocking or displacing the inflow zone can produce thrombosis of an aneurysm with preservation of the parent artery.

Collagen-coated acrylic microspheres for embolotherapy: in vivo and in vitro characteristics.

PURPOSE: To evaluate the in vivo and in vitro properties of collagen-coated acrylic microspheres and to compare them with polyvinyl alcohol (PVA) particles. METHODS: Samples of 100- to 300-microns, 300- to 500-microns, 500- to 700-microns, and 700- to 900-microns collagen-coated acrylic microspheres and 200- to 300-microns PVA particles were suspended in solutions of 50% saline and 50% contrast material. The samples were evaluated for quantitative and qualitative microscopic characteristics (shape, size, deformability); injectability via standardized microcatheters; degree of particulate penetration in the pig rete mirabile; and reaction of tissue to the particles in 48-hour- and 4-week-old specimens. RESULTS: The acrylic microspheres were spherical and deformable. The sample of 100- to 300-microns microspheres (n = 202) had a mean diameter of 210 microns (standard deviation, 43 microns). Hub accumulation, particle aggregation, and catheter occlusion were not observed with the microspheres (all sizes) but were noted with the PVA particles. The 200- to 300-microns PVA particles formed aggregates in the proximal rete. The 100- to 300-microns microspheres were found throughout the rete and beyond. Chronic transmural and perivascular inflammation was observed with the microspheres and the PVA particles. CONCLUSIONS: Particle aggregation and catheter occlusion do not complicate the transcatheter delivery of collagen-coated acrylic microspheres as they do with PVA particles. For a given particle and vessel size, acrylic microspheres penetrate to a much greater extent than the PVA particles. Tissue reaction to acrylic microspheres and PVA particles is similar.

An endovascular retrieving device for use in small vessels.

A new retrieval device for use in small vessels was evaluated in a canine model. Retrieval was attempted on 25 coils, three silicone balloons, and five catheter and guide wire fragments placed in the renal, pulmonary, hepatic, brachiocephalic, subclavian, carotid, and maxillary arteries. This was successful in 91% (30/33). The snare could also be selectively placed in the vertebral and proximal basilar arteries.

Treatment of carotid artery aneurysms with platinum coils: an experimental study in dogs.

The objectives of this study were to investigate the consistency, patency, and natural history of a vein graft canine aneurysm model and to determine the effectiveness of various coil designs on inducing aneurysm thrombosis. Twenty-one sacculuslike aneurysms were created in mongrel dogs by anastomosing a vein pouch to the common carotid artery. The model produced wide-neck aneurysms with 100% patency. The canine vein graft aneurysm provided an excellent model for the evaluation of endovascular devices. Three types of specially designed platinum coils were placed in the aneurysms: those with simple curves, those with complex curves, and those with flower petal curves and silk fibers. These coils were placed by the endovascular route by means of microcatheters. Flower petal coils with silk fibers were effective in producing thrombosis of the aneurysms, suggesting that coils of the appropriate design may be useful in the endovascular treatment of aneurysms. The other coil designs evaluated, those with simple and complex curves without silk fibers, demonstrated insufficient thrombogenicity and spatial stability.

Utility of CT angiography and MR angiography for the follow-up of experimental aneurysms treated with stents or Guglielmi detachable coils.

BACKGROUND AND PURPOSE: Previous studies have depicted arterial and aneurysmal anatomy with three-dimensional time-of-flight (3D-TOF) MR angiography before and after treatment with Guglielmi detachable coils (GDCs) and with CT angiography before and after treatment with stents and stent-grafts. We investigated the ability of time-resolved contrast-enhanced 3D MR angiography (3D MR digital subtraction angiography [DSA]) to accurately depict the anatomy of experimental lateral aneurysms before and after treatment with GDCs and a variety of stents or stent-grafts, and compared these findings with 3D-TOF MR angiography without and with contrast enhancement and CT angiography. METHODS: Two nitinol stents, two nitinol-polytetrafluoroethylene (PTFE) stent-grafts, and two stainless steel stents were deployed in three dogs with experimental carotid aneurysms. In a fourth animal, one of three aneurysms was completely occluded with GDCs. The other two aneurysms were loosely packed to ensure persistence of some residual aneurysmal lumen. Cut-film angiography, CT angiography, 3D-TOF MR angiography without and with contrast enhancement, and 3D MR DSA were performed in all dogs before and 3 weeks after treatment. RESULTS: 3D MR DSA was superior to conventional 3D-TOF MR angiography without and with contrast enhancement in accurately depicting experimental lateral aneurysms and superior to CT angiography in depicting aneurysms treated by GDCs. 3D MR DSA and CT angiography were comparable in depicting vessels treated with nitinol stents and stent-grafts, whereas CT angiography was superior for showing vessels treated by stainless steel stents. CONCLUSION: We recommend further development and clinical evaluation of 3D MR DSA for imaging cerebral aneurysms before and after treatment with GDCs. 3D MR DSA or CT angiography may be useful for evaluating vessels containing nitinol stents or nitinol-PTFE stent-grafts, whereas CT angiography should be used for follow-up of vessels treated by stainless steel stents.

Glutaraldehyde cross-linked collagen (GAX): a new material for therapeutic embolization.

In a search for a better agent for use in therapeutic embolization, a newly available bovine collagen product, glutaraldehyde cross-linked collagen (GAX), was evaluated to determine its effectiveness in causing arterial obstruction, its persistence after embolization, and the acute and chronic pulmonary toxicity resulting from direct pulmonary embolization. GAX is an effective agent for causing arterial obstruction: 3-4 ml caused prompt flow arrest when injected into the internal iliac artery of six dogs. In this canine model, the material persisted within embolized tissue for as long as 2 months, and at follow-up intervals of 2 days, 2 weeks, and 2 months, its presence did not produce any cellular response. Studies of both acute and chronic pulmonary toxicity reveal that when GAX is embolized directly into the pulmonary circulation it causes adverse effects only by mechanical blockage of pulmonary arteries. GAX offers several advantages over other currently available agents and is of sufficient safety that clinical trials in humans can be undertaken.

The effect of flow arrest on distal embolic events during arterial occlusion with detachable coils: a canine study.

PURPOSE: To determine the effect of proximal flow arrest on the frequency and timing of distal embolic events during occlusion of the common femoral artery with detachable coils. METHODS: Twenty-three complex fibered platinum coils were delivered into 10 common femoral arteries without proximal flow arrest. The arteries were continuously monitored for flow and embolic events by Doppler sonography during delivery and for at least 10 minutes after delivery of each coil. Thirty-four coils were delivered into 6 arteries after proximal flow arrest by inflation of a nondetachable balloon. After balloon deflation, each artery was monitored by Doppler sonography for 10 minutes. RESULTS: In the 10 arteries occluded without flow arrest, 87 events (8.7 per artery) occurred, of which 47 were embolic and 40 were indeterminate. In the 6 arteries with flow arrest, the number of emboli detected was 3 (0.5 per artery). Embolic events occurred only if there was residual flow. In those arteries that were occluded when the flow-arrest balloon was deflated, no emboli were detected. CONCLUSIONS: Proximal flow arrest virtually eliminates the risk of distal emboli during arterial occlusion with detachable fibered coils. The use of fibered coils, in conjunction with proximal flow arrest, allows for safe arterial occlusion when detachable balloons are not available or their use is not feasible.

Effect of Guglielmi detachable coil placement on intraaneurysmal pressure: experimental study in canines.

BACKGROUND AND PURPOSE: Treatment of ruptured aneurysms with Guglielmi detachable coils (GDCs) has been shown to prevent repeat bleeding. To assess whether GDC coiling alters aneurysmal pressure, we measured intraaneurysmal pressure in two canine types of carotid artery aneurysms before and after GDC placement. METHODS: A 0.014-inch guidewire with a pressure transducer was inserted into parent arteries and domes of surgically created aneurysms. Intravascular static pressures were recorded before and during saline power injections (10, 20, and 30 mL over 1 and 2 s), before and after GDC placement. Common femoral arterial pressure was monitored. RESULTS: Saline power injections reproducibly and abruptly increased pressure in parent arteries and aneurysms. Mean intraaneurysmal pressure varied (18 +/- 4 [10 mL] to 75 +/- 15 mm Hg [30 mL]), independent of injection duration. Intraaneurysmal baseline pressures were higher after GDC placement (111 +/- 10 versus 93 +/- 15 mm Hg; P =.05). Aneurysmal pressure increases with saline injections were slightly higher after GDC placement, which dampened intraaneurysmal pressure amplitude at baseline (26.5 +/- 5.6 versus 19.6 +/- 7.4 mm Hg; P =.003) and during hypertension (25.3 +/- 5.4 versus 19.8 +/- 7.5 mm Hg, P =.002). The pressure increase slope with saline injection was delayed with GDC placement (0.24 +/- 0.1 versus 0.38 +/- 0.19 s; P <.001). CONCLUSION: Graded saline power injections into parent arteries can rapidly increase intraaneurysmal pressure. GDC treatment did not attenuate mean intraaneurysmal pressures, but both dampened the pressure amplitude and delayed pressure increases during locally induced hypertension.