Patent Publication Number: US-2020276761-A1

Title: Multi-material microstereolithography using injection of resin

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to and the benefit of U.S. Provisional Patent Application Ser. No. 62/616,671, entitled “Multi-Material Microstereolithography Using Injection of Resin” and filed on Jan. 12, 2018, the entire contents of which are hereby incorporated by reference. 
    
    
     BACKGROUND 
     Ultra violet (UV) curable polymer based additive manufacturing is enabled by polymerization of liquid monomer into solid polymer when exposed to patterned UV light. In existing methods of microstereolithography, the bulk liquid monomer is contained in a tank before polymerization. During the growth process, the liquid monomer immediately adjacent to the solid boundary is polymerized to become a solid. The source of liquid monomer in the immediately adjacent layer is from the bulk liquid monomer contained in the tank. Applicant has identified a number of deficiencies and problems associated with conventional additive manufacturing. Through applied effort, ingenuity, and innovation, many of these identified problems have been solved by developing solutions that are included in embodiments of the present invention, many examples of which are described in detail herein. 
     BRIEF SUMMARY 
     Embodiments of the present disclosure provide novel and advantageous microstereolithography devices and methods that selectively inject a plurality of liquid monomers through a porous substrate. 
     Embodiments provided herein are directed to a device for additive manufacturing. The device may include a containment vessel and a substrate disposed in the containment vessel and having a first substrate surface. In some embodiments, at least a portion of the substrate is a porous substrate and the device is configured to inject a liquid monomer through the porous substrate such that the liquid monomer is polymerized to form a solid polymer on the portion of the substrate that is the porous substrate. In some embodiments, the device includes a substrate holder attached to the substrate, wherein the substrate holder includes one or more channels for the liquid monomer to flow through the substrate holder to the substrate. In some embodiments, the device further includes a liquid monomer reservoir accommodating the liquid monomer, at least one pump providing the liquid monomer to the substrate, and a channel connected to the pump and transferring the liquid monomer from the liquid monomer reservoir to the substrate. In some embodiments, the liquid monomer reservoir includes a first liquid monomer reservoir and a second liquid monomer reservoir. The first liquid monomer reservoir includes a first liquid monomer different from a second liquid monomer disposed in the second liquid monomer reservoir. 
     In some embodiments, the device is configured to inject a plurality of liquid monomers through the porous substrate. In some embodiments, the liquid monomer reservoir includes a first liquid monomer reservoir and a second liquid monomer reservoir and the pump is configured to provide a first liquid monomer from the first liquid monomer reservoir, a second liquid monomer from the second liquid monomer reservoir, or combinations thereof to the substrate. 
     In some embodiments, the device includes a solid boundary disposed opposite the substrate and configured to expose a portion of the liquid monomer to polymerization light passing through the solid boundary. In some embodiments, the solid boundary includes a photomask, is transparent, or is both transparent and includes a photomask. In some embodiments, the device includes a light source configured to emit polymerization light to the liquid monomer, wherein the light source spatially controls polymerization of the liquid monomer to the solid polymer. A variety of light sources as disclosed herein may be used in the device to emit polymerization light to the liquid monomer. The position of the light source, wavelength of polymerization light, type and location of solid boundary and containment vessel, etc. may allow the emitted polymerization light to polymerize the liquid monomer. 
     In some embodiments, the device includes one or more inlet/outlet ports disposed in the containment vessel, in the solid boundary, or combinations thereof. 
     In some embodiments, the device may be configured to form a solid polymer comprising one or more channels for liquid monomer to flow through the one or more channels. For instance, in some embodiments, the liquid monomer may be polymerized to solid polymer at certain locations along the porous substrate using a photomask, patterned light, laser, etc. to spatially control the polymerization light to form one or more channels for liquid monomer to flow through the one or more channels in the solid polymer. 
     In some embodiments, the porous substrate includes a plurality of pores disposed equally over the porous substrate and the solid polymer forms over pores of the porous substrate. In some embodiments, the solid polymer forms over a portion of the substrate that is non-porous. 
     Embodiments of the present disclosure are also directed to a method of additive manufacturing comprising. The method may include injecting a first liquid monomer through a porous substrate to a porous substrate surface disposed in a containment vessel; exposing the first liquid monomer injected to the porous substrate surface to a polymerization light to form a first solid polymer disposed on the porous substrate surface; injecting a second liquid monomer through the porous substrate to the porous substrate surface disposed in the containment vessel; and exposing the second liquid monomer injected to the porous substrate surface to the polymerization light to form a second solid polymer disposed on the porous substrate surface. In some embodiments, the first liquid monomer is different from the second liquid monomer. In some embodiments, the second liquid monomer is injected immediately following injection of the first liquid monomer or simultaneously with injection of the first liquid monomer. In some embodiments, the containment vessel includes a solid boundary and injection of the first liquid monomer through the porous substrate forms a liquid bridge disposed between the porous substrate and the solid boundary. In some embodiments, the porous substrate includes a plurality of pores to allow the first liquid monomer and the second liquid monomer to flow through the plurality of pores to multiple locations along the porous substrate surface. In some embodiments, the method further includes draining excess liquid monomer from the containment vessel through one or more inlet/outlet ports disposed in the containment vessel, a solid boundary disposed in the containment vessel, or combinations thereof. 
     Embodiments of the present disclosure are also directed to 3D objects formed using the present device and method. 
     Embodiments of the present disclosure are also directed to a device for additive manufacturing, the device including a containment vessel; a substrate disposed in the containment vessel; and a solid boundary disposed in the device and opposite the substrate. The solid boundary defines one or more inlet/outlet ports, such as a single inlet/outlet port or a plurality of inlet/outlet ports, disposed in the solid boundary for injection of liquid monomer into the containment vessel. A plurality of inlet/outlet ports may be strategically placed in the solid boundary. The plurality of inlet/outlet ports may direct the desired injected liquid monomer to the region where polymerization is desired. The solid boundary is configured such that liquid monomer injected into the one or more inlet/outlet ports disposed in the solid boundary is polymerized to form a solid polymer when exposed to polymerization light through the solid boundary. The inlet/outlet ports may be placed in a region different from where polymerization is desired. That is, the inlet/outlet ports may not block the polymerization light. The inlet/outlet ports may also be used to drain fluid from the device. 
     The details of one or more embodiments of the subject matter described in this specification are set forth in the accompanying drawings and the description below. Other features, aspects, and advantages of the subject matter will become apparent from the description, the drawings, and the claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Having thus described the disclosure in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein: 
         FIG. 1  illustrates a conventional microstereolithography device; 
         FIG. 2  illustrates a microstereolithography device in accordance with embodiments discussed herein; 
         FIG. 3  illustrates a microstereolithography device in accordance with embodiments discussed herein; 
         FIG. 4( a )  illustrates a microstereolithography device in accordance with embodiments discussed herein; 
         FIG. 4( b )  illustrates a microstereolithography device in accordance with embodiments discussed herein; 
         FIG. 5  illustrates a light source of the device for polymerization according to embodiments of the present disclosure; 
         FIG. 6  illustrates a microstereolithography device in accordance with embodiments of the present disclosure; 
         FIGS. 7 and 8  illustrate an example of creating a multi-material three-dimensional (3D) object using injection approach in a top-down orientation of the process in accordance with embodiments of the present disclosure; 
         FIGS. 9( a )-9( c )  show an example of the spatially selective UV light exposure scheme to fabricate a 3D object formed from multiple materials in accordance with embodiments of the present disclosure; 
         FIGS. 10 and 11  show another example of the injection method but in a bottom-up orientation for the process in accordance with embodiments of the present disclosure; 
         FIG. 12  shows an example of the separation, draining, and injection during a bottom-up orientation process in accordance with embodiments of the present disclosure; 
         FIG. 13  shows another example of a bottom-up method for injection in accordance with embodiments of the present disclosure; 
         FIG. 14  shows an example device and method incorporating an inert immiscible liquid in accordance with embodiments disclosed herein; 
         FIG. 15  illustrates an example of how the solid polymer may be utilized as an inlet/outlet port in accordance with embodiments disclosed herein; 
         FIG. 16  shows various examples of substrates and substrate holders in accordance with embodiments disclosed herein; 
         FIG. 17  illustrates an example using pump with multiple channels in accordance with embodiments disclosed herein; 
         FIG. 18  illustrates an example of passive draining in a top-down orientation in accordance with embodiments disclosed herein; 
         FIG. 19  illustrates an example in which the inlet/outlet port is a microfluidic channel connected to a tube that is connected to a pump and a third liquid monomer reservoir containing third liquid monomer in accordance with embodiments disclosed herein; 
         FIG. 20  shows an example of operations that may be performed during the present method in accordance with embodiments disclosed herein; 
         FIGS. 21( a ) and 21( b )  are examples of porous substrates attached to substrate holders in accordance with embodiments of the present disclosure; 
         FIG. 22  shows the injection of fluid through the substrate holder and porous substrate using a pump in accordance with embodiments of the present disclosure; 
         FIG. 23  shows an example apparatus for a bottom-up orientation in accordance with embodiments of the present disclosure; 
         FIG. 24  shows a bottom view of apparatus shown in  FIG. 23  in accordance with embodiments of the present disclosure; 
         FIGS. 25( a )-25( c )  show an example 3D object fabricated using the injection approach in accordance with embodiments of the present disclosure; 
         FIGS. 26( a )-26( d )  are a series of chronological pictures of the injection process in a top-down orientation in accordance with embodiments of the present disclosure; 
         FIGS. 27 and 28  show examples of a microstereolithography device according to embodiments of the present disclosure; 
         FIG. 29  shows an example of a substrate holder of a microstereolithography device according to embodiments of the present disclosure; 
         FIG. 30  shows an example of a solid polymer on a microstereolithography device according to embodiments of the present disclosure; 
         FIGS. 31 and 32  are computer-aided-design (CAD) models of a microstereolithography system for polymerization according to embodiments of the present disclosure; 
         FIGS. 33 and 34  are CAD models of a containment vessel of a microstereolithography device for polymerization according to embodiments of the present disclosure; 
         FIG. 35  shows a schematic of an image stitching of a microstereolithography device according to an embodiment of the present disclosure; 
         FIG. 36  shows a light source integrating galvo scanning mirrors for image stitching of  FIG. 35  in accordance with embodiments discussed herein; 
         FIGS. 37( a )-37( c )  show an example of a light source in accordance with embodiments discussed herein; and 
         FIG. 38  is a flowchart for an exemplary method in accordance with embodiments disclosed herein. 
     
    
    
     DETAILED DESCRIPTION 
     Microstereolithography is a process where complex 3D objects can be grown in a layer-by-layer fashion (additive manufacturing). Traditionally, a liquid monomer (e.g., resin) undergoes polymerization (e.g., curing or solidification) when exposed to UV light. The exposed UV light may be a patterned light, allowing the solidified polymer to take the shape of the patterned light. The growth process may be layer-by-layer where each layer has a discrete thickness, and the process may continue until the desired thickness is achieved. 
     As used herein, the term “resin” and “monomer” may be used interchangeably. In some embodiments, a “resin” may be composed of monomer, photoinitiator, dye, absorber, loaded micro/nano particles, any other component desired for polymerization or the resulting 3D object, or combinations thereof. As used herein, a “liquid monomer” will generally be used to refer to the fluid that is used to form the solid polymer and may include the components listed above for a resin and any other additional component desired for the resulting 3D object. For instance, the liquid monomer may include one or more types of monomers, photoinitiators, dyes, absorbers, loaded micro/nano particles, any other component desired for polymerization or the resulting 3D object, or combinations thereof. 
     As used herein, the term “polymerization” or “curing” may refer to the process of converting liquid monomer into a “solid polymer.” The method may not be limited to creating “polymers” (e.g., “plastic”). The disclosed devices and methods may be used to create any 3D object out of, for example, metal, ceramic, etc., and combinations thereof. The materials may be modified to prepare the desired object from the desired material. Thus, while the reaction process (e.g., the process of converting a liquid component to a solid component) is generally referred to as polymerization and with reference to a liquid monomer, the disclosed devices and methods may be used to create any 3D object out of, for example, metal, ceramic, etc., and combinations thereof, and thus, would use liquid forms of these materials and convert such forms to solid to form the 3D object. 
     Reference may be made throughout the present disclosure to “UV light” as the light that initiates polymerization. However, the polymerization light may be of any wavelength (e.g., narrow or broad spectrum). That is, the disclosure may be applied to light of any wavelength. Further, the disclosed devices and methods are not limited to only light initiated polymerization and may be applied to other curing processes. 
     Stereolithography and microstereolithography (μSL) is one type of additive manufacturing. Microstereolithography is generally used to refer to the fabrication of objects on a micrometer scale. However, the method and its basic principles may be scalable to a macro scale (that is, stereolithography). Thus, stereolithography and microstereolithography may be used interchangeably throughout the present disclosure. 
     Existing resin based 3D printing approaches, specifically stereolithography, may only manufacture parts using a single material, thus limiting the use of parts to mostly structural applications. The present device and method allows for fabrication of multi-material components, thus allowing fabrication of heterogeneous parts. The ability to fabricate heterogeneous parts may allow for the manufacture of functional parts within a single process similar to monolithic semiconductor fabrication processing methods. 
     Disclosed herein is an improved device and method for the manufacture of 3D objects. The present device and method may allow for the manufacture of 3D objects from multiple materials and complex geometries. The present device and method may allow for improved efficiency of such production with improved draining and injection of liquid monomer without the concern for cross-contamination of liquid monomers when alternating between materials. The present device and method may reduce excess liquid monomer as well as washing solvents or materials used to prevent cross-contamination of liquid monomers. For example, in some embodiments, washing solvent or other material for washing may be avoided completely. The present device and method is both flexible in the orientation of the production method, allowing for various orientations, and flexible in the geometries allowable for the resulting 3D object. The present device and method may have many applications in the additive manufacturing field. 
     As used herein, a “porous substrate” generally refers to a substrate sufficiently permeable to allow the injected liquid monomer to pass through the substrate. The porous substrate may be blocked by material disposed on one side of the substrate, for instance when the solid polymer is formed on a portion of the porous substrate over pores of the porous substrate, but would still be considered porous as the injected liquid monomer can enter the substrate and flow into the substrate and out through other pores of the porous substrate. Embodiments disclosed herein may utilize a porous substrate when injecting liquid monomer. Such porous substrate allows for flexibility of the formation of the solid polymer by providing various paths for the injected liquid monomer to flow, where such paths may be changed during formation of the solid polymer (e.g., as pores are blocked by formation of solid polymer, more of the liquid monomer may flow through other pores in the porous substrate). In some embodiments, the porous substrate may include interconnected pores providing a variety of connecting flowpaths for the liquid monomer to travel through the substrate. For instance, in some embodiments, injected liquid monomer may flow through the porous substrate where there is no solid polymer has been formed. In some embodiments, the porous substrate includes pores over the entire surface of the porous substrate and on the side edges of the substrate. Therefore, liquid monomer may flow from the sides and edges of the porous substrate. 
     The porous substrate may improve the efficiency of the method of manufacture by more efficiently directing the liquid monomer to the gap in which the liquid monomer is to be exposed to polymerization light thereby reducing excess liquid monomer waste. The porous substrate may reduce resources and time needed for draining the excess liquid monomer due to the reduced amount of excess liquid monomer. Further, a second liquid monomer may be injected into the same porous substrate without cleaning the substrate after using a first liquid monomer. The second liquid monomer sufficiently pushes out any residue of the first liquid monomer in the pores of the porous substrate. 
     The porous substrate may allow for fabrication of 3D objects in top-down or bottom-up orientation as well as left-right or right-left orientation. 
     The porous substrate may allow for an equally distributed flow of the injected liquid monomer throughout the entire substrate, as opposed, for example, to flow through a single channel which provides flow in only selected region where the channel is placed. When using a porous substrate, the liquid monomer may flow over the entire surface area of the porous substrate and the liquid monomer may flow through all the pores of the porous substrate equally, not just through a single channel. The porous substrate may allow for washing away unwanted oligomers or unwanted partially polymerized areas in the solid polymer (e.g., in channels formed in the solid polymer). Any residue on the porous substrate, 3D object, or solid boundary may be washed away by injection of new liquid monomer. In addition, when injected liquid monomer flowed through all the pores equally, stiction may be reduced during the separation of the solid polymer from the solid boundary due to fluidic pressure caused by the injected liquid monomer. The pores of the porous substrate may vary and may be less than about 200 microns in diameter, such as less than 100 microns, less than 50 microns, less than 20 microns, less than 2 microns in diameter. For instance, in some embodiments, the pores of the porous substrate are about 1 micron to about 200 microns in diameter, such as about 2 microns to about 100 microns in diameter, such as about 5 microns to about 50 microns in diameter. In some embodiments, the porous substrate is a porous stainless steel filters, foam metal, or other similar structure and may be a mesh or sieve type substrate, for example, a nylon mesh netting or fabric. 
     The porous substrate provides a surface for the solid polymer to bond or adhere to. The adhesion of the solid polymer to a substrate may be increased due to the use of a porous substrate because the solid polymer (e.g., the first layer of solid polymer formed) may be locked into the intricate random porous nature of the porous substrate. In additive manufacturing and microstereolithography, there may be a desire for strong adhesion of the solid polymer to the substrate due to issues with stiction between the solid polymer and a solid boundary. When liquid monomer becomes a solid polymer, the solid polymer may have strong stiction to the solid boundary. Due to the high stiction and repeated pulling/release operations for each layer, the solid polymer may debond or peel off from the substrate. When using a porous substrate in accordance with the present disclosure, there may be a stronger adhesion of the solid polymer to the porous substrate due to higher surface area and ability of the solid polymer to be interlocked into the pores of the porous substrate. 
     The porous substrate may also act as a filter and remove any unwanted solids or contaminants. For example, in a traditional approach, the liquid monomer may be expensive and the operator may want to reuse collected waste liquid monomer. The waste liquid monomer may contain partially polymerized solids or particles. When reusing waste liquid monomer (e.g., excess liquid monomer as disclosed herein), the porous substrate may filter unwanted contaminates. 
     As explained herein, the present device and method may allow for fabrication of heterogeneous 3D objects. That is, the present device and method may allow for the formation of 3D objects formed of multiple materials. The multiple materials may be different polymers formed into the same or different layers or features of the 3D object. Multiple types of liquid monomers may be injected without concern for cross-contamination if not desired. For instance, in some embodiments, the liquid monomers may be intentionally mixed. However, in some embodiments, it may be desired to not mix the liquid monomers such that a portion of the 3D object is formed of just the first liquid monomer (and not the second liquid monomer) while a portion of the 3D object is formed of just the second liquid monomer (and not the first liquid monomer). Thereby, heterogeneous 3D objects may be formed without concern for cross-contamination or additional washing or cleaning operations (beyond injecting the alternative liquid monomer). The porous substrate may allow for injection of the second liquid monomer without concern for cross-contamination. Due to the injection of the liquid monomer through the porous substrate, excess liquid monomer from both the first liquid monomer and the second liquid monomer may fall to the containment vessel. Embodiments disclosed herein may use the same containment vessel for the formation of the 3D object without concern for cross-contamination. In situ material changing may be performed. The first and second liquid monomer may sequentially be exposed to polymerization light to form portions of the 3D object. 
     In some embodiments, excess liquid monomer may be drained from the containment vessel. Such draining may occur by a variety of manners, both passive and active draining operations, and may further confirm the lack of cross-contamination over the various liquid monomers that may be injected into the containment vessel to form the 3D object. 
     In the present device and method, polymerization may occur at a solid-liquid interface (e.g., liquid monomer-solid boundary interface), a liquid-liquid interface (e.g., liquid monomer-inert immiscible liquid interface), or a liquid-gas interface (e.g., liquid monomer-air interface). Oxygen may be a polymerization inhibitor and, thus, an air-liquid monomer interface may not be used in some embodiments. 
     In the present device and method, different liquid monomers may be sequentially injected into the containment vessel without any intervening cleaning or washing step. With existing methods, there may be multiple containment vessels of liquid monomer, where each containment vessel contains a specific type of resin. With these traditional methods, one may need to 1) switch the containment vessel if it was desired to change the liquid monomer, and 2) one may need to wash or rinse away the 3D object before switching to the new liquid monomer and containment vessel. The 3D object may need to be washed to avoid any residue of the previous liquid monomer from appearing in the new containment vessel since the liquid monomer for polymerization is sourced from the bulk liquid monomer in the containment vessel. In the present device and method, different liquid monomers may be injected into the porous substrate sequentially and exposed to polymerization light to form a solid polymer including polymerized forms of the different liquid monomers without concern for cross-contamination. The source of liquid monomer (e.g., the liquid monomer reservoir) may not be contaminated when switching between liquid monomers. 
     Embodiments of the present disclosure provide methods and devices in which the liquid monomer layer immediately adjacent to the polymerization layer is sourced through injection of liquid monomer through a substrate (e.g., a porous substrate) and/or substrate holder. After flowing through the porous substrate, the injected liquid monomer may flow through any non-polymerized areas, including any channels in the solid polymer (if formed) and fill the finite gap between the solid boundary and the solid polymer. The excess liquid monomer may collect in the containment vessel. The freshly injected liquid monomer disposed in the gap between the solid boundary and previous solid polymer layer may be polymerized. 
     In the present device and method, a porous substrate may be used in addition to inlet/outlet ports disposed in the substrate and/or substrate holder. In some embodiments, inlet/outlet ports are disposed in various locations in the containment vessel to inject liquid monomer at these points in the containment vessel (see e.g.,  FIG. 12 , tank drain port  276  and drain port  274 ). In some embodiments, inlet/outlet ports are disposed in various locations in the solid boundary to inject liquid monomer at these points in the device (see e.g.,  FIG. 19 , microfluidic channel  286 ,  FIG. 12 , solid boundary drain port  275 ). In some embodiments, external flow paths or channels may be used to inject liquid monomer near the solid polymer (see e.g.,  FIG. 13 , first inlet/outlet tube  791  and second inlet/outlet tube  792 ). Direct injection of the liquid monomer to the location of polymerization may improve the efficiency of the device, reduce waste, reduce the concern for cross-contamination, and allow for the simultaneous and/or sequential injection of a plurality of liquid monomers. Further, in some embodiments, the liquid monomer may be drained from various inlet/outlet ports, such as those discussed herein. 
     The present device and method provides improved methods and devices for forming 3D objects composed of multiple different materials. The different materials may be in the same or different layer of the 3D object or feature of the 3D object. A variety of geometries and resulting objects may be prepared using the disclosed device and method. 
       FIG. 1  illustrates a microstereolithography device according to traditional methods where polymerization occurs in a containment vessel filled with liquid monomer. In traditional methods of microstereolithography, the liquid monomer  400  is contained in a containment vessel  200  as shown in  FIG. 1 . Referring to  FIG. 1 , the microstereolithography device  100  includes a containment vessel  200  (e.g., a containment vessel or monomer bath) and a substrate  300  disposed in the containment vessel  200 , wherein the containment vessel  200  includes a solid boundary  250  as a bottom plate such that a polymerization light  500  passes through the solid boundary  250 . Before polymerization by the polymerization light  500 , a liquid monomer  400  is poured into the containment vessel  200 . An immediate layer  430  of the liquid monomer  400  corresponding to the polymerization light  500  becomes a solid polymer  450  when the polymerization light  500  is applied to the liquid monomer  400 . When a substrate holder  350  connected to the substrate  300  moves in a vertical direction, the solid polymer  450  attached to the substrate  300  is pulled upwards, allowing more liquid monomer  400  to move under the solid polymer  450  becoming the immediate layer  430 . The solid polymer  450  has a layer-by-layer structure. That is, during the layer-by-layer growth process, the immediate layer  430  of the liquid monomer  400  adjacent to the solid boundary  250  polymerized to become the solid polymer  450 . The source of liquid monomer  400  in the immediate layer  430  is from the bulk liquid monomer  410  contained in the containment vessel  200 . After polymerization of the immediate layer  430 , the solid polymer  450  is mechanically separated from the solid boundary  250  using methods such as pulling, sliding, peeling, and tilting. The solid boundary  250  is often needed at the polymerization interface  425  to confine the polymerization boundaries and to prevent oxygen from diffusing from the environment into the polymerization reaction. Oxygen is a known inhibiter of the polymerization reaction of traditionally used UV curable polymer reactions. 
     In  FIG. 1 , when the solid polymer  450  needs to be made of more than one material, multiple types of monomers may be provided in the containment vessel  200  in sequence. That is, a first of liquid monomer  400  is provided into the containment vessel  200  as the bulk liquid monomer  401 . The bulk liquid monomer  401  is used to become the solid polymer  450 . The bulk liquid monomer  401  may be removed from the containment vessel  200 , the containment vessel  200  may be cleaned, and a new type of liquid monomer (not illustrated) may be inserted into the containment vessel forming a new bulk liquid monomer. A stop-rinse process may be performed repeatedly for a multi-material solid polymer. The process is complex and a significant amount of bulk liquid monomer may be wasted. 
     Existing additive manufacturing methods, specifically (micro)stereolithography (uSL) are limited to “single” material fabrication. As shown in  FIG. 1 , a containment vessel  200  may be filled with liquid monomer  400  for polymerization. After exposure of a layer and formation of a solid polymer  450 , the solid polymer  450  and substrate  300  are separated from the solid boundary  250  to form a new gap/layer. This gap gets filled with liquid monomer  400  from the rest of the liquid monomer  400  in the containment vessel  200 . Such prior processes allow for the manufacture of a 3D object made of a single type of material (homogenous). In addition, in such processes, the liquid monomer  400  may need to be protected from ambient light to prevent premature polymerization or degradation of the liquid monomer  400 . Such prior processes may involve a significant amount of wasted liquid monomer  400  as not all of the bulk liquid monomer  401  is used to form the solid polymer  450 . In addition, in such prior processes, it is difficult to control the state in which the liquid monomer  400  is exposed to polymerization light  500 . 
     The present device and method allow for the fabrication of multi-material 3D objects using stereolithography based additive manufacturing method. Stereolithography utilizes photo polymerization where a resin (typically liquid) is selectively exposed to UV light to create the 3D object. 
     The present device and method utilize an injection technique to carry out stereolithography and microstereolithography to fabricate 3D objects that are composed of multiple types of materials. The injection methods allow for delivery of multiple types of liquid monomer  400  at desired times and locations before exposure to polymerization light  500 . Unlike existing methods, in the present device and method, the liquid monomer  400  is injected to the containment vessel  200  as the process is occurring. 
     The present device and method may allow multiple types of liquid monomer  400  to be used to fabricate 3D objects thus allowing heterogonous fabrication in a single process. This in-situ approach to changing the liquid monomer  400  also acts as a rinsing or washing operation. In some embodiments, there may be no need for an additional step of rinsing or washing the containment vessel  200 . In some embodiments, there may be no need for cleaning of the containment vessel  200  separate from the injection of subsequent liquid monomer  400 . In some embodiments, the liquid monomer  400  exposed to polymerization light  500  may be in the desired state considering the liquid monomer  400  is delivered to the polymerization interface  425  when needed rather than being stagnant in the containment vessel  200 . This ensures the delivery of fresh liquid monomer  400  that is not degraded or does not include areas of pre-mature polymerization due to ambient exposure. For example, if a liquid monomer  400  needs to be maintained at 50° C., but the process is occurring at 25° C., the liquid monomer  400  may be injected at a temperature of 50° C. Or for example, if the liquid monomer  400  includes suspended nanoparticles, where the suspension is time varying, the liquid monomer  400  may be kept in an external reservoir in a state where the nanoparticles stay suspended and then injected when desired. 
       FIG. 2  illustrates a microstereolithography device according to embodiments of the present disclosure. Referring to  FIG. 2 , a microstereolithography device  210  includes a containment vessel  200  including a solid boundary  250 , and a porous substrate  310  disposed in the containment vessel  200 . The porous substrate  310  includes a first substrate surface  301  that faces the solid boundary  250 . 
     The porous substrate  310  is configured such that the liquid monomer  400  passes through the porous substrate  310  and then is provided toward the solid boundary  250 . The porous substrate  310  is attached to the substrate holder  350  and the porous substrate  310  can be moved in a vertical direction when the substrate holder  350  moves in the vertical direction. In the embodiments illustrated in  FIG. 2 , the substrate holder  350  includes a through hole  355  to provide the liquid monomer  400  to the porous substrate  310 . The solid polymer  450  is formed on the porous substrate  310 , specifically on the first substrate surface  301  of the porous substrate  310 . 
     The microstereolithography device  210  further comprises a liquid monomer reservoir  700  including a first liquid monomer reservoir  710  and a second liquid monomer reservoir  720 , a pump  730  selectively providing a first liquid monomer  400   a  of the first liquid monomer reservoir  710  or a second liquid monomer  400   b  of the second liquid monomer reservoir  720 , and a tube  750  connected to the pump  730  in order to transfer the liquid monomer  400  selected from the first liquid monomer  400   a  and the second liquid monomer  400   b  to the substrate holder  350 . The tube  750  passes through the through hole  355  of the substrate holder  350  or is connected to a pump  730  tubing injection port (not shown) of the substrate holder  350 . While a tube may be referred to throughout the disclosure, any flowpath may be used and may be interchangeable with a channel or other cavity for fluid to flow. As used herein, channel refers to flow pathways for fluid. 
     The first liquid monomer  400   a  and the second liquid monomer  400   b  can be selectively injected through the porous substrate  310  for each layer to be polymerized, thereby enabling a truly heterogeneous additive manufacturing process where the solid polymer  450  is made of multiple materials instead of a single homogenous material. That is, the first liquid monomer  400   a  of the first liquid monomer reservoir  710  is injected through the porous substrate  310  for a first solid polymer  451  and the second liquid monomer  400   b  of the second liquid monomer reservoir  720  is injected through the porous substrate  310  for a second solid polymer  452 . During manufacturing of the solid polymer  450  having multiple monomers, the manufacturing process may not need to be stopped to change the liquid monomer  400 , and the containment vessel  200  may not need to be cleaned. 
     Further, in some embodiments, the first liquid monomer  400   a  and the second liquid monomer  400   b  can be injected simultaneously from the first liquid monomer reservoir  710  and the second liquid monomer reservoir  720 , thereby providing a solid polymer  450  made of a mixture of the first and second liquid monomers  400   a,    400   b,  respectively. 
     In the embodiment illustrated in  FIG. 2 , the solid boundary  250  is positioned at a bottom portion of the containment vessel  200  and functions as a bottom plate of the containment vessel  200 . In this configuration, the liquid monomer  400  injected through the porous substrate  310  fills a gap between the solid boundary  250  and the porous substrate  310  and then the injected liquid monomer  400  is polymerized to become the solid polymer  450 . After a layer of the solid polymer  450  is formed, newly injected liquid monomer  400  fills a gap between the solid boundary  250  and the solid polymer  450  and then this liquid monomer  400  becomes an immediate layer  430  that is a liquid monomer  400  to become solid polymer  450  in presence of polymerization light  500 . When the immediate layer  430  is exposed to the polymerization light  500 , the immediate layer  430  is polymerized. That is, the growth surface (i.e., solid boundary  250 ) is at the bottom  764  of the containment vessel  200  and the growth occurs from the bottom to the top at the polymerization interface  425 . The excess liquid monomer  410  is washed away into the containment vessel  200 , and the excess liquid monomer  410  in the containment vessel  200  can be drained through an outlet port (not shown). 
     During the layer-by-layer growth process, while an empty channel  470  in the solid polymer  450  provides a passage for the injected liquid monomer  400 , the empty channel  470  may be filled with oligomers that are partially polymerized liquid monomer  400 . The empty channel  470  may be cleared by washing away the oligomers using injection of the liquid monomer  400  (e.g., the second liquid monomer  400   b  after injection of the first liquid monomer  400   b ), thereby maintaining the clear empty channel  470  as desired. With the use of the porous substrate  310 , the channel  470  may be cleared of unwanted residue. That is, the porous substrate  310  provides flow paths for the liquid monomer  400  to distribute along the substrate and enter any channels  470  disposed along the substrate to wash away any unwanted residue. Such may be difficult with a single injection flow path. 
       FIG. 3  shows a microstereolithography device according to an embodiment of the present device and method. Referring to  FIG. 3 , the containment vessel  200  includes a bottom plate  260  at a bottom  764  of the containment vessel  200  and the solid boundary  250  at a top portion. That is, the solid boundary  250  is at the top of the device  210 , and the growth of the solid polymer  450  occurs from the top to the bottom of the device  210 . The transparent solid boundary  250  can seal the containment vessel  200 , thereby inhibiting contaminants from interfering polymerization. In this embodiment, the solid boundary  250  is a solid boundary  250  of which an area corresponding to the porous substrate  310  is transparent, but the solid boundary  250  can be replaced by a solid boundary  250  including a patterned photomask corresponding to the porous substrate  310 . In some embodiments, the solid boundary  250  may be both transparent and include a patterned photomask. In addition, the solid boundary  250  can confine a layer of solid polymer  450  (e.g., first solid polymer  451  and second solid polymer  452 ) to a certain thickness. The porous substrate  310  and the substrate holder  350  are configured such that the liquid monomer  400  is injected from the bottom  764  to the top  763  of the containment vessel  200  through the porous substrate  310 . Similar to the microstereolithography device  210  of  FIG. 2 , the liquid monomer  400  is provided from the liquid monomer reservoir  700  through the pump  730  and the tube  750 . The liquid monomer reservoir  700  includes the first liquid monomer reservoir  710  and the second liquid monomer reservoir  720 , and a selected liquid monomer  400  between the first liquid monomer  400   a  and the second liquid monomer  400   b  is injected through the pump  730 , the tube  750 , and the porous substrate  310 . 
     The liquid monomer  400  to be polymerized is injected from the porous substrate  310 , passes through the empty channel  470 , and then reaches the solid boundary  250 . As a result, the injected liquid monomer  400  fills a gap between the solid boundary  250  and the solid polymer  450 , and becomes the immediate layer  430  that is the liquid monomer  400  to become the solid polymer  450  in the presence of polymerization light  500 . The excess monomer  410  falls into the containment vessel  200 , thus injection of another liquid monomer  400  after polymerization ensures a fresh and uncontaminated liquid monomer  400  at the immediate layer  430  for polymerization. That is, even if multiple liquid monomers  400  are sequentially injected as the immediate layer  430 , each layer of the solid polymer  450  can remain high quality. Thus, multi-material monomer types are feasible without a stop-rinse-repeat process. Further, the present device and method may reduce excess liquid monomer  410  contained in the containment vessel  200 . In addition, this configuration may reduce the exposure of the liquid monomer  400  to external contaminants by allowing it to be contained in a protected external reservoir, such as the liquid monomer reservoir  700 . 
       FIG. 4( a )  shows a microstereolithography device according to an embodiment of the present disclosure. Referring to  FIG. 4( a ) , the microstereolithography device  210  comprises a inert immiscible layer  230  between the solid boundary  250  and the immediate layer  430  by separating the injected liquid monomer  400  from the solid boundary  250 . The inert immiscible layer  230  can be a vacuum or a gas such as nitrogen or air. The containment vessel  200  further includes inlet/outlet ports such as a first general purpose inlet/outlet port  270  and a second general purpose inlet/outlet port  280  for injecting and withdrawing solid, liquid, gas, or vacuum. For example, the first general purpose inlet/outlet port  270  may provide nitrogen for the inert immiscible layer  230  and the second general purpose inlet/outlet port  280  may be used for vacuum or for draining excess gas. When the second general purpose inlet/outlet port  280  is placed adjacent to the bottom plate  260 , the excess liquid monomer  410  dripped into the containment vessel  200  may be drained through the second general purpose inlet/outlet port  280 . 
       FIG. 4( b )  illustrates a microstereolithography device according to an embodiment of the present disclosure. Referring to  FIG. 4( b ) , the inert immiscible layer  230  is formed between the solid boundary  250  and the immediate layer  430  and between the solid boundary  250  and the excess liquid monomer  410 . The inert immiscible layer  230  is a liquid in this embodiment. In addition, the inert liquid for the inert immiscible layer  230  can be provided by the pump  730 . The microstereolithography device  210  can further include an inert immiscible reservoir (not shown) including a source of inert immiscible liquid that is configured to be provided to the containment vessel  200  through the pump  730  to form the inert immiscible layer  230 . 
     With respect to  FIGS. 2-4 ( b ), the immediate layer  430  of the liquid monomer  400  is exposed to the polymerization light  500  for polymerization provided by light source  510 . The polymerization light  500  is a light having wavelength that initiates photo polymerization of liquid monomer  400 . In particular, an Ultraviolet (UV) light can be used for polymerization. The UV light can be a non-patterned collimated light projected from a mercury arc lamp or an array of LED, or a patterned light projected from a projection system, such as a DLP projector. In addition, a laser can be used for a light source providing the polymerization light  500 . Optics can be provided for collimating optics for the non-patterned light, for semi-collimating optics for the patterned light, and for magnifying or de-magnifying. The optics can include mirrors, prisms, and beam splitters.  FIG. 5  illustrates a light source  510  of the device  210  for polymerization according to an embodiment of the present disclosure. Referring to  FIG. 5 , the light source  510  is a DPL  820  fitted with a projection lens  830 . The DLP  820  emits a light  505  through a projection lens  830 . Galvo scanning mirrors  810  reflect the emitted light  505  and provide the projected UV polymerization light  500 . The galvo scanning mirrors  810  allow XY positioning of projected polymerization light  500  and may be used with a laser light instead of DLP projection light  505 . 
     After each layer is polymerized in the microstereolithography device of  FIGS. 2 and 3 , the solid polymer  450  may be separated from the solid boundary  250  by applying mechanical force. When the liquid monomer  400  is injected through the channel  470 , fluidic pressure force may be provided at the polymerization interface  425  of the solid polymer  450  and the solid boundary  250 , and this fluidic pressure force may help separate the solid polymer  450 . 
     The liquid monomer  400  may be injected in a vertical direction, such as from top to bottom or from bottom to top, as shown for instance in  FIGS. 2-4 ( b ). However, the injection direction of the liquid monomer  400  is not limited to a particular direction. For example, the liquid monomer  400  may be injected from left to right or from right to left. 
       FIG. 6  illustrates an example approach to an injection method as disclosed herein. In the embodiment illustrated in  FIG. 6 , the liquid monomer  400  (e.g., first liquid monomer  400   a  and/or second liquid monomer  400   b ) is injected from pump  730 . The injection occurs through the porous substrate  310  and substrate holder  350 . The porous substrate  310  is porous to allow liquid monomer  400  to flow through the substrate. In the embodiment illustrated in  FIG. 6 , the containment vessel  200  also has inlet/outlet ports, including first and second general purpose inlet/outlet ports  270 ,  280 , respectively, as well as a third general purpose inlet/outlet port  271 . These inlet/outlet ports may be general purpose inlet/outlet ports for various functions such as injecting liquid monomer  400 , draining excess liquid monomer  410 , vacuum, injecting gasses, etc. When injected, the liquid monomer  400  flows into gap  760  formed between the solid boundary  250  and the solid polymer  450 . Gap  760  may be filled with liquid monomer  400  due to surface forces (e.g., capillary forces) and the wettability of the solid boundary  250 . 
     The liquid monomer  400  may also flow through channel  470  formed in the solid polymer  450  and any unpolymerized area on the porous substrate  310 . Channel  470  may be intended or unintended and may be formed to direct the flow of liquid monomer  400  to the desired location (e.g., gap  760 ) and/or to control the liquid bridge  762 . In some embodiments, channel  470  may be used when a second of liquid monomer  400  (e.g., first liquid monomer  400   a  or second liquid monomer  400   b ) is injected into the containment vessel  200 . 
     In some embodiments, the liquid monomer  400  may also be injected through an inlet/outlet tube  766  placed near the solid boundary  250 , the gap  760 , and/or the solid polymer  450 . The inlet/outlet tube  766  may be an inlet/outlet port for easier injection or draining excess liquid monomer  410 . The injected liquid monomer  400  may form a liquid bridge  762  around the gap  760 , solid boundary  250 , solid polymer  450 , and the porous substrate  310 . This liquid bridge  762  may occur due to surface forces. Any excess liquid monomer  410  that is not part of the liquid bridge  762  may flow to the bottom  764  of the containment vessel  210 . In some embodiments, when a second of liquid monomer  400  (e.g., first liquid monomer  400   a  or second liquid monomer  400   b ) is injected into the containment vessel  200 , the first of liquid monomer  400  (e.g., first liquid monomer  400   a  or second liquid monomer  400   b ) may fall to the bottom  764  of containment vessel  210 . 
       FIGS. 7 and 8  illustrate an example of creating multi-material 3D object using an injection approach in a top-down orientation in accordance with embodiments disclosed herein. In the embodiment illustrated in  FIGS. 7 and 8 , for a given layer, first liquid monomer  400   a  is exposed to spatially patterned polymerization light  500  to form a solid polymer  450 . After formation of the solid polymer  450 , the solid polymer  450  may be separated (not shown) from the solid boundary  250  resulting in the formation of a gap  760 . Second liquid monomer  400   b  may be injected. Excess liquid monomer  410  of first liquid monomer  400   a  may be washed away and may fall to the bottom  764  of the containment vessel  200 . The second liquid monomer  400   b  may be injected sufficiently such that the gap  760  has no remaining first liquid monomer  400   a  residue. After injection of second liquid monomer  400   b,  the second liquid monomer  400   b  may be exposed to polymerization light  500  to form solid polymer  450 . As discussed herein for  FIG. 6 , any suitable mode of injection may be used. The collected excess liquid monomer  410  in the containment vessel  200  at the bottom  764  may be drained, for example, using another pump (not shown) or vacuum. 
     As shown in  FIG. 7 , there may be unpolymerized area  767  which may be intended to be polymerized after injection of a second of liquid monomer  400  (e.g., second liquid monomer  400   b ). That is, the polymerization light  500  may be patterned such that portions of the first liquid monomer  400   a  are not exposed to polymerization light  500  leaving unpolymerized area  767  along the surface of the solid polymer  450 . As shown in  FIG. 8 , the unpolymerized area  767  may be filled with the second liquid monomer  400   b,  which is then exposed to polymerization light  500  to form new areas of solid polymer  450 . 
     As also shown in  FIGS. 7 and 8 , the embodiment illustrated in  FIGS. 7 and 8  includes inlet/outlet ports including first substrate holder inlet/outlet port  272  and second substrate holder inlet/outlet port  273  disposed in the substrate holder  350 . These inlet/outlet ports may be flow paths for liquid monomer  400  to travel. The first substrate holder inlet/outlet port  272  and the second substrate holder inlet/outlet port  273  may be connected to a pump for injecting liquid monomer  400  and/or draining excess liquid monomer  410 . 
       FIGS. 9( a )-9( c )  show an example of the spatially selective UV light exposure scheme to fabricate a 3D object formed from multiple materials. In the embodiment illustrated in  FIGS. 9( a )-9( c ) , the exposure method uses a projection or photomask based approach.  FIG. 9( a )  shows a cross section of the solid polymer  450  formed according to the embodiment illustrated in  FIGS. 9( a )-9( c ) . In particular, the solid polymer  450  includes a first solid polymer  450   a  and a second solid polymer  450   b,  where first solid polymer  450   a  is formed of first liquid monomer  400   a  and second solid polymer  450   b  is formed of second liquid monomer  400   b.    FIG. 9( a )  also includes channel  470  where no solid polymer forms.  FIG. 9( b )  illustrates an exposure pattern for the first liquid monomer  400   a.  As shown in  FIG. 9( b ) , the exposure pattern includes an exposure region for the first liquid monomer  480   a  and an unexposed region for the first liquid monomer  481   a.    FIG. 9( c )  illustrates an exposure pattern for the second liquid monomer  400   b.  As shown in  FIG. 9( c ) , the exposure pattern includes an exposure region for the second liquid monomer  480   b  and an unexposed region for the second liquid monomer  481   b.  Use of the exposure patterns illustrated in  FIGS. 9( b ) and 9( c )  result in the solid polymer  450  of  FIG. 9( a ) . 
       FIGS. 10 and 11  show another example of the injection method but in a bottom-up orientation in accordance with embodiments of the present disclosure. In the embodiment illustrated in  FIG. 10 , first liquid monomer  400   a  is selectively polymerized resulting in unpolymerized area  767  forming gap  760 . Excess liquid monomer  410  of first liquid monomer  400   a  may fall to the bottom  764  of the containment vessel  200 . The solid polymer  450  may be separated from the solid boundary  250  (e.g., as shown in  FIG. 12 ) to form a larger gap  760 . During and/or after separation, second liquid monomer  400   b  may be injected and first liquid monomer  400   a  may be drained (not shown). The injected second liquid monomer  400   b  may fall to the bottom  764  of containment vessel  200  resulting in the gap  760  being filled with second liquid monomer  400   b.  The solid polymer  450  may be moved back to its previous position of the layer, which is the same position where first liquid monomer  400   a  was exposed. The spatially selective exposure process may be repeated to solidify the second liquid monomer  400   b  in gap  760  as shown in  FIG. 11 . 
     In the embodiments illustrated in  FIGS. 10 and 11 , a plurality of channels  470  may be present in the solid polymer  450 . The liquid monomer  400  (e.g., first liquid monomer  400   a  and/or second liquid monomer  400   b ) may flow through these channels  470 . 
       FIG. 12  shows an exemplary separation process in accordance with embodiments disclosed herein. The solid polymer  450  may be separated from the solid boundary  250  by any suitable method, such as peeling, and may include draining of fluids and injection of additional fluids. The embodiment illustrated in  FIG. 12  is a bottom-up orientation. Excess liquid monomer  410  may be drained and/or washed through any number of inlet/outlet ports (e.g., first, second, and third general purpose inlet/outlet ports  270 ,  280 , and  271 , respectively as disclosed herein) disposed throughout the device  210 . The device  210  may include holes, voids, cavities, grooves, etc. (which all may be referred to as inlet/outlet ports) for draining and/or washing the device  210 . The inlet/outlet ports may be strategically placed in the device, e.g., in the solid boundary  250  and/or the containment vessel  200  to guide and drain fluid. 
     In some embodiments, a squeegee or wiper blade  783  may move relative to the device  210 . In some embodiments or the apparatus moves relative to the squeegee or wiper blade to help wipe off excess liquid monomer  410  and direct the excess liquid monomer  410  to an inlet/outlet port. In the embodiment illustrated in  FIG. 12 , the device  210  includes drain port  274  that may be an inlet/outlet port as discussed herein and may be connected to drain pump  785  which is connected to waste reservoir  786 . As the first liquid monomer  400   a  is drained (e.g., using drain pump  785  into waste reservoir  786 ), injection of the second liquid monomer  400   b  may be started, which may operate to help rinse or wash away any residue of the first liquid monomer  400   a.  In some embodiments, the containment vessel may be tilted (shown by arrow  790 ) to help guide or push the liquid monomer  400  for drainage. 
     In the embodiment illustrated in  FIG. 12 , the containment vessel  200  includes tank drain port  276  defined in the bottom  764  of containment vessel  200  that drains the excess liquid monomer  410  to the liquid collection bin  787 . The liquid collection bin  787  may be part of the containment vessel  200  or may be an attachment to the containment vessel  200 . As shown in  FIG. 12 , the solid boundary  250  includes a solid boundary drain port  275  defined in the solid boundary  250  that drains the excess liquid monomer  410  to the liquid collection bin  787 . Both drain port  276  and solid boundary drain port  275  may be inlet/outlet ports as discussed herein and may be disposed in various locations in the device  210 . 
       FIG. 13  shows another example of a bottom-up method for injection. In this example, the solid boundary  250  is coated with a low surface energy coating  251  (e.g., TEFLON™ AF, polydimethylsiloxane (PDMS), CYTOP®, or combinations thereof). In some embodiments, the coating  251  is hydrophobic and thus reduces the wettability of the solid boundary  250 . For instance, when liquid monomer  400  is injected into the containment vessel  200 , the liquid monomer  400  may not spread over the entire surface of the solid boundary  250 . Instead, the liquid monomer  400  may form a liquid bridge  762  around the solid polymer  450  and the porous substrate  310  due to poor wettability of the solid boundary  250  due to the coating  251 . The liquid bridge  762  may also fill any gaps  760  between the solid polymer  450  and the solid boundary  250  coated with coating  251 . The gap  760  may be filled with liquid monomer  400  to be polymerized after polymerization light  500  exposure. The gap  760  may be filled with liquid monomer  400  due to surface forces (e.g., capillary forces) and the surrounding liquid bridge  762 . To maintain the shape of the liquid bridge  762 , in some embodiments, each individual inlet/outlet ports or tubes (e.g., first and second substrate holder inlet/outlet ports  272  and  273 , respectively; through hole  355 ; and drain port  274 , tank drain port  276 , and solid boundary drain port  275 ) may be continuously or intermittently inject or drain liquid monomer  400 . In the embodiment illustrated in  FIG. 13 , the device  210  includes first inlet/outlet tube  791  and second inlet/outlet tube  792  disposed near the solid boundary  250  and near the substrate holder  350 , respectively. The first inlet/outlet tube  791  may be used to drain excess liquid monomer  410 . 
     In some embodiments, injection of the liquid monomer  400  as described herein may allow for in-situ dispensing of liquid monomer  400  (e.g., first liquid monomer  400   a  and/or second liquid monomer  400   b ) at a desired time. In addition, the disclosed method of injection allows for rinsing, washing, cleaning, and purging of the containment vessel  200 . In some embodiments, there may be no need for manual material change over. In some embodiments, there may be no need for manual or external cleaning or rinsing beyond the injection of the next liquid monomer  400 . However, in some embodiments, manual or external cleaning or rinsing may be performed in addition to the injection of the next liquid monomer  400 . 
     In some embodiments, injection of the liquid monomer  400  (e.g., first liquid monomer  400   a  and/or second liquid monomer  400   b ) may allow for direct delivery of the desired liquid monomer  400  at the desired location or near the desired location where exposure to polymerization light  500  may take place. The desired location is typically the position along the porous substrate  310  where exposure will take place. The location may be the gap  760  between the solid boundary  250  and the solid polymer  450 . When the gap  760  is filled with the desired liquid monomer  400 , the liquid monomer  400  may be exposed with polymerization light  500  for solidification to occur. In some embodiments, the gap  760  may be filled because the liquid monomer  400  is a liquid and thus takes the form of the area it is injected into. 
     In some embodiments, injection may allow for delivery of the liquid monomer  400  at the time when needed. The liquid monomer  400  may be injected from the respective liquid monomer reservoir  700  (e.g., first liquid monomer reservoir  710  and/or second liquid monomer reservoir  720 ). That is, in some embodiments, the liquid monomer  400  may not be injected from the excess liquid monomer  410  contained in the containment vessel  200 . The state of the liquid monomer  400  may be thus maintained or nearly the same as the state in which the liquid monomer  400  was in when disposed in the respective liquid monomer reservoir  700 . For instance, the liquid monomer  400  may be maintained at 40° C. in the liquid monomer reservoir  700 . When the liquid monomer  400  is injected from the liquid monomer reservoir  700  to the porous substrate  310 , the liquid monomer  400  may still be at the same temperature at which it was contained in the respective liquid monomer reservoir  700 . When the liquid monomer  400  is sourced from the containment vessel  200 , the liquid monomer  400  may be at the temperature of the containment vessel  200  (e.g., ambient or the process operating temperature) (e.g., 25° C.) rather than a temperature specific for the liquid monomer  400 . In some embodiments, the liquid monomer  400  may include suspended nanoparticles, which may be time or temperature varying. In such embodiments, the liquid monomer  400  may be continuously heated, cooled, stirred, or combinations thereof such that the liquid monomer  400  may be injected to the porous substrate  310  with the desired state of the suspended nanoparticles or other additives, such that the liquid monomer  400  is in this state when exposed to polymerization light  500 . 
     In some embodiments, during injection, the liquid monomer  400  may be sourced from the respective liquid monomer reservoir  700  and injected at the desired location or near its desired location using pump  730 . The flow of the liquid monomer  400  occurs through the various inlet/outlet ports, holes, tubes, channels, cavities, and porous substrate  310 . These flow paths and inlet/outlet are chosen so that the delivery of the liquid monomer  400  occurs at the near location or near the desired location. For instance, the liquid monomer  400  may be injected from the liquid monomer reservoir  700  to the substrate holder  350  to the porous substrate  310 , to the solid polymer  450 , to the solid boundary  250 , other locations in the containment vessel  200 , or combinations thereof. 
     In some embodiments, a plurality of liquid monomers  400  (e.g., first liquid monomer  400   a,  second liquid monomer  400   b,  a third liquid monomer  400   c,  a forth liquid monomer (not illustrated), etc., and combinations thereof) may be injected simultaneously rather than a single liquid monomer (e.g., first liquid monomer  400   a  or second liquid monomer  400   b,  third liquid monomer  400   c,  a forth liquid monomer (not illustrated), etc.). In some embodiments, it may be desired to form a solid polymer  450  have a mixture of liquid monomers  400  (e.g., a heterogenous feature of the 3D object) in a portion of the solid polymer  450  or over the whole solid polymer  450 . 
     In some embodiments, injecting the liquid monomer  400  may act as a purging, washing, rinsing, or cleaning operation. For instance, injecting the liquid monomer  400  may operate to rinse and/or wash away another liquid monomer (e.g., first liquid monomer  400   a  and/or second liquid monomer  400   b ) from the desired location. In some embodiments, injecting the liquid monomer  400  may operate to wash away oligomers (e.g., undesired reaction byproducts) or partially reacted liquid monomer  400  (e.g., partial solidification in locations where solidification is not desired). For example, when forming a 3D object that includes a dense array of tightly packed channels (holes), there may be partial solidification (e.g., gel-like features) in the channels. The partial solidification may be created due to reflection, diffraction, poor collimation, poor focusing, or combinations thereof of the polymerization light  500 . In such embodiments, injecting liquid monomer  400  through the channels may help wash away any wanted residue. 
     In some embodiments, the injected components may not be reacted to form the solid polymer  450 . For instance, in some embodiments, non-liquid monomers, such as solvents, may be injected to rinse away undesired liquid monomer  400  and/or residue. After injection of the solvent, for instance, the desired liquid monomer  400  may be injected such that that liquid monomer  400  may be exposed to polymerization light  500  to form solid polymer  450 . 
     In some embodiments, any type of material may be injected to the containment vessel  200  (e.g., through the porous substrate  310 ). For instance, injection may be of other fluids (e.g., liquids or gasses), such as other resins, monomers, polymers, slurries, etc. that may be desired. These fluids may be reacted to form the solid polymer  450  or may be desired to be included within the solid polymer  450  as is. For example, 1,6-hexanediol (HDDA), poly(ethylene glycol) diacrylate (PEGDA), or combinations thereof may be added. A photoinitiator, such as 4,4′-bis(dimethylamino)benzophenone, may be added. An absorber or dye, such as 2-hydroxy-4-(octyloxy)benzophenone, may be added. Solvents or unreactive fluids may be injected as cleaning agents. For example, suitable solvents may include ethyl acetate, methanol, isopropyl alcohol, ethanol, and combinations thereof. Gasses may be injected to help purge the injection path or may act as additives in the liquid monomer  400  to control polymerization. For example, oxygen, nitrogen, argon, or combinations thereof may be injected into the containment vessel  200 . For instance, as liquid monomer  400  (e.g., first liquid monomer  400   a  and/or second liquid monomer  400   b ) is injected, nitrogen (N2) gas may be injected in combination with the liquid monomer  400  to reduce the oxygen (O2) concentration. Oxygen may be an inhibitor of polymerization. Thus, decreasing the concentration of oxygen in the containment vessel  200  may increase the rate at which polymerization occurs. 
     In some embodiments, solids may be mixed with the fluids (e.g., liquid monomer  400 ). For example, solid nanoparticles may be suspended in the liquid monomer  400 . While the liquid monomer  400  is injected into the containment vessel  200 , the nanoparticles may be mixed with the liquid monomer  400  to form a slurry. 
     In some embodiments, the liquid monomer  400  is injected into a specific location along the solid polymer  450  for formation of the desired features. The porous substrate  310  as well as any inlet/outlet ports (e.g., first and second substrate holder inlet/outlet ports  272  and  273 , respectively) may be configured to inject the liquid monomer  400  to the solid polymer  450  at a desired location. 
     In some embodiments, polymerization may occur between the solid boundary  250  and the solid polymer  450 . In some embodiments, polymerization may occur at a liquid-liquid interface. For instance, polymerization may occur at a liquid monomer-inert immiscible liquid interface as disclosed in U.S Provisional Application No. 62/616,655. The disclosure of U.S Provisional Application No. 62/616,655, filed on Jan. 12, 2018, is incorporated herein in its entirety. 
     The gap  760  may be where the liquid monomer  400  is injected. The concept of the gap  760  is to bound the injected liquid monomer  400  to a desired location with a given height (layer thickness). Therefore, filling the gap  760  with liquid monomer  400  can be between two solids or can be between a liquid and solid. 
       FIG. 14  shows an example device and method incorporating an inert immiscible liquid in accordance with embodiments disclosed herein. In the embodiment illustrated in  FIG. 14 , an inert immiscible liquid  230  is disposed between the solid boundary  250  and the liquid monomer  400  (e.g., the liquid monomer  400  filling the gap  760  and forming the liquid bridge  762 ). In the embodiment illustrated in  FIG. 14 , the liquid monomer  400  may not fully spread over the inert immiscible liquid  230  due to surface forces (e.g., surface tension). If more liquid monomer  400  is injected into the containment vessel  200 , the liquid monomer  400  may spread over the inert immiscible liquid  230 . The liquid monomer  400  fills the gap  760  and is then exposed to polymerization light  500  to become solid polymer  450 . 
     In some embodiments, inlet/outlet ports may be used to guide fluid (e.g., liquid monomer  400 ) in a desired direction. As used herein, inlet/outlet ports may be a connection or pathway for fluid to flow through and may be bi-directional. The inlet/outlet ports may have an inlet and an outlet, where the inlet is an entrance into the flow path and the outlet is an exit from the flow path. In embodiments wherein the inlet/outlet ports are bi-directional, the inlet may be an entrance and an exit for the flow path through the inlet/outlet port and the outlet may be an entrance and an exit for the flow path through the inlet/outlet port. For instance, the direction of fluid flow may be in any direction (e.g., in or out). In the context of “injection”, the inlet or outlet may be considered the injection point. For example, the substrate holder  350  may have an inlet where tubing  750  for the pump  730  is connected. In the flow path through the substrate holder  350  (e.g., through hole  355 ), the liquid monomer  400  may flow from an inlet and finally end at the outlet. The outlet may be disposed where the liquid monomer  400  is desired to end. Depending on the orientation of the device  210 , the liquid monomer  400  may then flow from the outlet to the actual desired location (e.g., the gap  760 ). 
     In the context of “draining”, the inlet or outlet may be considered as a drain point. For example, if pump  730  is running in a reverse direction, then all the excess liquid monomer  410  may flow from the porous substrate  310  through the substrate holder  350  and finally back to the liquid monomer reservoir  700 . 
     In some embodiments, the inlet and/or outlet is a hole, port, void, connection point, tube, channel, tunnel, attachment point for tubes, hose, valves, etc., or combinations thereof. For instance, the inlet and/or outlet may be anywhere flow is injected and/or drained from. In some embodiments, the inlet and/or outlet may be incorporated into the 3D object. 
     In some embodiments, the inlet/outlet ports may be interconnected or disconnected to each other. For instance, if the inlet/outlet ports are disconnected from each other, each flow path may be used for only a specific liquid monomer  400  (e.g., first liquid monomer  400   a  or second liquid monomer  400   b ). Such configuration may provide spatial and/or direction control over where the liquid monomer  400  is injected or drained. 
     In some embodiments, the inlet/outlet ports may be interconnected. In some embodiments, interconnected the inlet/outlet ports may be used to mix liquid monomers  400  that may have been injected in different inlet and/or outlets. In some embodiments, the inlet/outlet ports may be disposed in the porous substrate  310 , the substrate holder  350 , the solid polymer  450 , and combinations thereof. 
     In some embodiments, the solid polymer  450  may operate as an inlet/outlet port. For instance, as shown in  FIG. 8 , the solid polymer  450  includes channel  470 . This channel  470  is an unsolidified area so that the porous substrate  310  may not be blocked and may allow fluid to flow through to the porous substrate  310  during injection. 
       FIG. 15  illustrates an example of how the solid polymer  450  may be utilized as an inlet/outlet port.  FIG. 15  shows a cross-section or top view of solid polymer  450  (including first solid polymer  450   a,  second solid polymer  450   b,  and third solid polymer  450   c ) being fabricated. As shown in  FIG. 15 , the substrate holder  350  has inlet/outlet ports disposed along the substrate holder  350 . In particular, the substrate holder  350  includes first and second substrate holder inlet/outlet ports  272 ,  273 , respectively, and third substrate holder inlet/outlet port  278  and forth substrate holder inlet/outlet port  279 . The solid polymer  450  is created is on the porous substrate  310 . Third solid polymer  450   c  includes an array of holes or channels  277  to direct the flow of injected liquid monomer  400  (e.g., first liquid monomer  400   a  and/or second liquid monomer  400   b ) through the porous substrate  310 . The first and second solid polymers  450   a,    450   b  may be blocking the flow of injected liquid monomer  400 . The third solid polymer  450   c  may be connected to the second solid polymer  450   b  and the first solid polymer  450   a  or may be disconnected to the second solid polymer  450   b  and/or the first solid polymer  450   a.  For instance, if it is undesired to have the third solid polymer  450   c  as part of the resulting 3D object, the third liquid monomer  400   c  may be selected such that the resulting third solid polymer  450   c  may be easily removed after forming the first solid polymer  450   a  and the second solid polymer  450   b.  For example, the third liquid monomer  400   c  may be water soluble such that the third liquid monomer  400   c  may be dissolved away while first and second solid polymer  450   a,    450   b,  remain in solid form. 
     Using the solid polymer  450  as the inlet/outlet ports may improve the injection efficiency. In the embodiment of  FIG. 6 , the fabrication orientation is top-down. Due to the liquid bridge  762  formed in  FIG. 6 , the gap  760  may be filled with the desired liquid monomer  400  after injection. In some embodiments, the liquid bridge  762  may not form. For instance, when the height of the solid polymer  450  becomes too large, the liquid bridge  762  may not form due to gravitational forces pulling the liquid monomer  400  down and causing it to drain to the bottom  764  of the containment vessel  200 . In this case, a flow path for the injected liquid monomer  400  through, for instance, the channel  470  in the solid polymer  450  may help ensure the injected liquid monomer  400  reaches the top location where it is desired. 
     The inlet/outlet ports may be strategically placed to control the fluid (e.g., liquid monomer  400 ) flow. For instance, the inlet/outlet ports may be placed such that the fluid flows through the porous substrate  310  and/or the substrate holder  350 . The inlet/outlet ports may be placed such that the fluid flows through the containment vessel  200 . For example, the containment vessel  200  may include a plurality of inlet/outlet ports disposed within the walls of the containment vessel  200  to provide desired fluid in various locations in the device  210 . In some embodiments, the inlet/outlet ports may be placed such that the fluid flows through the solid boundary  250 . For instance, the solid boundary  250  may include an array of holes. The holes in the solid boundary  250  may not overlap areas where polymerization and UV exposure are to take place. 
     In some embodiments, the porous substrate  310  and substrate holder  350  may be disposed where the solid polymer  450  is intended to grow and be attached.  FIG. 16  shows various examples of porous substrates  310  (e.g., porous substrates  310   d  and  310   e ) and substrate holders  350  (e.g., substrate holders  350   a  and  350   b ). In some embodiments, the porous substrate  310  and the substrate holder  350  may be a single component. For instance, in some embodiments, the substrate holder  350  may be a porous substrate  310 . The configuration of the substrate holder  350  and porous substrate  310  may be adjusted depending on the desired 3D object and the configuration of the inlet/outlet ports. 
     In some embodiments, the porous substrate  310  may be porous. In some embodiments, the porous substrate  310  is porous and is disposed along the flow path of the liquid monomer  400 . As used herein, the substrate is generally referred to as a porous substrate (e.g., porous substrate  310 ). However, the substrate may have non-porous portions or in some embodiments, may be a non-porous substrate (e.g., a silicon wafer). The disclosure provided herein may be applied to non-porous substrates.  FIG. 16  illustrates example non-porous substrates  310   a - 310   c.  In such embodiments, the substrate holder  350  may be configured such that inlet/outlet ports for injecting the liquid monomer  400  (e.g., first and second substrate holder inlet/outlet ports  272 ,  273 ) are disposed in the substrate holder  350  such that these inlet/outlet ports are not blocked by the non-porous substrate  310 . In  FIG. 16 , substrate holder inlet  281  is shown as well as substrate holder outlet  282  where liquid monomer  400  may pass through the substrate holder  350   b.    
     In some embodiments, the porous substrate  310  and substrate holder  350  may include single or multiple inlet/outlet ports, channels, or other flow paths. In some embodiments, the porous substrate  310  may be porous stainless steel filters, stainless steel mesh, a woven stainless steel filter, woven nylon filter, or combinations thereof. In some embodiments, the non-porous substrate  310  may include aluminum, delrin, glass, silicon wafer, stainless steel, or combinations thereof. In some embodiments, combinations of porous and non-porous material may be used such that the substrate includes portions of porous material and portions of non-porous material. 
     In some embodiments, the pump  730  may be a device used to push or direct the liquid monomer  400  from one location to another location. For instance, the pump  730  may be used to direct the liquid monomer  400  from an area of high pressure to low pressure and may be used to dispense the liquid monomer  400 . The pump  730  may be any suitable pump, for instance, a peristaltic pump, HPLC pump, syringe pump, similar pumps, or combinations thereof. In some embodiments, a plurality of pumps  730  may be used.  FIG. 17  illustrates an example using a single pump  730  that can be connected to multiple liquid monomer reservoirs  700 . For instance, as shown in  FIG. 17 , the pump  730  includes reservoir inlets  731   a,    731   b,  and  731   c  for connecting to various liquid monomer reservoirs  700 .  FIG. 17  also illustrates tube  750  that can be connected to the substrate holder  350  as discussed herein to inject liquid monomer  400  selectively from the various liquid monomer reservoirs  700  connected to pump  730 . 
     In some embodiments, draining of the liquid monomer  400  may include collecting, removing, isolating liquid monomer  400 , or combinations thereof, away from the desired location. In some embodiments, draining may be performed to prevent cross contamination of the liquid monomer  400  and ensure only the desired liquid monomer  400  is provided at a given location. In some embodiments, draining may be performed to ensure there is no residual or mixture of the previous liquid monomer  400 . In some embodiments, the drained fluids (e.g., liquid monomer  400 ) can be recollected and reused or recycled back into the respective reservoir (e.g., liquid monomer reservoir  700 ). 
     In some embodiments, draining may be passive. For example, in some embodiments, draining may be performed using solid boundary drain port  275  or tank drain port  276 , for example, in  FIG. 12 . For example, excess liquid monomer  410  may be drained at the bottom  764  of the containment vessel  200  as shown in  FIG. 7 .  FIG. 18  shows an experimental example of various passive drain points as conveyed in  FIG. 7 . In particular,  FIG. 18  illustrates an example of passive draining in a top-down orientation. In  FIG. 18 , inlet/outlet ports including first top drain  284 , second top drain  285 , and main drain  283  are illustrated as well as tubing  750   c  and z-axis stage  803 . In the embodiment illustrated in  FIG. 18 , first top drain  284  and second top drain  285  are draining holes (e.g., flow paths) that may allow excess liquid monomer  410  to fall to the bottom  764  of the containment vessel  200 . The main drain  283  is connected to the containment vessel  200  to prevent the containment vessel  200  from overflowing. The device  210  may include a variety of inlet/outlet ports disposed in the device  210 . 
     In some embodiments, draining may be active. For example, in some embodiments, draining may be performed through the inlet and outlet port that&#39;s connected to a pump  730 . Pump  730  may act as a vacuum to suck out excess fluid (e.g., excess liquid monomer  410 ). In some embodiments, the injection process in reverse flow direction may operate as a draining process. For example in  FIG. 13 , when liquid monomer  400  is injected, the liquid monomer  400  flows through the porous substrate  310 . In some embodiments, the direction of the flow of pump  730  may be changed to drain the liquid monomer  400  from the containment vessel  200 . 
     In some embodiments, the mechanical design of the device  210  may be designed in a way to maximize the draining efficiency. In some embodiments, the orientation may be modified. For instance, in  FIG. 7 , the orientation of the device  210  is top-down, which may have a better draining efficiency as opposed to, for instance, the embodiment illustrated in  FIG. 10 , which has a bottom-up orientation and may need additional active or passive draining methods. 
     In some embodiments, the draining process may be mechanically assisted using other elements in the device  210  to improve the efficiency of the active or passive draining process. For example, the containment vessel  200  or device  210  may slide, tilt, rotate, spin, vibrate, etc. to help direct excess fluid (e.g., excess liquid monomer  410 ) out of the containment vessel  200  or device  210  to improve the efficiency of the active or passive draining process as shown, for instance, in  FIG. 12 . 
     In some embodiments, for example in  FIG. 12 , a wiper blade or squeegee  783  may sweep through the bottom  764  of the containment vessel  200  to help push the fluid (e.g., liquid monomer  400 ) in one direction or help wipe away any residue from the solid boundary  250  during the draining process. In some embodiments, the wiper blade or squeegee  783  may be used to help push one liquid monomer (e.g., first liquid monomer  400   a  and/or second liquid monomer  400   b ) in one direction or help wipe away any residue from the solid boundary  250  during the draining process while another liquid monomer  400  (e.g., first liquid monomer  400   a  and/or second liquid monomer  400   b ) is injected into the porous substrate  310 . 
     In some embodiments, the solid boundary  250  may operate as a containment for fluid (e.g., liquid monomer  400 ) in the containment vessel  200 . For instance, the solid boundary  250  may operate as containment for the fluid at the desired location in the device  210 . In some embodiments, the solid boundary  250  may operate as a boundary for the gap  760  in which the injected liquid monomer  400  is filled. In some embodiments, the solid boundary  250  is optional and may not be present or needed. In some embodiments, the solid boundary  250  may be permeable. For example, in some embodiments, the solid boundary  250  may be permeable to certain desired gasses, such as oxygen, air, etc. In some embodiments, the solid boundary  250  may contain embedded or printed electronics. For example, in some embodiments, an array of micro heaters (e.g., microheater  260 ) may be disposed on or in the solid boundary  250  to control the temperature of the process. In some embodiments, the solid boundary  250  may contain an array of spiral conductive coils to generate magnetic field. In some embodiments, the solid boundary  250  may contain an array of capacitive electrodes to generate fringe electromagnetic fields. In some embodiments, the solid boundary  250  may contain an array of electrical contacts or electrodes or embedded sensors to detect temperature, pressure, etc. The solid boundary  250  can be a Light Emitting Device (LED) or a Liquid Cristal Display (LCD) type screen that emits patterned light itself. The solid boundary  250  can include a ground glass diffuser or holographic diffuser. 
     In some embodiments, the solid boundary  250  may be transparent to the wavelength of the polymerization light  500 . In some embodiments, the solid boundary  250  may operate as a patterned photomask. In some embodiments, the solid boundary  250  may be attached to the containment vessel  200 . In some embodiments, the solid boundary  250  may be a separate component from the containment vessel  200 . In some embodiments, the solid boundary  250  may be integrated into the containment vessel  200 . In some embodiments, the solid boundary  250  may move relative to the containment vessel  200 , the porous substrate  310 , the wiper blades, etc. In some embodiments, the configuration of the solid boundary  250  may be used to control or modify the draining process. For example, the solid boundary  250  may move relative to the wiper blade  783  and containment vessel  200  instead of the wiper blade  783  and containment vessel  200  moving relative to the solid boundary  250 . 
     The solid boundary  250  may be of any suitable thickness. In some embodiments, the solid boundary  250  may be very thin and may operate as a permeable membrane or diaphragm. 
     In some embodiments, the solid boundary  250  may include inlet/outlet ports for injection or draining as long as the solid boundary  250  does not block, hinder, or negatively impact polymerization in the presence of polymerization light  500 . Therefore, these inlet/outlet ports may be placed in a different locations away from where polymerization may take place.  FIG. 19  illustrates an example in which the solid boundary  250  includes a microfluidic channel  286  connected to tube  750   c  that is connected to pump  730   b  and third liquid monomer reservoir  725  containing third liquid monomer  400   c.  In the embodiment illustrated in  FIG. 19 , the microfluidic channel  286  allows injection and/or draining of third liquid monomer  400   c  from the bottom  764  of the containment vessel  200  rather than through the substrate holder  350  and porous substrate  310 . 
     The solid boundary  250  may include any suitable materials and may include a plurality of materials. In some embodiments, the solid boundary  250  may be coated with one or more materials. In some embodiments, the solid boundary  250  may be designed such that the surface properties of the solid boundary  250  may favor the injection process. For instance, in some embodiments, the solid boundary  250  may be coated with a low surface energy coating such as PDMS, TEFLON™ AF, CYTOP®, a silane, or combinations thereof to decrease the wettability (see e.g., coating  251  of  FIG. 19 ). 
     In some embodiments, a liquid bridge  762  may be formed as shown, for instance, in  FIG. 13 . For instance, the solid boundary  250  may have a rough surface or may be coated with another material to increase wettability of the surface. For instance, such configuration may be favorable in a top-down fabrication orientation as shown in  FIG. 6  because such configuration may help ensure the gap  760  is filled with the desired liquid monomer  400 . 
     As shown, for instance, in  FIG. 6  and  FIG. 13 , a liquid bridge  762  may form due to surface forces and wetting characteristics of the surfaces. A liquid bridge  762  is generally a liquid formation or connection of liquid between two solid objects. 
     In some embodiments, for instance, despite the coating  251  in  FIG. 13 , the liquid bridge  762  may disappear. For instance, in some embodiments, all of the fluid (e.g., liquid monomer  400 ) may fall to the bottom  264  of the containment vessel  200  depending on multiple factors. In some embodiments, these factors affect which forces (e.g., surface or gravitational forces) dominate. Thus, the liquid bridge  762  may or may not form. Factors include: the geometry of the 3D object (e.g., the height), the distance the porous substrate  310  and substrate holder  350  is away from the solid boundary  250 ; and injected fluid (e.g., liquid monomer  400 ) properties such as surface energy, viscosity, density, etc. 
     In some embodiments, the liquid bridge  762  may allow for a reduced amount of liquid monomer  400  needed for injection. In some embodiments, the liquid bridge  762  may not fully spread over the entire bottom  764  of the containment vessel  200  (e.g.,  FIG. 13 ). Such formation may reduce the need for excessive draining or actions needed for draining. The liquid bridge  762  may also provide a better determination of where the inlet/outlet ports may need to be placed. 
     In some embodiments, the formation of the liquid bridge  762  may be controlled or maintained. For example, the inlet/outlet ports may be used to continuously or intermittently inject and/or drain fluid (e.g., liquid monomer  400 ) from the containment vessel  200 . The rate at which these occur and the placement of the inlet/outlet ports may provide better control over the liquid bridge. The liquid bridge  762  may be formed of one liquid monomer (e.g., first liquid monomer  400   a ) and then formed of another liquid monomer (e.g., second liquid monomer  400   b ) during formation of the solid polymer  450 . In embodiments utilizing a porous substrate  310 , the liquid bridge  762  may be easily prepared and maintained for multiple liquid monomers  400  over the course of formation of the 3D object. 
     The fabrication orientation may be in any direction: top-down, bottom-up, left-right, right-left. The injection or draining of liquid monomer  400  and other fluids may occur at any suitable time during the process and may be simultaneous in some embodiments. Injection and/or draining of the liquid monomer  400  may be continuous or intermittent. For instance, the liquid monomer  400  may be continuously injected and/or drained while the liquid monomer  400  is exposed to polymerization light  500 . The substrate holder  350  may be continuously moved to allow for new liquid monomer  400  to be exposed to polymerization light  500 . In such embodiments, multi-material solid polymer  450  may still be prepared with different liquid monomers  400  (e.g., first liquid monomer  400   a  and second liquid monomer  400   b ) injected and/or drained such that at least one of the liquid monomers  400  is being injected and/or drained while exposing the appropriate liquid monomer  400  to polymerization light  500 . 
       FIG. 20  shows an example of various operations that may be performed during the present method. As shown, any point during the present method (e.g., before and/or after exposure) and during the separation or peeling process, injection and/or draining may occur. In the embodiment illustrated in  FIG. 20 , the z-axis position of the porous substrate  310  is shown over time. First liquid monomer  400   a  is injected; then second liquid monomer  400   b  is injected; and then first liquid monomer  400   a  is injected again. Time  1  relates to the injection of the current liquid monomer  400  before exposure, time  2  relates to the patterned UV exposure, time  3  relates to after exposure, the injection of alternate liquid monomer  400  and draining of the previous liquid monomer  400 , time  4  relates to moving the porous substrate  310  up while injecting alternate liquid monomer  400  and draining the previous liquid monomer  400 , time  5  relates to injecting the alternate liquid monomer  400  and draining the previous liquid monomer  400  before moving the porous substrate  310  down, and time  6  relates to moving the substrate down while injecting the alternate liquid monomer  400 . The new layer thickness is shown by T 1 . 
     In some embodiments, injection and/or draining may not occur. For instance, in embodiments where it is desired to use the same liquid monomer  400  for the next exposure to polymerization light  500 , injection and/or draining may not occur. In some embodiments, the liquid monomer  400  may be sourced from excess liquid monomer  410  in containment vessel  200 . In some embodiments, whether injection and/or draining is performed may depend on the orientation of the device  210 . For example, in a top-down orientation, draining may not be needed, while injection may be needed. In a bottom-up orientation, neither draining nor injection may be needed since the liquid monomer  400  may be sourced from excess liquid monomer  410  in the containment vessel  200  when it is desired to use the same liquid monomer  400  for the next exposure to polymerization light  500 . 
       FIGS. 21( a ) and 21( b )  are examples of porous substrates  310   f,    310   g  attached to substrate holders  350   c,    350   d.  The substrate holder  350   c  of  FIG. 21( a )  includes through hole  355  and the substrate holder  350   d  of  FIG. 21( b )  includes a side substrate holder inlet/outlet port  287 . 
       FIG. 22  shows the injection of fluid through the substrate holder  350  and porous substrate  310  using an external pump  730 . The substrate holder  350  is connected to the pump  730  using tube  750 . 
       FIG. 23  shows an example apparatus for a bottom-up orientation, for instance, as shown in schematics of  FIG. 10 ,  FIG. 13 , and  FIG. 14 . In the embodiment illustrated in  FIG. 23 , attachment device  807  is used to initiate mechanical manipulation of the containment vessel  200  (e.g., tilt, vibrate, rotate, etc.).  FIG. 24  shows a bottom view of apparatus, for instance, as shown in  FIG. 23 . In particular,  FIG. 24  shows a bottom view through the solid boundary  250  of an example device  210  with a liquid bridge  762 . In this view, a liquid bridge  762  is visible, for instance, as conveyed in  FIG. 13  and  FIG. 14 , and has not spread over the entire solid boundary  250 . The liquid monomer  400  fills the gap  760  and is exposed to polymerization light  500 . 
       FIGS. 25( a )-25( c )  show an example 3D object  600  fabricated using the injection approach. This 3D object  600  has an array of channels  470  of around 1 mm in length/width. The channels  470  may also operate as an inlet/outlet port allowing liquid monomer  400  from the porous substrate  310  through the channels  470 . 
       FIGS. 26( a )-26( d )  are a series of chronological pictures (time-lapse) of the injection process in a top-down orientation. Time increases from  FIG. 26( a )  to  FIG. 26( d ) . In the embodiment illustrated in  FIGS. 26( a )-26( d ) , liquid monomer  400  is injected through the porous substrate  310  and through the channels  470  (not shown) formed in the solid polymer  450 . Once the liquid monomer  400  reaches the top, the liquid monomer  400  forms a curved shape until gravitational forces dominate, resulting in excess liquid monomer  410  being drained to the bottom  764  of containment vessel  200 . The series of pictures in  FIGS. 26( a )-26( d )  also convey how injection/draining can be used as a method to clean, rinse, wash, or combination thereof. For example, if the liquid monomer  400  used was first liquid monomer  400   a,  then second liquid monomer  400   b  can be injected, which causes first liquid monomer  400   a  to be washed away leaving only second liquid monomer  400   b  remaining. This operation ensures that when second liquid monomer  400   b  is being exposed to polymerization light  500  for solidification, the second liquid monomer  400   b  may have no residual or cross-contamination of the previous liquid monomer  400  (e.g., first liquid monomer  400   a ). In addition, the act of injection also ensure any residual oligomer (e.g., partially reacted liquid monomer  400 ) is washed away. 
       FIGS. 27 and 28  show examples of a microstereolithography device  210  according to an embodiment of the present disclosure. Referring to  FIGS. 27 and 28 , the microstereolithography device  210  is similar to the device of  FIG. 3 . The solid boundary  250  is placed over the containment vessel  200  and may comprise a photomask  255  for patterning polymerization light  500 . The porous substrate  310  is attached to the substrate holder  350  and the substrate holder  350  includes an injection port  357  for the tube  750 . The photomask  255  is arranged to correspond to the porous substrate  310 . 
       FIG. 29  shows an example of a substrate holder  350  of a microstereolithography device  210  according to an embodiment of the present disclosure. Referring to  FIG. 29 , the porous substrate  310  is disposed on a top surface of the substrate holder  350 , and the pump  730  tubing injection port  357  for the tube  750  is also disposed on the same top surface of the substrate holder  350 . The through hole  355  (not shown) can be provided between the pump  730  tubing injection port and the porous substrate  310  such that the liquid monomer  400  provided from the pump  730  tubing injection port is ejected from the porous substrate  310 . 
       FIG. 30  shows an example of a solid polymer on a microstereolithography device  210  according to an embodiment of the present disclosure. Referring to  FIG. 30 , the solid polymer  450  is grown from the porous substrate  310  and is attached to the porous substrate  310 . In the embodiment illustrated in  FIG. 30 , the solid polymer  450  includes a plurality of dense empty channels  470 . 
       FIGS. 31 and 32  are CAD models of a microstereolithography system for polymerization according to an embodiment of the present disclosure. Referring to  FIGS. 31 and 32 , the system  1000  comprises a substrate  300  and a substrate holder  350  securing the substrate  300  in a fixed desired position. The substrate holder  350  allows 3 axes (X, Y, and Z) movement of the substrate  300  during setup for auto leveling and fixing the final position prior to fabrication. The substrate  300  can have electrical potential, thereby inducing adhesion of growing solid polymer  450  onto the substrate  300 , and reducing stiction at the solid boundary  450 . The substrate  300  and the substrate holder  350  can include sensors; such as pressure sensor, force sensor, temperature sensor, accelerometer, and position sensor to detect various fabrication conditions; actuators, and combinations thereof. 
     The system  1000  shown in  FIGS. 31 and 32  includes a lead screw  1050 , a stepper motor  1080 , Z-axis wheeled rail support system  1070  for reducing deflections, and Z-axis rail movement system  1060  that enable the substrate holder  350  to move in a vertical direction along a Z-axis. 
     The system  1000  shown in  FIGS. 31 and 32  also includes a containment vessel  200  configured to be attached to a bath sliding rail  1010 , a bath detachment unit  1020  between the containment vessel  200  and the bath sliding rail  1030 , and a bath sliding rail motor  1030  for moving the containment vessel  200 . The system  1000  further includes a projection lens  830 , a DLP projection system  820  for providing a polymerization light  500 , and two axes (X and Y) linear stage  1040 . 
       FIGS. 33 and 34  are CAD models of a containment vessel  200  of a microstereolithography device  210  for polymerization according to an embodiment of the present disclosure. Referring to  FIGS. 33 and 34 , the containment vessel  200 , which is configured to contain liquid monomer  400  and secure a solid boundary  250 , can be modular and the modular comprises a sealing O-ring  1130 , and a solid boundary clamping plate  1120 , providing flexibility in choosing and replacing the solid boundary  250 . The containment vessel in  FIGS. 33 and 34  include inlet/outlet ports such as containment vessel drain ports  291 ,  292 ,  293 , and  294 . The containment vessel  200  can include sensors, such as pressure sensor, force sensor, displacement sensor, temperature sensor, accelerometer, or combinations thereof. In addition, the containment vessel  200  can further include actuators such as piezoelectric actuator and motors. 
       FIG. 35  shows a schematic of an image stitching of a microstereolithography device according to an embodiment of the present disclosure. Referring to  FIG. 31 , the liquid monomer  400  is polymerized by sequentially exposing unit areas of exposure  1320  to polymerization light  500  (e.g., moving from unit area of exposure  1320  numbered 0 to unit area of exposure  1320  numbered 11), and thus, a high resolution large area image can be manufactured by the image stitching method stitching multiple small area high resolution images into a larger area image. As shown in  FIG. 35 , the unit areas of exposure  1320  include un-polymerized monomer area  1300  and polymerized monomer area  1310 . In an embodiment, the two axes linear stage of  FIG. 31  on which the DPL projection system  820  and the projection lens  830  are attached moves in the X axis and Y axis, thereby achieving image stitching. In another embodiment, the XY galvo scanning mirrors  810  of  FIG. 5  change the direction of projected polymerization light  500 , thereby accomplishing image stitching. In yet another embodiment, the movement of the substrate  300  or the containment vessel  200  in two axes (X and Y) with respect to the projected polymerization light  500  provides image stitching. 
       FIG. 36  shows a light source  510  integrating galvo scanning mirrors for image stitching of  FIG. 35 , and  FIGS. 37( a )-37( c )  show an example of a light source  510 . Referring to  FIGS. 36 and 37 ( a )- 37 ( c ), when the DLP  820  emits the light through the projection lens  830 , the XY galvo scanning mirrors  810  change the direction of the projected light  505  to polymerization light  500 , thereby accomplishing image stitching. After image stitching for one layer of the solid polymer  450 , the solid polymer  450  is pulled upwards by Z-axis movement of the porous substrate  310  and then another image stitching for the next layer of the solid polymer  450  is performed by adjusting the XY galvo scanning mirrors  810 .  FIGS. 37( a )-37( c )  also show light  505  being projected onto the XY galvo scanning mirrors  810 , the galvo scanning mirror device  813 , diffuser  811  to see the projected image, and the projected patterned image  812  which is visible on the diffuser that is being repositioned by the XY galvo scanning mirrors  810 . 
       FIG. 38  is a flowchart for an exemplary method in accordance with embodiments disclosed herein. In particular,  FIG. 38  illustrates method  3800  which includes injecting a first liquid monomer through a porous substrate to a porous substrate surface disposed in a containment vessel  3801 , exposing the first liquid monomer injected to the porous substrate surface to a polymerization light  3802 , injecting a second liquid monomer through the porous substrate to the porous substrate surface disposed in the containment vessel  3803 , and exposing the second liquid monomer injected to the porous substrate surface to the polymerization light  3804 . The method  3800  may also include draining excess liquid monomer from the containment vessel  3805  at any point during the method  3800  (as shown by dotted lines). 
     In some embodiments, the first and second liquid monomer may be injected simultaneously or sequentially. Further, the first and second liquid monomer may be exposed to polymerization light simultaneously or sequentially. 
     The present disclosure includes, but is not limited to, the following exemplified embodiments. 
     Embodiment 1. A device for additive manufacturing, comprising: 
     a containment vessel; and 
     a substrate disposed in the containment vessel and having a first substrate surface, 
     wherein at least a portion of the substrate is a porous substrate and the device is configured to inject a liquid monomer through the porous substrate such that the liquid monomer is polymerized to form a solid polymer on the portion of the substrate that is the porous substrate. 
     Embodiment 2. The device according to embodiment 1, further comprising a substrate holder attached to the substrate, wherein the substrate holder comprises one or more channels for the liquid monomer to flow through the substrate holder to the substrate. 
     Embodiment 3. The device according to any of embodiments 1-2, further comprising a liquid monomer reservoir accommodating the liquid monomer, at least one pump providing the liquid monomer to the substrate, and a tube connected to the pump and transferring the liquid monomer from the liquid monomer reservoir to the substrate. 
     Embodiment 4. The device according to any of embodiments 3, wherein the liquid monomer reservoir comprises a first liquid monomer reservoir and a second liquid monomer reservoir, wherein the first liquid monomer reservoir comprises a first liquid monomer different from a second liquid monomer disposed in the second liquid monomer reservoir. 
     Embodiment 5. The device according to any of embodiments 1-4, wherein the device is configured to inject a plurality of liquid monomers through the porous substrate. 
     Embodiment 6. The device according to any of embodiments 3-4, wherein the liquid monomer reservoir comprises a first liquid monomer reservoir and a second liquid monomer reservoir and the pump is configured to provide a first liquid monomer from the first liquid monomer reservoir, a second liquid monomer from the second liquid monomer reservoir, or combinations thereof to the substrate. 
     Embodiment 7. The device according to any of embodiments 1-6, further comprising a solid boundary disposed opposite the substrate and configured to expose a portion of the liquid monomer to polymerization light passing through the solid boundary. 
     Embodiment 8. The device according to any of embodiments 1-7, further comprising a light source configured to emit polymerization light to the liquid monomer, wherein the light source spatially controls polymerization of the liquid monomer to the solid polymer. 
     Embodiment 9. The device according to embodiment 7, wherein the solid boundary includes a photomask, is transparent, or is both transparent and includes a photomask. 
     Embodiment 10. The device according to any of embodiments 7 and 9, wherein the device includes one or more inlet/outlet ports disposed in the containment vessel, in the solid boundary, or combinations thereof. 
     Embodiment 11. The device according to any of embodiments 1-10, wherein the device is configured to form a solid polymer comprising one or more channels for liquid monomer to flow through the one or more channels. 
     Embodiment 12. The device according to any of embodiments 1-11, wherein the porous substrate comprises a plurality of pores disposed equally over the porous substrate and the solid polymer forms over pores of the porous substrate. 
     Embodiment 13. The device according to any of embodiments 1-12, wherein the solid polymer forms over a portion of the substrate that is non-porous. 
     Embodiment 14. A method of additive manufacturing comprising: 
     injecting a first liquid monomer through a porous substrate to a porous substrate surface disposed in a containment vessel; 
     exposing the first liquid monomer injected to the porous substrate surface to a polymerization light to form a first solid polymer disposed on the porous substrate surface; 
     injecting a second liquid monomer through the porous substrate to the porous substrate surface disposed in the containment vessel; and 
     exposing the second liquid monomer injected to the porous substrate surface to the polymerization light to form a second solid polymer disposed on the porous substrate surface. 
     Embodiment 15. The method according to embodiment 14, wherein the first liquid monomer is different from the second liquid monomer. 
     Embodiment 16. The method according to any of embodiments 14-15, wherein the second liquid monomer is injected immediately following injection of the first liquid monomer or simultaneously with injection of the first liquid monomer. 
     Embodiment 17. The method according to any of embodiments 14-16, wherein the containment vessel comprises a solid boundary and injection of the first liquid monomer through the porous substrate forms a liquid bridge disposed between the porous substrate and the solid boundary. 
     Embodiment 18. The method according to any of embodiments 14-17, wherein the porous substrate comprises a plurality of pores to allow the first liquid monomer and the second liquid monomer to flow through the plurality of pores to multiple locations along the porous substrate surface. 
     Embodiment 19. The method according to any of embodiments 14-18, further comprising draining excess liquid monomer from the containment vessel through one or more inlet/outlet ports disposed in the containment vessel, a solid boundary disposed in the containment vessel, or combinations thereof. 
     Embodiment 20. A 3D object formed using the device according to any of embodiments 1-13. 
     Embodiment 21. A 3D object formed using the method according to any of embodiments 14-19. 
     It should be understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application. 
     All patents, patent applications, provisional applications, and publications referred to or cited herein are incorporated by reference in their entirety, including all figures and tables, to the extent they are not inconsistent with the explicit teachings of this specification.