Nanofiber Mats and production methods thereof

• US 20060264140A1
• Filed: 05/17/2005
• Published: 11/23/2006
• Est. Priority Date: 05/17/2005
• Status: Active Grant

• Current Assignee: Research Triangle Institute
• Original Assignee: Research Triangle Institute
• Inventors: Ensor, David S., Walker, Teri A., Andrady, Anthony L., Prabhu, Purva
• Granted Patent: US 7,592,277 B2
• Time in Patent Office: Days
• US Class Current: 442/341
• CPC Class Codes: D01D 5/0061 characterised by the electro-spinning apparatus
• CPC Class Codes: D04H 1/4291 Olefin series
• CPC Class Codes: Y10T 428/29 Coated or structually defined flake, particle, cell, strand, strand portion, rod, filament, macroscopic fiber or mass thereof
• CPC Class Codes: Y10T 428/2913 Rod, strand, filament or fiber
• CPC Class Codes: Y10T 428/2915 including textile, cloth or fabric
• CPC Class Codes: Y10T 428/298 Physical dimension
• CPC Class Codes: Y10T 442/60 Nonwoven fabric [i.e., nonwoven strand or fiber material]
• CPC Class Codes: Y10T 442/614 Strand or fiber material specified as having microdimensions [i.e., microfiber]
• CPC Class Codes: Y10T 442/615 Strand or fiber material is blended with another chemically different microfiber in the same layer
• CPC Class Codes: Y10T 442/625 Autogenously bonded
• CPC Class Codes: Y10T 442/626 Microfiber is synthetic polymer
• CPC Class Codes: Y10T 442/659 Including an additional nonwoven fabric
• CPC Class Codes: Y10T 442/66 Additional nonwoven fabric is a spun-bonded fabric
• CPC Class Codes: Y10T 442/681 Spun-bonded nonwoven fabric
• CPC Class Codes: Y10T 442/69 Autogenously bonded nonwoven fabric
• CPC Class Codes: Y10T 442/692 Containing at least two chemically different strand or fiber materials
• CPC Class Codes: Y10T 442/696 Including strand or fiber material which is stated to have specific attributes [e.g., heat or fire resistance, chemical or solvent resistance, high absorption for aqueous compositions, water solubility, heat shrinkability, etc.]
• CPC Class Codes: Y10T 442/697 Containing at least two chemically different strand or fiber materials
• CPC Class Codes: Y10T 442/699 Including particulate material other than strand or fiber material

Abstract

An apparatus and method in which the apparatus includes a first electrospinning device configured to electrospin first fibers of a first substance, a second electrospinning device configured to electrospin second fibers of a second substance such that first and second fibers combine in a mat formation region, and a biasing device configured to bias the first electrospinning device with a first electric polarity and to bias the second electrospinning device with a second electric polarity of opposite polarity to the first electric polarity to promote attraction and coalescence between the first and second fibers. The method electrospins under the first electric polarity first fibers from the first substance, electrospins under the second electric polarity fibers from the second substance, and coalesces the first and second fibers to form the fiber mat.

Claims

1. An apparatus comprising:

a first electrospinning device configured to electrospin first fibers of a first substance;

a second electrospinning device configured to electrospin second fibers of a second substance; and

a biasing device configured to bias the first electrospinning device with a first electric polarity and to bias the second electrospinning device with a second electric polarity of opposite polarity to the first electric polarity to promote attraction and coalescence between the first and second fibers such that first and second fibers combine in a mat formation region.

2. The apparatus of claim 1, further comprising:

an enclosure configured to enclose at least the mat formation region; and

a control device configured to control an atmosphere in the enclosure so as to control at least one of an evaporation rate of a solvent from the first and second fibers and an electrical resistance of the atmosphere in the enclosure.

3. The apparatus of claim 2, wherein the control device comprises:

a vapor pool container disposed in the enclosure, configured to contain a liquid and provide a vapor pressure of the liquid to the atmosphere in the enclosure.

4. The apparatus of claim 3, wherein the control device comprises:

a temperature controller configured to control a temperature of the liquid in the vapor pool container.

5. The apparatus of claim 4, wherein the vapor pool container is configured to contain at least one of dimethylformamide, formamide, dimethylacetamide, methylene chloride, chlorobenzene, chloroform, carbon tetrachloride, chlorobenzene, chloroacetonitrile, carbon disulfide, dimethylsulfoxide, toluene, benzene, styrene, acetonitrile, tetrahydrofuran, acetone, methylethylketone, dioxanone, cyclohexanone, cyclohexane, dioxane, 1-nitropropane, tributylphosphate, ethyl acetate, phosphorus trichloride, methanol, ethanol, propanol, butanol, glycol, phenol, diethylene glycol, polyethylene glycol, 1,4butanediol, water, other acid, other alcohol, other ester alcohol, other ketone, other ester, other aromatic, other amide, and other chlorinated hydrocarbon.

6. The apparatus of claim 2, wherein the control device comprises:

a gas supply configured to supply a gaseous species to the enclosure.

7. The apparatus of claim 6, wherein the gas supply comprises:

a flow controller configured to control a flow rate of the gaseous species to the enclosure.

8. The apparatus of claim 6, wherein the gas supply comprises:

a supply of at least one of electronegative gases, non-electronegative gases, ions, and energetic particles.

9. The apparatus of claim 6, wherein the gas supply comprises:

a supply of at least one of CO₂, CO, SF₆, CF₄, N₂O, CCl₄, CCl₃F, and CCl₂F₂.

10. The apparatus of claim 2, wherein the control device is configured to control the solvent content of the first and second fibers in said mat formation region from 0% to 80 weight %.

11. The apparatus of claim 10, wherein the control device is configured to control the solvent content of the first and second fibers in said mat formation region to less than 2 weight %.

12. The apparatus of claim 10, wherein the control device is configured to control the solvent content of the first and second fibers in said mat formation region to be between 20 and 30 weight %.

13. The apparatus of claim 10, wherein the control device is configured to control the solvent content of the first and second fibers in said mat formation region to be between 20 and 80 weight %.

14. The apparatus of claim 1, wherein:

the first and second electrospinning devices are configured to electrospin a same material for the first and second substances or different materials for the first and second substances.

15. The apparatus of claim 1, wherein:

the first electrospinning device comprises a first extrusion element;

the second electrospinning device comprises a second extrusion element; and

the apparatus further comprises a gas shroud in a vicinity of the first and second extrusion elements.

16. The apparatus of claim 15, further comprising:

a gas supply connected to the gas shroud to supply at least one of electronegative gases, non-electronegative gases, ionized gases, non-ionized gases, and energetic particles.

17. The apparatus of claim 16, wherein the gas supply comprises:

a supply of at least one of CO₂, CO, SF₆, CF₄, N₂O, CCl₄, CCl₃F, and CCl₂F₂.

18. The apparatus of claim 1, further comprising:

a collection electrode disposed in the mat formation region and configured to collect the first and second fibers.

19. The apparatus of claim 1, further comprising:

a collection electrode disposed in the mat formation region and configured to rotate about an axis between the first electrospinning device and the second electrospinning device.

20. The apparatus of claim 18, wherein the collection electrode comprises:

at least one of a loop, a net, a hook, and a web.

21. The apparatus of claim 18, wherein the collection electrode comprises:

a grounded electrode.

22. The apparatus of claim 1, wherein:

the first electrospinning device comprises a first extrusion element;

the second electrospinning device comprises a second extrusion element; and

the biasing device comprises a power source connected to the first and second extrusion elements such that the first and second extrusion elements have opposite electric polarities.

23. The apparatus of claim 1, wherein:

the first electrospinning device comprises a first extrusion element;

the second electrospinning device comprises a second extrusion element; and

the biasing device comprises a first power source connected to the first extrusion element and a second power source connected to the second extrusion element.

24. The apparatus of claim 1, wherein the first electrospinning device comprises:

a compartment in which the first substance is stored; and

plural extrusion elements mounted in a wall of the compartment.

25. The apparatus of claim 24, further comprising:

an electrode inside the compartment and configured to radiate an electric field into said mat formation region.

26. The apparatus of claim 1, wherein the biasing device comprises:

a power supply configured to supply an electric field strength of 10,000 to 100,000 V/m in a vicinity of at least one of the first and second electrospinning devices to produce nanofibers having average diameters less than 1 μ

m along lengths thereof.

27. The apparatus of claim 1, wherein the biasing device comprises:

a power supply configured to supply an electric field strength of 50,000 to 200,000 V/m in a vicinity of at least one of the first and second electrospinning devices to produce nanofibers having average diameters less than 500 nm along lengths thereof.

28. The apparatus of claim 1, wherein the biasing device comprises:

a power supply configured to supply an electric field strength of 150,000 to 400,000 V/m in a vicinity of at least one of the first and second electrospinning devices to produce nanofibers having average diameters less than 100 nm along lengths thereof.

29. The apparatus of claim 1, further comprising:

a particle delivery device configured to deliver particles in a vicinity of the first and second electrospinning devices.

30. The apparatus of claim 29, wherein the particle delivery device comprises:

at least one of a nebulizer, an atomizer, and an electrospray device.

31. The apparatus of claim 29, wherein the particle delivery device comprises:

a particle source including a supply of at least one of a metallic material, an organic material, an oxide material, a semiconductor material, an electroluminescent material, a phosphorescent material, a medical compound, and a biological material.

32. A mat of fibers, comprising:

a plurality of intermixed first and second fibers; and

a first region including said plurality of intermixed fibers and having a cross section fiber density of at least (2.5×

10¹³)/d² fibers/cm², where a value of d is given in nm, is less than 500 nm, and comprises an average of diameters of the first and second fibers in a cross section of the mat.

33. The mat of claim 32, wherein said intermixed first and second fibers have an average separation distance less than d.

34. The mat of claim 32, wherein the first fibers comprise a material different than that of the second fibers.

35. The mat of claim 32, wherein the first fibers have a first elastic modulus and the second fibers have a second elastic modulus at least twice the first elastic modulus.

36. The mat of claim 35, wherein the first fibers comprise a material different than that of the second fibers.

37. The mat of claim 35, wherein the second elastic modulus is at least five times the first elastic modulus.

38. The mat of claim 37, wherein the first fibers comprise a material different than that of the second fibers.

39. The mat of claim 34, wherein the first fibers comprise hydrophobic fibers and the second fibers comprise hydrophilic fibers.

40. The mat of claim 39, wherein the hydrophobic fibers comprise at least one of poly(alkyl acrylate), polybutadiene, polyethylene, polylactones, polystyrene, polyacrylonitrile, polyethylene terephthalate), polysulfone, polycarbonate, and poly(vinyl chloride).

41. The mat of claim 39, wherein the hydrophilic fibers comprise at least one of poly(acrylic acid), poly(ethylene glycol), poly(vinyl alcohol), poly (vinyl acetate), cellulose, poly(acrylamide), proteins, poly (vinyl pyrrolidone), and poly(styrene sulfonate).

42. The mat of claim 32, wherein the first fibers have a first average diameter along a length thereof and the second fibers have a second average diameter along a length thereof different from the first average diameter.

43. The mat of claim 42, wherein the first average diameter is less than 10 μ

m and the second average diameter is less than 500 nm.

44. The mat of claim 42, wherein the first average diameter is less than 1 μ

m and the second average diameter is less than 100 nm.

45. The mat of claim 32, wherein the first region comprises:

a region in which the first fibers vary in number relative to a number of the second fibers along a predetermined direction of the mat.

46. The mat of claim 45, wherein a relative number of the first fibers to the second fibers varies along the predetermined direction.

47. The mat of claim 46, wherein the first fibers comprise a material different than that of the second fibers.

48. The mat of claim 47, wherein a relative number of the first fibers to the second fibers varies linearly along the predetermined direction.

49. The mat of claim 46, wherein the first fibers have an average diameter along a length thereof different than that of the second fibers.

50. The mat of claim 49, wherein a relative number of the first fibers to the second fibers varies linearly along the predetermined direction.

51. The mat of claim 32, further comprising:

a second region having more first fibers than second fibers; and

a third region having more second fibers than first fibers.

52. The mat of claim 51, wherein

the first fibers of said second region comprise hydrophobic fibers;

and

the second fibers of said third region comprise hydrophilic fibers.

53. The mat of claim 51, wherein

the first fibers of said second region have an average diameter less than 10 μ

m along lengths thereof; and

the second fibers of said third region have an average diameter less than 500 m along lengths thereof.

54. The mat of claim 51, wherein

the first fibers of said second region have an average diameter less than 1 μ

m along corresponding lengths thereof; and

the second fibers of said third region have an average diameter less than 100 nm along corresponding lengths thereof.

55. The mat of claim 32, wherein the first and second fibers comprise a same material.

56. The mat of claim 32, wherein the first and second fibers have the same average diameter.

57. The mat of claim 32, further comprising:

particles included with the mat.

58. The mat of claim 57, wherein the particles include at least one of a metallic material, an organic material, an oxide material, a semiconductor material, an electroluminescent material, a phosphorescent material, a medical compound, and a biological material.

59. A method of forming a fiber mat, comprising:

electrospinning under a first electric polarity first fibers from a first substance;

electrospinning under a second electric polarity of opposite polarity to the first electric polarity second fibers from a second substance; and

coalescing the electrospun first and second fibers to form the fiber mat.

60. The method of claim 59, wherein the electrospinning under a first electric polarity comprises:

biasing an extrusion element containing the first substance with the first electric polarity.

61. The method of claim 59, further comprising:

providing for the first and second substances materials of different chemical compositions.

62. The method of claim 59, wherein the coalescing comprises:

electrostatically attracting said first and second fibers to each other.

63. The method of claim 59, further comprising:

controlling an atmosphere in a vicinity of the electrospun first and second fibers so as to adjust at least one of an evaporation rate of a solvent from the first and second fibers and an electrical resistance of the atmosphere.

64. The method of claim 63, wherein the controlling an atmosphere comprises:

providing a vapor under pressure to the vicinity.

65. The method of claim 64, wherein the providing a vapor under pressure comprises:

controlling a temperature of a liquid to be vaporized.

66. The method of claim 65, wherein the controlling a temperature of a liquid comprises:

controlling a temperature of at least one of dimethylformamide, formamide, dimethylacetamide, methylene chloride, chlorobenzene, chloroform, carbon tetrachloride, chlorobenzene, chloroacetonitrile, carbon disulfide, dimethylsulfoxide, toluene, benzene, styrene, acetonitrile, tetrahydrofuran, acetone, methylethylketone, dioxanone, cyclohexanone, cyclohexane, dioxane, 1-nitropropane, tributylphosphate, ethyl acetate, phosphorus trichloride, methanol, ethanol, propanol, butanol, glycol, phenol, diethylene glycol, polyethylene glycol, 1,4butanediol, water, other acid, other alcohol, other ester alcohol, other ketone, other ester, other aromatic, other amide, and other chlorinated hydrocarbon.

67. The method of claim 63, wherein the controlling an atmosphere comprises:

providing a gas supply of at least one of electronegative gases, non-electronegative gases, ionized gases, non-ionized gases, and energetic particles.

68. The method of claim 63, wherein the controlling an atmosphere comprises:

supplying at least one of CO₂, CO, SF₆, CF₄, N₂O, CCl₄, CCl₃F, and CCl₂F₂.

69. The method of claim 63, wherein the controlling an atmosphere comprises:

controlling the solvent content of the first and second fibers in said mat formation region from 0% to 80 weight %.

70. The method of claim 69, wherein the coalescing the first and second fibers comprises:

combining fibers of the first and second fibers that have a solvent content less than 2 weight %.

71. The method of claim 69, wherein the coalescing the first and second fibers comprises:

combining fibers of the first and second fibers that have a solvent content between 20 and 30 weight %.

72. The method of claim 69, wherein the coalescing the first and second fibers comprises:

combining fibers of the first and second fibers that have a solvent content between 20 and 80 weight %.

73. The method of claim 59, further comprising:

collecting the first and second fibers on a collection electrode.

74. The method of claim 59, further comprising:

collecting the first and second fibers on a collection electrode that rotates about an axis between the first electrospinning device and the second electrospinning device.

75. The method of claim 73, wherein the collecting comprises:

collecting the first and second fibers on at least one of a loop, a net, a hook, and a web.

76. The method of claim 73, wherein the collecting comprises:

collecting the first and second fibers on a grounded electrode.

77. The method of claim 59, wherein the electrospinning steps comprise:

extracting the first and second fibers in opposing directions towards each other.

78. The method of claim 59, further comprising:

storing at least one of the first and second substances in a compartment having extrusion elements mounted in a wall of the compartment.

79. The method of claim 78, further comprising:

radiating an electric field from the compartment by an electrode disposed inside the compartment.

80. The method of claim 59, further comprising:

providing the first substance in a first solvent; and

providing the second substance in a second solvent.

81. The method of claim 80, further comprising:

providing as at least one of the first and second substances a polymeric compound included in one of the first and second solvents.

82. The method of claim 80, comprising:

providing a common solvent or different solvents for the first and second solvents.

83. The method of claim 59, wherein the electrospinning steps comprise:

electrospinning fibers of different average diameters along a length thereof.

84. The method of claim 59, wherein at least one of the electrospinning steps comprises:

applying an electric field strength of 10,000 to 100,000 V/m in a vicinity of at least one of the first electrospinning device and the second electrospinning device to produce nanofibers having an average diameter less than 1 μ

m along a length thereof.

85. The method of claim 59, wherein at least one of the electrospinning steps comprises:

applying an electric field strength of 50,000 to 200,000 V/m in a vicinity of at least one of the first electrospinning device and the second electrospinning device to produce nanofibers having an average diameter less than 500 nm along a length thereof.

86. The method of claim 59, wherein at least one of the electrospinning steps comprises:

applying an electric field strength of 150,000 to 400,000 V/m in a vicinity of at least one of the first electrospinning device and the second electrospinning device to produce nanofibers having an average diameter less than 100 nm along a length thereof.

87. The method of claim 59, further comprising:

delivering particles in a vicinity of the electrospun first and second fibers.

88. The method of claim 87, further comprising:

combining the particles with at least one of the electrospun first and second fibers.

89. The method of claim 87, wherein the delivering comprises:

combining the particles with at least one of the electrospun first and second fibers when at least one of the first and second fibers include a solvent content.

90. The method of claim 87, wherein the delivering comprises:

delivering particles formed by at least one of a nebulizer, an atomizer, and an electrospray device.

91. The method of claim 87, wherein the delivering comprises:

providing a source of the particles;

mixing the particles from the source with a gaseous carrier; and

entraining the particles in a regulated flow of the gaseous carrier.

92. The method of claim 87, wherein the delivering comprises:

providing particles including at least one of a metallic material, an organic material, an oxide material, a semiconductor material, an electroluminescent material, a phosphorescent material, a medical compound, and a biological material.

93. The method of claim 92, wherein the providing particles comprises:

providing nanoparticles having an average diameter less than 500 nm.

94. The method of claim 87, wherein the delivering comprises:

electrospraying said particles from a third substance.

95. The method of claim 59, wherein at least one of the electrospinning steps comprises:

electrospinning nanofibers having an average diameter less than 1 μ

m along the lengths thereof.

96. The method of claim 59, wherein at least one of the electrospinning steps comprises:

electrospinning nanofibers having an average diameter less than 500 nm along the lengths thereof.

97. The method of claim 59, wherein at least one of the electrospinning steps comprises:

electrospinning nanofibers having an average diameter less than 100 nm along the lengths thereof.

98. The method of claim 59, wherein the coalescing comprises:

combining the first and second fibers to produce a region in the fiber mat in which adjacent fibers have a separation less than an average diameter d of the first and second fibers.

99. The method of claim 59, wherein the coalescing comprises:

combining the first and second fibers to produce a region in the fiber mat in which the first and second fibers have a cross section fiber density of at least (2.5×

10¹³)/d² fibers/cm², where d is given in nm, and comprises an average of diameters of the first and second fibers in a cross section of the mat.

100. A composite filter comprising:

a plurality of intermixed first and second fibers defining composite intermixed fibers; and

the composite intermixed fibers having a cross section fiber density of at least (2.5×

10¹³)/d² fibers/cm², where d is given in nm, and comprises an average of diameters of the first and second fibers in a cross section of the filter.

101. A skin substitute comprising:

a membrane comprising plural hydrophilic fibers and plural hydrophobic fibers intermixed to form composite intermixed fibers;

the composite intermixed fibers having a cross section fiber density of at least (2.5×

10¹³)/d² fibers/cm², where d is given in nm, and comprises an average of diameters of the first and second fibers in a cross section of the skin substitute.

102. A filtration medium comprising:

a plurality of intermixed first and second nanofibers defining composite intermixed nanofibers; and

the composite intermixed nanofibers having a cross section fiber density of at least (2.5×

10¹³)/d² fibers/cm², where a value of d is given in nm, and comprises an average of diameters of the first and second fibers in a cross section of the medium,

wherein some of the first and second nanofibers have an average diameter less than 1 μ

m along the lengths thereof.

103. The mat of claim 102, wherein the first nanofibers comprise a material different than that of the second nanofibers.

104. The mat of claim 102, wherein the first nanofibers have a first elastic modulus and the second nanofibers have a second elastic modulus at least twice the first elastic modulus.

105. The mat of claim 102, wherein the first nanofibers comprise a material different than that of the second nanofibers.

106. An apparatus for forming fibers, comprising:

an electrospinning device including an extrusion element and configured to electrospin a fiber base material from the extrusion element to a fiber-extraction region removed from the extrusion element; and

a particle delivery device configured to deliver particles to the fiber-extraction region such that the delivered particles collide and combine with the electrospun fiber base material to form said fibers.

107. The apparatus of claim 106, further comprising:

an enclosure enclosing the electrospinning device, the particle delivery device, and the fiber extraction region.

108. The apparatus of claim 106, wherein the electrospinning device comprises a first longitudinal axis and the particle delivery device comprises a second longitudinal axis that intersects the first longitudinal axis in the fiber-extraction region.

109. The apparatus of claim 106, wherein the particle delivery device comprises:

at least one of a nebulizer, an atomizer, and an electrospray device.

110. The apparatus of claim 106, wherein the particle delivery device comprises:

a particle source; and

a gaseous carrier source in communication with particles output by the particle source.

111. The apparatus of claim 110, wherein the particle source comprises:

a supply of at least one of a metallic material, an organic material, an oxide material, a semiconductor material, an electroluminescent material, a phosphorescent material, a medical compound, and a biological material.

112. The apparatus of claim 106, wherein the particle source comprises:

a liquid having the particles suspended therein; and

a dryer configured to receive and dry the particles expelled from the liquid.

113. The apparatus of claim 106, further comprising:

a biasing device configured to bias the electrospinning device with a first electric polarity and to bias the particle delivery device with a second electric polarity opposite in sign to the first electric polarity.

114. The apparatus of claim 106, wherein the particle delivery device comprises:

a nanoparticle source configured to deliver particles having an average diameter less than 1 μ

m.

115. The apparatus of claim 106, wherein the particle delivery device comprises:

a nanoparticle source configured to deliver particles having an average diameter less 100 nm.

116. A method for forming fibers, comprising:

providing a fiber base material to an extrusion element of an electrospinning device;

electrospinning the fiber base material from the extrusion element into a fiber-extraction region removed from the extrusion element; and

delivering particles into the fiber-extraction region such that the particles collide and combine with the electrospun fiber base material during formation of the fibers.

117. The method of claim 116, wherein the electrospinning comprises applying a first electric potential to the electrospinning device, and the delivering particles comprises charging the particles to a second electric potential having an opposite polarity than the first electric polarity.

118. The method of claim 116, wherein the delivering particles comprises:

supplying particles from at least one of a nebulizer, an atomizer, and an electrospray device.

119. The method of claim 116, wherein the delivering particles comprises:

delivering particles having an average diameter less than 1 μ

m.

120. The method of claim 116, wherein the delivering particles comprise:

delivering particles having an average diameter less than 100 nm.

121. The method of claim 116, wherein the delivering particles comprise:

delivering particles of at least one of a metallic material, an oxide material, a semiconductor material, an electroluminescent material, a phosphorescent material, a medical compound, and a biological material.

122. A composite fiber mat comprising:

plural intermeshed fibers; and

particles directly attached to a fiber material of the at least one of the plural fibers along a length of the at least one of the fibers.

123. The mat of claim 122, wherein the particles are attached by the fiber material of the at least one of the plural fibers.

124. The mat of claim 122, wherein the particles comprise at least one of a metallic material, an organic material, an oxide material, a semiconductor material, an electroluminescent material, a phosphorescent material, a medical compound, and a biological material.

125. The mat of claim 122, wherein the particles comprise electrosprayed particles.

126. The mat of claim 122, wherein the at least one of the first and second fibers comprises a cross section fiber density of at least (2.5×

10¹³)/d² fibers/cm², where a value of d is given in nm, is less than 500 nm, and comprises an average of diameters of the first and second fibers in a cross section of the composite fiber mat.