Abstract:
The invention features a method for producing replicas of a desired structure, a master and a mold obtained from it for use in such a method. The master includes a desired structure ( 200 ) to be reproduced and an additional structure ( 206, 208 ) that assists in removal of the master from a mold, e.g. by permitting portions of the mold on opposite sides of the additional structure to flex away from one another when the master is removed from the mold. The method includes immersing the master comprising the desired structure ( 200 ) and the additional structure ( 206, 208 ) in a molding material ( 142 ), solidifying the molding material ( 142 ) to make a mold, removing the master from the mold and using the mold to produce replicas of the desired structure ( 200 ). For example, the desired structure of the master may include a loop that defines a fluid flow pathway, and the additional structure includes a membrane configured to prevent the molding material from completely passing through the loop when the mold is made from the master.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from U.S. Provisional Patent Application No. 60/658,192, filed on Mar. 2, 2005, the contents of which is incorporated herein by reference in its entirety. 
    
    
     STATEMENT OF FEDERALLY SPONSORED RESEARCH 
     This invention was made with Government support under National Science Foundation (NSF) Grant Numbers ECS-0088438 and ECS-0210497. Thus, the Government has certain rights in the invention. 
    
    
     TECHNICAL FIELD 
     This invention relates to structures, and to methods of replicating the same, or portions of the same. 
     BACKGROUND 
     Three-dimensional structures can be fabricated by a variety of techniques, including machining, stereo-lithography and multi-photon absorption polymerization (MAP). Such three-dimensional structures are often referred to as a “masters” when they are used in a replication process. Replicating masters can be achieved by using transfer molding, or micro-transfer molding (μ-TM) when features to be replicated are small, e.g., less than 500 μm. Micro-transfer molding has been reviewed by Whitesides in  Accounts of Chemical Research,  35 (7), 491-499 (2002). 
     Referring to  FIGS. 1A, 1B, 1C and 1D , a mold  30  can be made from a master  10 , by pouring a first molding material, e.g., a silicone molding material, e.g., poly(dimethylsiloxane) (PDMS), about master  10 , and then allowing the molding material to set, e.g., vulcanize, producing a separable master-mold complex  20 . Separation of complex  20  frees master  10  and mold  30 . Replica  50  can be prepared from mold  30  by pouring a second molding material, e.g., a polyurethane or an acrylic molding material, into mold  30 , and then allowing the second molding material to set, producing a separable replica-mold complex (not shown). Separation of the replica-mold complex frees mold  30  and replica  50 . Master  10  can be used a number of times to produce a number of molds, and the resulting molds can be used a number of times to produce a number of replicas. 
     SUMMARY 
     Generally, structures and methods of replicating the same, or portions of the same, are disclosed. For example, in some methods for producing replicas of a desired structure, a master is provided that includes the desired structure and an additional structure or feature that assists in removal of the master from a mold. For example, the desired structure can define a fluid flow pathway and the additional structure a membrane, e.g., a thin wall, blocking the fluid flow pathway, or the desired structure can have reentrant features and the additional structure a projection extending outwardly from the desired structure. 
     In general, in one aspect, the invention features a method for producing replicas of a desired structure. The method includes: (i) providing a master including the desired structure and additional structure different from the desired structure; (ii) immersing the master in a molding material; (iii) solidifying the molding material to make a mold of the master; (iv) removing the master from the mold, wherein the additional structure in the master assists in the removal of the master from the mold; and (v) using the mold to produce the replicas of the desired structure. 
     Embodiments of the method may include any of the following features. 
     The desired structure may define a fluid flow pathway and the additional structure may include a membrane blocking the fluid flow pathway. 
     For example, the fluid flow pathway may pass through the desired structure, or the desired structure may include a loop that defines the fluid flow pathway, and the membrane may be configured to prevent the molding material from completely passing through the loop. For example, the loop may be a closed loop. Alternatively, the additional structure may include a membrane extending away from the desired structure. 
     The additional structure may be a membrane that is thin compared to a thickness of the desired structure. For example, the membrane may have a thickness less than about 2 micron. In another example, a ratio of the thickness of the desired structure to that of the membrane may be greater than about 5:1. 
     The desired structure may have a cross-sectional dimension less than about 50 microns. 
     The master may be removed from the mold along a first direction, and the additional structure may assist in the removal of the master from the mold by preventing the molding material from completely surrounding a portion of the desired structure in any plane including the first direction. For example, the additional structure may be a membrane preventing the molding material from completely passing through a loop in the desired structure. Furthermore, the additional structure may assist in the removal of the master from the mold by permitting portions of the mold on opposite sides of the additional structure to flex away from one another when the master is removed from the mold. 
     The master may be removed from the mold along a first direction, and the additional structure may extend into the mold in a direction opposite the first direction. For example, the additional structure may assist in the removal of the master from the mold by permitting portions of the mold on opposite sides of the additional structure to flex away from one another when the master is removed from the mold. 
     Using the mold to produce the replicas may include filling regions of the mold corresponding to the desired structure of the master with a replication material while not substantially filling regions of the mold corresponding to the additional structure of the master with the replication material. For example, the regions of the mold corresponding to the desired structure may be substantially larger than the regions of the mold corresponding to the additional structure, and the replication material may have a viscosity that prevents the substantial filling of the regions of the mold corresponding to the additional structure. For example, a ratio of a dimension of the desired structure to a dimension of the additional structure may be greater than about 5:1. 
     Using the mold to produce the replicas may further include filling selected regions of the mold with a replication material by applying a force to the mold to preferentially fill the selected regions. For example, the force may be applied through a vacuum or by a mechanical clamp. The selected regions of the mold may correspond to the desired structure, and not the additional structure. 
     Using the mold to produce the replicas of the desired structure may include using the mold to produce a replica of the master including the desired structure and the additional structure, and removing the additional structure from the replica of the master. For example, removing the additional structure from the replica may include sonicating the replica to weaken an attachment of the additional structure to the desired structure in the replica. 
     In another aspect, the invention includes a mold or a replica made by the above method. 
     In general, in another aspect, the invention includes a master for use in a method for producing replicas of a desired structure. The master includes the desired structure and additional structure different from the desired structure, wherein the additional structure in the master assists in a removal of the master from a mold used to produce the replicas of the desired structure. 
     The master may include any of the following features. 
     The desired structure may have a cross-sectional dimension less than about 50 microns. 
     The desired structure may define a fluid flow pathway and the additional structure may include a membrane blocking the fluid flow pathway. For example, the desired structure may include a loop that defines the fluid flow pathway, and the membrane may be configured to prevent a molding material from completely passing through the loop when the mold is made from the master. 
     The additional structure may include a membrane extending away from the desired structure. 
     The additional structure includes a membrane having a thickness less than about 2 micron. A ratio of a thickness of the desired structure to that of the membrane may be greater than about 5:1. 
     The master may farther include any of the features described above in connection with the method. 
     In general, in another aspect, the invention features a mold of a master for use in a method for producing replicas of a desired structure, wherein the master includes the desired structure and additional structure different from the desired structure, wherein the additional structure in the master assists in a removal of the master from a mold used to produce the replicas of the desired structure, wherein the mold includes a cavity in a shape of the master. 
     The mold may include any of the following features. 
     A region of the cavity corresponding to the desired structure may have a cross-sectional dimension less than about 50 microns. 
     The desired structure may define a fluid flow pathway and the additional structure may include a membrane blocking the fluid flow pathway. For example, the desired structure of the master may include a loop that defines the fluid flow pathway, and the membrane may be configured to prevent molding material from completely passing through the loop when the mold is made from the master. 
     The additional structure may include a membrane extending away from the desired structure. 
     The additional structure may include a membrane having a thickness less than about 2 micron. A ratio of a thickness of the desired structure to that of the membrane may be greater than about 5:1. 
     The mold may further include any of the features described above in connection with the method. 
     Embodiments and/or aspects may include any one or combination of the following advantages. The structures and methods described herein allow for replication of small, complex three-dimensional structures. For example, structures having a maximum transverse dimension of less than 100 μm, e.g., 75 μm, 50 μm or less, e.g., 25 μm, can be replicated with high fidelity and in large numbers. Structures having fluid flow pathways defined therethrough can be produced and masters can be replicated that have high aspect ratios and/or reentrant features. 
     The structures and methods described herein are useful, e.g., in the fields of microelectronics and microfluidics. For example, the structures can be used in microelectromechanical systems (MEMS) devices, e.g., micro-inductors, and medical devices. 
     All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specifications control. 
     Other features and advantages of the invention will be apparent from the following detailed description, and from the claims. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIGS. 1A, 1B, 1C and 1D  schematically illustrate making a replica from a master. 
         FIGS. 2A and 2B  are perspective and a side views, respectively, of a structure having two fluid flow pathways defined therethrough. 
         FIG. 3A  is a schematic side view of a structure having two fluid flow pathways defined therethrough, the structure being positioned above a molding material contained by a mold frame. 
         FIGS. 3B and 3C  are schematic side and cross-sectional views, respectively, of the structure of  FIG. 3A  immersed in the molding material. 
         FIGS. 4A, 4B and 4C  are perspective, side and cross-sectional views, respectively, of a structure having walls blocking fluid flow pathways through the structure. 
         FIG. 5A  is a schematic side view of the structure of  FIG. 4B  positioned above a molding material contained by a mold frame. 
         FIGS. 5B and 5C  are schematic side and cross-sectional views, respectively, of the structure of  FIG. 4B  immersed in the molding material. 
         FIGS. 6A and 6B  are side and cross-sectional views, respectively, of a mold fabricated using the structure of  FIG. 4B . 
         FIG. 7A  is a scanning electron micrograph at 2500 times magnification of a structure made by multi-photon absorption polymerization (MAP) having walls blocking fluid flow pathways through the structure. 
         FIG. 7B  is a scanning electron micrograph at 2500 times magnification of a replica of the structure of  FIG. 7A  without replication of blocking walls. 
         FIG. 8A  is a scanning electron micrograph at 1700 times magnification of a replica in which a wall blocking a fluid flow pathway through the structure is partially replicated. 
         FIG. 8B  is a scanning electron micrograph at 1900 times magnification of the replica of  FIG. 8A  after removal of the partially replicated blocking wall. 
         FIG. 9A  is a scanning electron micrograph at 1500 times magnification of a stool-shaped master having interior walls blocking fluid flow pathways through the stool-shaped structure. 
         FIG. 9B  is a cross-sectional view of the master of  FIG. 9A , taken along  9 B- 9 B. 
         FIG. 9C  is a scanning electron micrograph at 1500 times magnification of a replica of the stool-shaped master of  FIG. 9A  after walls have been removed. 
         FIGS. 10A and 10B  are side and front views, respectively, of a hook-shaped master. 
         FIGS. 11A and 11B  are side and front views, respectively, of a hook-shaped master having a blocking wall. 
         FIG. 12A  is a schematic side view of a triangular structure extending from a base positioned above a molding material contained by a mold frame. 
         FIG. 12B  is a schematic side view of the structure of  FIG. 12A  immersed in the molding material. 
         FIG. 13A  is a schematic side view of a structure that includes a thin membrane positioned above a liquid molding material contained by a mold frame. 
         FIG. 13B  is a schematic side view of the structure of  FIG. 13A  in a set molding material. 
         FIG. 13C  is a schematic side view illustrating removal of the structure of  FIGS. 13A and 13B  from a mold. 
     
    
    
     DETAILED DESCRIPTION 
     In general, structures, methods and molds are disclosed. For example, masters useful for producing replicas of a desired structure include the desired structure and an additional structure or feature different from the desired structure. The additional structure or feature in the master, e.g., a membrane blocking a fluid flow pathway, assists in removal of the master from a mold, e.g., by preventing mold-lock or by making the mold more flexible. 
     Referring to  FIGS. 2A and 2B , which are perspective and side views, respectively, an A-shaped structure  90  having flat sides includes a first and a second fluid flow path  100 ,  110  defined through the structure. First fluid flow path  100  is a closed flow path in that it is entirely bounded by structure  90 . Walls  101 ,  103 ,  105  and  107  of flow path  100  define a closed loop  111 . Second fluid flow path  110  is an open flow path in that it is not completely bounded by structure  90 , but is open at a side  120 . Walls  113 ,  115  and  117  of second flow path  110  define an open loop  121 . 
     Referring to  FIG. 3A , structure  90  extends downwardly from and is integral with, e.g., bonded to or contiguous with, a front surface  129  of a base  130 , such that two closed fluid flow paths (closed loops) are defined. The first flow path  100  is bounded entirely by structure  90 , and a second fluid flow path  135  is bounded by structure  90  and the front surface  129  of the base  130 .  FIG. 3A  shows structure  90  positioned above an unset, e.g., unvulcanized, liquid molding material  142 , e.g., poly(dimethylsiloxane), contained within a mold frame  140 . Referring now to  FIGS. 3B and 3C , which are side and cross-sectional views, respectively, after immersion of structure  90  into the unset, liquid molding material  142 , the liquid molding material  142  flows into fluid flow paths  100 ,  135 , and through structure  90 . After the liquid molding material  142  sets and solidifies, e.g., vulcanizes, a solid material, e.g., cross-linked poly(dimethylsiloxane), passes though structure  90  via flow paths  100 ,  135 , making it impossible to remove structure  90  from the set material by applying a force in an upwardly direction (indicated by arrow  151 ) without breaking the solid mold material or structure  90 . This phenomenon is often referred to as “mold-lock.” We have discovered certain structures and methods that effectively circumvent this mold-lock phenomenon, as described below. 
     Referring to  FIGS. 4A, 4B and 4C , which are perspective, side and cross-sectional views, respectively, structure  200  extends upwardly from and is integral with a front surface  202  of a base  204 , e.g., a base made of glass or plastic. Structure  200  has two walls  206  and  208  that bisect each flow path, preventing fluid flow through structure  200 . 
     Referring particularly to  FIGS. 4B and 4C , in some embodiments, structure  200  has a maximum height (H) of less than less than about 250 μm, e.g., 200 μm, 150 μm, 100 μm, 75 μm, 50 μm, 25 μm or less, e.g., 10 μm, a maximum lateral dimension (W) of less than about 200 μm, e.g., 150 μm, 100 μm, 75 μm, 50 μm, 25 μm or less, e.g., 10 μm, and a maximum cross-sectional dimension (T), taken perpendicular to base  204 , of less than about 100 μm, e.g., 75 μm, 50 μm, 25 μm or less, e.g., 10 μm. In some embodiments, each wall has a thickness (t) of less than 25 μm, e.g., 20 μm, 15 μm, 10 μm, 7.5 μm, 5.0 μm, 2.5 μm, 2.0 μm, 1.0 μm, 0.5 μm, 0.25 μm or less, e.g., 0.1 μm. 
     In particular embodiments, a ratio of the cross-sectional dimension (T) to the wall thickness (t) is greater than about 5, e.g., 7.5, 10, 15, 25 or more, e.g., 50. For example, in some implementations, the maximum cross-sectional dimension (T) is less than 50 μm and each wall  206 ,  208  has a wall thickness (t) of less than 2 μm, e.g., 1 μm, 0.5 μm or less, e.g., 0.25 μm. 
     In some embodiments, structure  200  is fabricated using multi-photon absorption polymerization (MAP). In a typical procedure, a cross-linkable resin, e.g., an acrylic resin, e.g., a mixture of ethoxylated trimethylolpropane triacrylate and tris(2-hydroxyethyl)isocyanurate, both available from Sartomer Company, Exton, Pa., is mixed with a photo-initiator, e.g., ethyl-2,4,6-trimethylbenzoylphenylphosphinate (Lucirin® TPO-L) available from BASF. The mixture of the cross-linkable resin and photo-initiator is then placed atop a substrate, e.g., a glass microscope slide that has been treated with adhesion promoter, e.g., (3-acryloxypropyl)-trimethoxysilane. The adhesion promoter promotes adhesion between the cross-linkable resin and the substrate. In such embodiments, the substrate is base  204  shown in  FIG. 4B . Polymerization of the cross-linkable resin is performed using a tunable laser oscillator, e.g., a Ti:Sapphire laser oscillator, such as the Mira™ Optima™ 900-F laser oscillator with 100 fs pulses available from Coherent, Inc., Santa Clara, Calif. Typically, laser output is directed through a Faraday isolator, a prism dispersion compensator and a beam expander before being introduced through a reflected light port to an upright microscope, e.g., a Zeiss AxioPlan2. Position of a focal volume is controlled with a computerized stage, e.g., a LUDL BioPrecision motorized microscope stage available from SpectraCore, Inc. If desired, the sample can be visualized during fabrication by using a charge-coupled camera. After polymerization is complete, unexposed resin that is not part of the fabricated structure  200  is washed away with a solvent, e.g., ethanol. Additional details regarding MAP have been described by Fourkas in  J. Appl. Phys.,  95(11), 6072-6076 (2004) and  J. Phys. Chem. B,  108, 11256-11258 (2004). 
       FIG. 5A  shows inverted structure  200  positioned above unset, liquid molding material  142  contained within mold frame  140 . Referring now to  FIGS. 5B and 5C , which are side and cross-sectional views, respectively, when structure  200  is immersed into the liquid molding material  142 , the liquid molding material is prevented from flowing through structure  200  by blocking walls  206  and  208 . As a result, after the liquid molding material sets, it is possible to remove structure  200  from the set material when the set material is relatively soft, e.g., has a Shore A hardness of less than about 100, by applying a force in an upwardly direction (indicated by arrow  211 ). Walls  206  and  208  effectively circumvent the mold-lock phenomenon described above. 
     Referring to  FIGS. 6A and 6B , which are front and cross-sectional views, respectively, after removal of structure  200  from the set molding material  262 , a mold  260  is produced that can be used to make replicas. Mold  260  includes a cavity  264  surrounded by set molding material  262 . Cavity  264  in cross-section includes two relatively thin regions  300  and  302 , and two relatively thick regions  303  and  305 . Thin regions  300  and  302  correspond to walls  206  and  208 , respectively, of structure  200 , while thick regions  303  and  305  correspond to regions  201  and  203 , respectively of structure  200  ( FIG. 4C ). 
     Referring to  FIGS. 4C, 6A and 6B , dimensions of cavity  264  are approximately equal to the corresponding dimensions of the master from which the cavity is formed. Namely, (H) is approximately equal (H′), W is approximately equal to (W′), (T) is approximately equal to (T′) and (t) is approximately equal to (t′). Any differences in dimensions arise from shrink in the molding material. For a cure at room temperature, typical shrinkage observed in an amorphous molding material, e.g., poly(dimethylsiloxane), is between about 0.25 percent and about 0.5 percent, and typical shrinkage observed in a crystalline molding material is between about 0.5 percent and about 2 percent. At higher cure temperatures, typical shrinkage can be up to about 5%. 
     In some embodiments, the liquid molding material is poly(dimethylsiloxane). Poly(dimethylsiloxane) is often supplied as two components, a base and a curing agent. A suitable poly(dimethylsiloxane) is Sylgard® 184 silicone elastomer available from Dow Corning. Cross-linking occurs when silicon hydride groups present in the curing agent react with vinyl groups present in the base. Sylgard® 184, mixed at 10 volume parts base to 1 volume part curing agent, has an initial viscosity of about 3,900 centipoise, an initial specific gravity of about 1.03 g/cm 3 , a room temperature working time of about 2 hours, a room temperature cure time of about 48 hours and a heat cure time of about 45 minutes at 100° C. The cured elastomer has a Shore A hardness durometer of about 50. We have found that Sylgard® 184 replicates with high fidelity features of a master, and its low surface energy of reduces sticking to the master. In addition, its softness and flexibility aids in removal of masters without damaging the mold or the master. 
     Referring particularly to  FIG. 6B , mold  260  can be used to mold replicas. When it is desired to produce structures having fluid flow pathways defined through the structure, molding conditions and/or the mold are adjusted such that regions  300  and  302  of cavity  264  are not filed with unset, liquid replication material. For example, structure  90  of  FIG. 2A  can be produced by filling all regions of mold cavity  264 , except regions  300  and  302 . When it is desired to produce structures having walls blocking fluid flow pathways through the structure, molding conditions are adjusted such all regions of mold cavity  264  are filled, including regions  300  and  302  of cavity  264 , with unset, liquid replication material. For example, structure  200  of  FIG. 4A  can be replicated by filling entire mold cavity  254 . In embodiments where regions  300  and  302  are filled, the resulting replicas can be used to produce structures having fluid flow pathways defined through the structure by removing the walls in a secondary operation. For example walls can be removed by sonicating the replica having walls blocking fluid flow pathways to weaken an attachment of the walls to the structure, or by ablating the undesired walls with a laser. Additional removal techniques include plasma etching and removal by solvent evaporation induced capillary forces. 
     In an exemplary embodiment, Sylgard® 184 silicone elastomer molding material is prepared, and then centrifuged to remove entrapped air bubbles. An O-ring that has been greased, e.g., with silicone grease, is placed around a master that extends outwardly from a base. Silicone molding material is poured in over the master to fill the volume encompassed by the O-ring, the O-ring becoming the mold frame in the set up mold. In an exemplary embodiment in which a single master is used, a final mold is 3 mm deep and has a transverse cross-sectional area of approximately 3 cm 2 . To cure the mold, the entire master-molding material assembly is placed in an oven at 150° C. for 15 minutes, and then the assembly is removed from the oven to cool. The set up mold is carefully peeled off the master. Replicating material, e.g., acrylic replicating material, is poured into the mold and a substrate, e.g., a glass slide, is pressed against the mold so that the replica becomes integrally bound to the substrate when the replicating material sets. Setting the replicating material can be achieved by exposing the replicating material in the mold to radiation, e.g., a ultra-violet light source. Removal of the replica is achieved by pulling on the substrate. 
     In some embodiments, should it be desired to not fill regions  300  and  302  of cavity  264 , hydrophobicity of the set molding material, as determined by contact angle θ (ASTM D 5946-04), viscosity of the liquid replication material, thickness (t′) of regions  300  and  302 , and pressure within the cavity are adjusted such that the liquid replication material is not drawn into regions  300  and  302  of cavity  264  by capillary forces. For example, regions  300  and  302  of cavity  264  are not filled when the set molding material has a contact angle θ of greater than 95 degrees, e.g., 105, 110, 125, 140 or greater, e.g., 170 degrees, the viscosity of the molding material is greater than 20,000 centipoise, the thickness (t′) of regions  300  and  302  is less than about 2 μm, e.g., 1.50 μm, 1.25 μm, 1.00 μm, 0.75 μm or less, e.g., 0.5 μm, and pressure within the cavity is approximately nominal atmospheric pressure. Mechanical forces can also be applied to mold  264  when it is desired not to fill regions  300  and  302  of mold  264 . For example, mold  264  can be clamped to force regions  300  and  302  to collapse, or a vacuum source can be applied cavity  264  such that regions  300  and  302  collapse. Should it be desired to fill regions  300  and  302  of cavity  264 , hydrophobicity of the set molding material, viscosity of the replication material, thickness (t′) of regions  300  and  302 , and pressure within the cavity can be adjusted to achieve this goal. For example, regions  300  and  302  of cavity  264  are filled when the molding material has a contact angle θ of between about 150 degrees and about 176 degrees, the viscosity of the replication material is less than 5000 centipoise, the thickness (t′) of regions  300  and  302  is greater than about 2 μm, e.g., 2.50 μm, 3.00 μm, 4.00 μm, 5.00 μm or more, e.g., 10.0 μm, and pressure within the cavity is approximately nominal atmospheric pressure. 
       FIG. 7A  is a scanning electron micrograph taken at 2500 times magnification of a master  320  made by multi-photon absorption polymerization (MAP) of a mixture of ethoxylated trimethylolpropane triacrylate and tris(2-hydroxyethyl)isocyanurate in the presence of ethyl-2,4,6-trimethylbenzoylphenylphosphinate (Lucirin® TPO-L) as photo-initiator. Master  320  has fluid flow pathway blocking walls  324  and  326  that are each about 0.5 μm thick. Master  320  extends upwardly from a top surface  322  of a glass microscope slide approximately 15 μm, has a maximum lateral dimension of approximately 20 μm, and a maximum thickness of about 3 μm.  FIG. 7B  is a scanning electron micrograph taken at 2500 times magnification of a replica of master  320 . In this particular embodiment, replication occurred without replication of blocking walls  324  and  326 . 
       FIG. 8A  is a scanning electron micrograph taken at 1700 times magnification of a master structure  340  having a wall  342  intend to block a fluid flow pathway. In this particular example, the wall had some imperfections and included some small holes. Nonetheless, a mold of the master structure was set and the master was released from the set mold without damaging the mold. The mold was then used to produce replica  345  shown in  FIG. 8B , which is a scanning electron micrograph taken at 1900 times magnification. The replica  345  was cast from a mixture of ethoxylated trimethylolpropane triacrylate and tris(2-hydroxyethyl)isocyanurate in the presence of ethyl-2,4,6-trimethylbenzoyl-phenylphosphinate (Lucirin® TPO-L) as photo-initiator. Replication occurred without replication of blocking wall  342 , which is an opening  346 . Opening  346  is defined by a closed loop formed by replica  345  and base  347 . 
       FIG. 9A  is a scanning electron micrograph taken at 1500 times magnification of a stool-shaped master  360  having interior walls blocking fluid flow pathways through the structure. Referring particularly to  FIG. 9B , master  360  includes four walls  370 ,  372 ,  374  and  376  that block four fluid flow entrance ways  380 ,  382 ,  386  and  388 . Walls  370 ,  372 ,  374  and  376  were formed from two intersecting diagonal membranes. The first diagonal membrane  381  connects legs  380  and  382 , and the second diagonal membrane  383  connects legs  384  and  386  of the stool-shaped master  360 . The membranes  381  and  383  of master  360  are each about 0.5 μm thick.  FIG. 9C  is a scanning electron micrograph taken at 1500 times magnification of a replica of the stool-shaped master  360 , without replication of walls  370 ,  372 ,  374  and  376 . 
     Referring to  FIG. 10 , a hook-shaped structure  400  extends outwardly from a base  402  and includes a head  404  that extends from a stem portion  406  to an engagement tip  410 . Head  404  overhangs the base  402 , forming an arched crook  409 . 
     Attempting to create a mold from structure  400  to produce replicas can be problematic in that during production of the mold from master  400 , liquid molding material flows into and through an open loop fluid flow pathway  412 , defined by a surface  411  from point  409  to engagement tip  410 . After the liquid molding material sets, fluid flow pathway  412  can create a mold-lock condition when an overhang distance (D), measured from an apex of the crook  408  to a forward-most extent of the head  421 , is sufficiently large, e.g., 5 μm or more, e.g., 10 μm, 15 μm, 25 μm or 50 μm. 
     Referring to  FIGS. 11A and 11B , master  401  has a fluid-blocking wall  420  that bisects the crook  409 , preventing fluid flow under the crook  409  of the structure. Structure  400  can be produced from structure  401  by first making a mold from structure  401 , and then adjusting replication conditions such that a region of the mold that produces the fluid-blocking wall  420  is not filled. 
     Referring to  FIG. 12A , a triangular structure  500  positioned above a molding material  142  extends downwardly from and is integral with a front surface  502  of a base  504 . Front surface  502 , together with sides  510  and  512  form two open loop fluid flow pathways  514  and  516 . Referring now to  FIG. 12B , upon immersion of structure  500  into the unset, liquid molding material  142 , the liquid molding material  142  flows into and through the open loop fluid flow paths  514  and  516 . Upon solidification, mold-lock can occur, making it impossible to remove structure  500  from the set material by applying a force in an upwardly direction (indicated by arrow  520 ) without breaking the solid mold material or structure  520 . 
     Referring to  FIGS. 13A and 13B , a triangular structure  550  positioned above a molding material  142  extends downwardly from and is integral with a front surface  552  of a base  554 . Structure  550  includes a membrane  560  extending away from a side  562  of structure  550 . Front surface  552 , together with sides  564  and  566  form two open loop fluid flow pathways  570  and  572 . Upon immersion of structure  550  into the unset, liquid molding material  142 , the liquid molding material  142  flows into and through open loop fluid flow paths  570  and  572 . 
     Referring now to  FIG. 13C , after removal of mold frame  140 , membrane  560  of structure  550  assists in the removal of structure  550  from set molding material by permitting portions  580  and  590  of mold  591  on opposite sides of membrane  560  to flex away from one another when the structure is removed. Replicas can now be produced from the mold. 
     Replicas can be produced in which the membrane is replicated, producing structure  550 , or structures can be produced in which the membrane is not replicated, producing structure  500 . Methods for replicating membranes, or not replicating membranes were described above. Alternatively, structures devoid of membrane  560  can be produced by sonicating structures having membrane  560  to weaken an attachment of the membrane to structure, or by ablating the undesired membrane with a laser. 
     OTHER EMBODIMENTS 
     A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. 
     For example, while embodiments have been shown that have two blocking walls, embodiments can include any number of walls, e.g., one, three, four, five, six or more, e.g., ten. 
     While certain embodiments have been shown in which a blocking wall bisects a fluid flow pathway, in other embodiments, blocking walls may be positioned along any portion of the fluid flow pathway. For example, a blocking wall can be positioned at an end of a fluid flow pathway such that it is substantially co-planar with an outside surface of the structure. 
     While embodiments have been shown in which a single master extends outwardly from a base, in other embodiments, multiple masters, e.g., two, four, ten, twenty-five, fifty or more, e.g., one hundred masters, can each extend outwardly from a single base. In such embodiments, multiple replicas can be produced from a single molding of a single mold. 
     While certain embodiments have been described in which a replicating material is polyacrylate, other replicating materials are possible. For example, a pre-ceramic precursor polymer, e.g., a silicon-containing polymer, can be used. After replication and de-molding, the replica can be pyrolyzed to produce ceramic replicas, e.g., made of silicon carbide. 
     Accordingly, other embodiments are within the scope of the following claims.