Patent Publication Number: US-9833978-B2

Title: Monolithic fabrication of three-dimensional structures

Description:
RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 13/961,510, filed on 7 Aug. 2013, which is a continuation of PCT/US2012/024682, filed on 10 Feb. 2012, and a continuation-in-part of U.S. application Ser. No. 13/519,901, which is the National Stage of International Application No. PCT/US2011/024479, filed on 11 Feb. 2011, the entire contents of each of which are incorporated herein by reference. This application also claims the benefit of U.S. Provisional Application No. 61/303,629 61/467,765, filed on 25 Mar. 2011, and of U.S. Provisional Application No. 61/561,144, filed on 17 Nov. 2011, the entire contents of each of which are incorporated herein by reference. 
    
    
     GOVERNMENT SUPPORT 
     The invention was supported, in whole or in part, by grants under Contract Number W911NF-08-2-0004 from the Army Research Laboratory, Contract Numbers CMMI-07466 38 and CCF-0926148 from the National Science Foundation, and Contract Number FA9550-09-1-0156-DOD35CAP from the Air Force Office of Scientific Research. The U.S. Government has certain rights in the invention. 
    
    
     BACKGROUND 
     Micron-scale device fabrication is dominated by micro-electro-mechanical structures (MEMS) technology, which typically involves a single planar substrate and serial processes. Meanwhile, centimeter-scale manufacturing is covered by a multitude of conventional machining processes. Manufacturing at the millimeter scale, however, is plagued with fabrication and assembly issues that greatly impact the cost and performance of micro-robots and other functional mechanical devices at this scale. 
     SUMMARY 
     Three-dimensional structures and methods for their fabrication are described herein. Various embodiments of the structures and methods may include some or all of the elements, features and steps described below. 
     A three-dimensional structure can be formed by stacking a plurality of patterned layers and bonding the plurality of patterned layers (i.e., layers having a patterned shape/features formed, e.g., by machining) at selected locations to form a laminate structure with inter-layer bonds. The laminate structure can then be expanded into an expanded three-dimensional configuration by selectively distorting at least one of the layers to produce gaps between layers while maintaining at least some of the inter-layer bonds. 
     The layers in the structure can include at least one rigid layer and at least one flexible layer, wherein the rigid layer includes a plurality of rigid segments, and the flexible layer can extend between the rigid segments to serve as a joint. The flexible layers are substantially less rigid than the rigid layers and can have a rigidity that is at least an order of magnitude (i.e., greater than 10× or greater than 100×) greater than the rigidity; likewise, the flexible layer can have at least 10 times or at least 100 times the flexibility of the rigid layers. The layers can then be stacked and bonded at selected locations to form a laminate structure with inter-layer bonds, and the laminate structure can be distorted or flexed to produce an expanded three-dimensional structure, wherein the layers are joined at the selected bonding locations and separated at other locations. 
     The methods and structures described herein can be used, for example, across and beyond the entire millimeter-scale manufacturing process (e.g., for apparatus with dimensions from 100 μm to 10 mm), and they enable mass production of precisely fabricated mechanisms, machines, and autonomous robots at this scale. Some of the manufacturing techniques described herein may be the same as or similar or analogous to those used for multi-layer printed circuit board (PCB) manufacturing, while some of the assembly techniques may be the same as or similar or analogous to assembly techniques used for MEMS, paper origami, and pop-up books; and additional techniques used in those contexts may be adapted for use with this invention. 
     When compared to traditional MEMS, the methods described herein are extremely versatile with respect to the materials that can be used. In addition, traditional MEMS are largely limited to bulk addition of materials, whereas the methods described herein can be used not only to add precisely patterned layers, but also full sub-components, such as integrated circuits, flex circuits, actuators, batteries, etc. The thermal requirements of the multi-layer, super-planar structures described herein can also be much lower, and the fabrication-equipment costs can be much lower, as well. Further still, processing steps in these methods on the various layers can be performed simultaneously in parallel, while much of MEMS processing takes place sequentially in series. 
     These methods can also be used to fabricate a wide variety of devices and structures within a device, including I-beams; large actuated mirrors for use as optical switches; steerable antennae; high-speed, high-power physical switching; and robotic flying devices. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows the fabrication, assembly and release steps in a schematic representation of the fabrication of a PC-MEMS machine. 
         FIG. 2  illustrates formation of folding joints. 
         FIGS. 3 a - c    shows sequential lamination steps in mobee fabrication using a midplate to support the lead zirconate titanate (PZT) plates. 
         FIG. 4  is an illustration of a single layer made of 0-90-0 carbon-fiber composite. 
         FIG. 5  is an exploded perspective view of a 15-layer layup for press curing. 
         FIG. 6  is a perspective photographic view of all 15 layers from  FIG. 5 , as fabricated; two PZT plates for actuation are visible near the bottom of the image. 
         FIG. 7  is a perspective photographic view of the layup of  FIG. 5  ready for press curing; one piezoelectric plate is visible after being picked and placed; the second actuator plate is in an internal cavity in the layup. 
         FIG. 8  is a close-up photographic view of the spring clip holding the actuator in place. 
         FIGS. 9 a  and 9 b    offer a schematic perspective view showing the functioning of the assembly scaffold to realize desired assembly trajectories. 
         FIG. 10 a    includes a perspective view of a monolithic robotic bee (mobee) with solder-locked joints highlighted via arrows; and  FIGS. 10 b - d    provide schematic illustrations of the folding and locking steps with brass plates and solder. 
         FIG. 11  is a perspective photographic view of the released mobee before pop-up assembly 
         FIG. 12  is a perspective photographic view of the released mobee after pop-up assembly. 
         FIG. 13  is a back-angle perspective photographic view of a mobee in a surrounding frame assembled from a multi-layer laminate via a method of this disclosure. 
         FIG. 14  is a front-angle perspective photographic view of the mobee of  FIG. 13 . 
         FIG. 15  is an elevated front perspective photographic view of the mobee of  FIG. 13 . 
         FIG. 16  is a right-side perspective photographic view of the mobee of  FIG. 13 . 
         FIG. 17  is a side-zoom perspective photographic view of the mobee of  FIG. 13 . 
         FIG. 18  is a back-angle perspective photographic view of the mobee of  FIG. 13  before complete assembly with the layers only partially folded. 
         FIG. 19  is a back-angle perspective photographic view of the mobee of  FIG. 13  against a white background. 
         FIG. 20  is a perspective photographic view of the mobee after release from the surrounding scaffold. 
         FIG. 21  is an illustration of the cuts in a shared layer of titanium and brass to form the wings and solder pads in a mobee. 
         FIG. 22  is a resulting wing for the mobee with the membrane layers attached. 
         FIG. 23  is an illustration of a laminate stack and alignment tool for monolithic fabrication. 
         FIGS. 24-27  illustrate interlocking chain-link structures fabricated via monolithic fabrication. 
         FIGS. 28-30  illustrate the folding mechanism and layers of a Wright Flyer model. 
         FIGS. 31-36  illustrate a spring-loaded monolithic structure for assembly into an eight-sided enclosure. 
         FIGS. 37-42  show a sequence of film captures and animated clips showing the unfolding assembly of an icosahedron. 
         FIGS. 43-48  show the sequence of clips from the unfolding assembly of the icosahedron with most of the scaffold removed from view. 
         FIG. 49  is a side view of the assembled icosahedron amidst the scaffold. 
         FIG. 50  shows the underside of the scaffold and rotating plate that serve to assemble the icosahedron. 
     
    
    
     In the accompanying drawings, like reference characters refer to the same or similar parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating particular principles, discussed below. 
     DETAILED DESCRIPTION 
     The foregoing and other features and advantages of various aspects of the invention(s) will be apparent from the following, more-particular description of various concepts and specific embodiments within the broader bounds of the invention(s). Various aspects of the subject matter introduced above and discussed in greater detail below may be implemented in any of numerous ways, as the subject matter is not limited to any particular manner of implementation. Examples of specific implementations and applications are provided primarily for illustrative purposes. 
     Unless otherwise defined, used or characterized herein, terms that are used herein (including technical and scientific terms) are to be interpreted as having a meaning that is consistent with their accepted meaning in the context of the relevant art and are not to be interpreted in an idealized or overly formal sense unless expressly so defined herein. For example, if a particular composition is referenced, the composition may be substantially, though not perfectly pure, as practical and imperfect realities may apply; e.g., the potential presence of at least trace impurities (e.g., at less than 1 or 2% by weight or volume) can be understood as being within the scope of the description; likewise, if a particular shape is referenced, the shape is intended to include imperfect variations from ideal shapes, e.g., due to machining tolerances. 
     Although the terms, first, second, third, etc., may be used herein to describe various elements, these elements are not to be limited by these terms. These terms are simply used to distinguish one element from another. Thus, a first element, discussed below, could be termed a second element without departing from the teachings of the exemplary embodiments. 
     Spatially relative terms, such as “above,” “upper,” “beneath,” “below,” “lower,” and the like, may be used herein for ease of description to describe the relationship of one element to another element, as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the apparatus in use or operation in addition to the orientation described and/or depicted in the figures. For example, if the apparatus in the figures is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the exemplary term, “above,” may encompass both an orientation of above and below. The apparatus may be otherwise oriented (e.g., rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly. 
     Further still, in this disclosure, when an element is referred to as being “on,” “connected to” or “coupled to” another element, it may be directly on, connected or coupled to the other element or intervening elements may be present unless otherwise specified. 
     The terminology used herein is for the purpose of describing particular embodiments and is not intended to be limiting of exemplary embodiments. As used herein, the singular forms, “a,” “an” and “the,” are intended to include the plural forms as well, unless the context clearly indicates otherwise. Additionally, the terms, “includes,” “including,” “comprises” and “comprising,” specify the presence of the stated elements or steps but do not preclude the presence or addition of one or more other elements or steps. 
     The apparatus of this disclosure, as described herein, can be utilized in a variety of applications, including the PARITy drivetrain described in PCT Application No. US2011/24479, entitled, “Passive Torque Balancing in a High-Frequency Oscillating System.” 
     The apparatus is a multi-layer, super-planar structure. By “planar,” we mean a layer or plane that can be distorted, flexed or folded (these terms may be used interchangeably herein). An embodiment of this structure can be achieved, for example, by forming a five-layer composite with the following sequence of layers: rigid layer, adhesive layer, flexible layer, adhesive layer, rigid layer. Alternatively, a thinner composite can be formed from a stacking of just a rigid layer, an adhesive layer, and a flexible layer, though this structure is not symmetric. The rigid layers are machined to have gaps that correspond to fold lines, while the flexible layer is continuous, thereby providing a joint where the flexible layer extends across the gaps machined from the rigid layers. 
     Characterization of the structure as being “super-planar” means taking multiple planar layers and selectively connecting them. An analogy here can be drawn to circuit boards, where electrical vias connect circuits on different layers. Here, in contrast, the structure is made with “mechanical vias.” By stacking multiple planar layers, the range of achievable devices is greatly expanded. The super-planar structure also enables features and components to be packed into the structure that would not fit if the device could only be made out of one planar sheet. Advantageously, super-planar structures with mechanisms that operate normal to the plane can now be made with these techniques. In practice, forming Sarrus linkages between planar layers is an advantageous strategy for designing an assembly mechanism/scaffold. Other mechanisms can attach to the Sarrus links to effect the intended component rotations. 
     The multi-layer super-planar structure can be fabricated via the following sequence of steps, which are further described, below: (1) machine each planar layer, (2) machine or pattern adhesives, (3) stack and laminate the layers under conditions to effect bonding, (4) post-lamination machining of the multi-layer structure, (5) post-lamination treatment of the multi-layer structure, (6) freeing an assembly degree of freedom in each structure, (7) locking connections between structural members, (8) freeing any non-assembly degrees of freedom, and (9) separating finished parts from a scrap frame. 
     A schematic representation of the PC-MEMS process is provided in  FIG. 1 , illustrating how the basic operations of micromachining  101 , lamination and pick-and-place  102 , folding  103 , locking  104 , and additional micromachining  105  can be arranged to manufacture PC-MEMS machines  106 . 
     These assembly techniques can include the formation of folding joints  21 , as illustrated in  FIG. 2 , wherein (a) features are first micro-machined  101  in individual material layers, and the resulting chips  13  are removed; (b) during lamination, dowel pins  20  align material layers while heat and pressure are applied; here, two rigid carbon-fiber layers  12  bonded to a flexible polyimide-film layer  16  with adhesive  14  form a five-layer laminate  15  referred to as a “linkage sub-laminate”; (c) micro-machining cuts mechanical bridges  17  that constrain individual elements, allowing the creation of articulated structures; and (d) a completed folding joint  21  is formed and removed from the surrounding scaffold  19 . The castellated pattern allows this flexure joint  21  to approximate an ideal revolute joint. All assembly folds in a more-complex assembly can be incorporated into a single “pop-up” degree of freedom, which can be locked in place by a soldering process after pop up and then released by micro-machining. 
     A) Mobee 
     In one embodiment, this fabrication method can produce a monolithic bee (mobee) structure  26 , shown, e.g., in  FIG. 12 , including a rigid airframe  27 , a piezoelectric actuator  24 , a single-degree-of-freedom power transmission, and two wings  28 . The mobee  26  can include eight sub-components (e.g., an actuator  24 , an airframe  27 , a slider-crank, a transmission, two wing hinges, and two wings  28 ) all co-fabricated in a single multi-layer laminate, integrating a diverse palette of layers of materials including carbon-fiber-reinforced polymer, polyimide film, piezoelectric lead zirconate titanate (PZT) for the actuator  24 , brass for pads  47  for locking the joints, and titanium for the wing spars  52 . In one embodiment, the mobee  26  has a 39-mm wingspan, an 18-mm length, an out-of-plane height of 2.4 mm, and a mass of 90 mg; and the wings  28  can reciprocate at a large angle (e.g., exceeding 120°). Furthermore, the parallel processes of “pop-up” folding and dip-solder locking replace the manual folding, assembling, and glueing used in previous processes. Elimination of all skilled manual steps enables panelized mass production of the mobee  26 . The processes described herein can likewise be used to form a wide variety of other structures, in addition to the mobee  26 . 
     1) Machining of Layers 
     In one embodiment, the multi-layer structure is formed from a multitude of thin (1.5-μm to about 150-μm thick) layers of various materials, as shown in  FIGS. 3 a - c   . As shown in  FIG. 3 a   , layers of carbon fiber  12 , adhesive  14 , and polyimide  16  are stacked with a spacer layer  39  separating respective linkages  35  and  37 , above and below. A mid-plate  23  with a raised section  33  for supporting the piezoelectric actuator  24  is also inserted between the spacer layer  39  and the lower linkage  37 . 
     These layers are laser micro-machined (e.g., by a diode-pumped solid-state pulsed laser) with desired features, usually cutting all the way through the layer to create individual planar structures, as shown, e.g., in  FIGS. 6-8 . Each layer is micro-machined so as to leave a unified (contiguous) part with robust connections to surrounding alignment holes. The micro-machining can produce complex in-plane features with dimensions as small as 10 μm. In particular embodiments, many copies of the mobee device  26  are formed on a laminate panel, and the machining process removes sufficient material to form each part and part feature, while leaving thin tabs to connect each device to the surrounding laminate; in this regard, the arrangement of devices in a laminate panel can be similar to that of a batch of circuit boards attached to a surrounding laminate structure by thin, easily breakable tabs. 
     In this case, the tabs (bridges)  17  (shown in  FIG. 2 ) connecting the devices to the surrounding laminate will be removed after lamination or assembly. Layers of metal, composite, polymer, etc., are machined or formed by virtually any method; and virtually any material may be used. Example machining methods include laser cutting from sheet material, photo-chemical etching, punching, electroforming, electric discharge machining, etc.—basically any method that has appropriate resolution and compatibility with the desired material. Machined layers may then be subjected to additional processes, such as cleaning/etching to remove machining debris, plating (e.g., plating fluxed copper on a layer to facilitate adhesion of solder thereto), preparation for bonding, annealing, etc. The unified nature of each layer makes handling and post-processing easy. Advantageously, each layer can be a different material and can be machined and treated differently from each of the other layers. 
     Each layer can also advantageously be formed of a material that is sufficiently rigid, strong and tough to allow holes  22  for alignment pins  20  and other features to be machined into the layer, to facilitate easy handling, and to not distort when placed into the layup and when restrained by alignment pins  30 . In other embodiments, layers that do not have the structural stability to support alignment features can nevertheless be used by attaching such layers, in bulk form, to a rigid frame that meets these objectives without introducing enough additional thickness to disturb the other layers or parts in the laminate. 
     In particular examples, a very thin polymer film (e.g., 2-5 microns thick) is included among the layers. Due to its thinness and insulating qualities, the thin polymer film is prone to wrinkling and electrostatic handling issues. To address this tendency, the thin polymer film can be lightly stretched, in bulk form, to a flat and controlled state and then bonded to a thin frame that is made, for example, of thin metal or fiberglass composite. Next, the thin polymer layer can be machined with the fine part features (e.g., tiny holes in the polymer at precise locations), and the alignment hole features can be machined into the frame material. 
     In additional embodiments, the device can be designed to mitigate thin-layer handling issues. For example, a part within the device can be designed such that all machining pertinent to a fragile layer is performed post-lamination; and, thus, this layer will not require precision alignment when put into the laminate, though the material is advantageously capable of being placed into the laminate sufficiently flat and extending over a sufficient area to cover the desired parts of the device. 
     In exemplary embodiments, bulk polymer films (formed, e.g., of polyester, polyimide, etc.); metal sheets and foils [formed, e.g., of stainless steel, spring steel, titanium, copper, invar (FeNi36), nickel-titanium alloy (nitinol), aluminum, etc.]; copper-clad laminates; carbon fiber and glass fiber composites; thermoplastic or thermoset adhesive films; ceramic sheets; etc.; can be laser machined to make the layers that are laminated to form the multi-layer structure. The laser machining can be performed, e.g., with a 355-nm laser (from DPSS Lasers Inc. of Santa Clara, Calif.) with a spot size of about 7 microns on materials with typical thicknesses of 1-150-μm, although thicker layers can be machined with such a laser, well. Accordingly, this type of laser allows for very high resolution and an ability to machine almost any type of material. 
     2) Machining or Patterning Adhesives 
     Adhesion between layers is achieved by patterning adhesive onto one or both sides of a non-adhesive layer or by using free-standing adhesive layers (“bondplies”)  14 . In the latter case, an intrinsically adhesive layer  14 , e.g., in the form of a sheet of thermoplastic or thermoset film adhesive, or an adhesive laminate, such as a structural material layer with adhesive pre-bonded to one or both sides. The adhesive layer  14  is machined like the other layers. Specific examples of sheets that can be used as the adhesive layer  14  include sheet adhesives used in making flex circuits (e.g., DuPont FR1500 adhesive sheet) or polyimide film  16  coated with FEP thermoplastic adhesive  14  on one or both sides. Free-standing sheet adhesives can be acrylic-based for thermosets; alternatively, the adhesive can be thermoplastic, wherein the thermoplastic film can be formed of polyester, fluorinated ethylene propylene (or other fluoropolymer), polyamide, polyetheretherketone, liquid crystal polymer, thermoplastic polyimide, etc. Any of these adhesives can also be applied on one or both sides to a non-adhesive carrier. In additional embodiments, a layer may serve both as a structural layer  12  and as a thermoset adhesive  14 —for example, liquid crystal polymer or thermoplastic polyimide. Furthermore, for special types of structural layers, a variety of wafer bonding techniques that do not require an adhesive may be employed, such as fusion bonding. 
     In another technique for achieving adhesion between layers, adhesive  14  is applied and patterned directly on a non-adhesive layer  12 . This technique can be used where, for example, the type of adhesive desired may not be amenable to free-standing form. Examples of such an adhesive  14  include solders, which are inherently inclined to form a very thin layer, or adhesives that are applied in liquid form (by spraying, stenciling, dipping, spin coating, etc.) and then b-stage cured and patterned. B-staged epoxy films are commonly available, but they usually cannot support themselves unless they are quite thick or reinforced with scrim. 
     The resulting bond can be a “tack bond,” wherein the adhesive  14  is lightly cross-linked to an adjacent layer before laser micromachining with sufficient tack to hold it in place for subsequent machining and with sufficient strength to allow removal of the adhesive backing layer. The tack bonding allows for creation of an “island” of adhesive  14  in a press layup that is not part of a contiguous piece, which offers a significant increase in capability. Another reason for tacking the adhesive  14  to an adjacent structural layer is to allow for unsupported “islands” of adhesive  14  to be attached to another layer without having to establish a physical link from that desired adhesive patch to the surrounding “frame” of material containing the alignment features. In one embodiment, a photoimagable liquid adhesive, such as benzocyclobutene, can be applied in a thin layer, soft baked, and then patterned using lithography, leaving a selective pattern of adhesive. Other photoimagable adhesives used in wafer bonding can also be used. 
     The adhesive  14  is patterned while initially tacked to its carrier film, aligned to the structural layer  12  using pins  20 , and then tacked to at least one adjoining layer in the layup  29  with heat and pressure (e.g., at 200° C. and 340 kPa for one hour). Alternatively, the adhesive layer can be patterned by micro-machining it as a free sheet. Tack bonding can involve application of heat and pressure at a lower intensity and for less time than is required for a complete bond of the adhesive. In yet another embodiment, the adhesive film  14  can be tack bonded in bulk, and then machined using, for example, laser skiving/etching. Advantageously, use of this variation can be limited to contexts where the machining process does not damage the host layer. Both of these variations were tried using DuPont FR1500 adhesive sheet and laser skiving. 
     3) Stacking and Laminating the Layers 
     To form the multi-layer laminate structure  31 , a multitude of these layers (e.g., up to 15 layers have been demonstrated) are ultrasonically cleaned and exposed to an oxygen plasma to promote bonding and aligned in a stack by passing several vertically oriented precision dowel pins  20  respectively through several alignment apertures  22  in each of the layers, as shown in  FIGS. 4, 5, and 7 , and by using a set of flat tooling plates with matching relief holes for the alignment pins  20 . In other embodiments, other alignment techniques (e.g., optical alignment) can be used. All layers can be aligned and laminated together. 
     Linkages in the laminated layers can be planar (where all joint axes are parallel); or the joint axes can be non-parallel, allowing for non-planar linkages, such as spherical joints. 
     In the fifteen-layer example, the final layup  29  included the following layers, which formed a pair of linkages (i.e., structures wherein flexible layers  16  are bonded to rigid segments  12  and extend across the gaps between the rigid segments  12 ), thereby enabling flexure of the rigid segments  12  relative to one another at the flexible layer  16  in the gaps between the rigid segments  12 , wherein those exposed sections of the flexible layer  16  effectively serve as joints. In the embodiment of  FIG. 5 , the layers are identified in the sequence of their stacking order as follows, wherein the “rigid” layers  12  comprise carbon and “flexible” layers  16  are formed of polyimide: 
     Linkage  1 :
         1) carbon layer  12     2) acrylic sheet adhesive  14     3) polyimide film  16     4) acrylic sheet adhesive  14     5) carbon layer  12     6) acrylic sheet adhesive  14         

     Lone Carbon Layer for Spacing:
         7) carbon layer  12     8) acrylic sheet adhesive  14         

     Linkage  2 :
         9) carbon layer  12     10) acrylic sheet adhesive  14     11) polyimide film  16     12) acrylic sheet adhesive  14     13) carbon layer  12     14) acrylic sheet adhesive  14         

     Wing Membrane:
         15) wing membrane (polyimide or polyester film)  18         

     The choice of the flexible layers  16 , which can be formed of a polymer—polyimide in this example—is based upon compatibility with the matrix resin in the carbon fiber. The cure cycle can reach a maximum temperature of 177° C. using a curing profile of four hours. Polyimide film (available, e.g., as KAPTON film from E.I. du Pont de Nemours and Company), for example, has a sufficiently high service temperature (up to 400° C.) to survive the curing step. The polyimide film can have a thickness of, e.g., 7.5 μm. 
     The rigid layers  12  in this embodiment are standard cured carbon fiber sheets (e.g., with three layers of unidirectional fibers, wherein the fiber layers are oriented at 0°, 90°, and 0° to provide thickness in two orthogonal directions) having a thickness of, e.g., 100 μm. Fifteen layers are used because the adhesive sheet  14  (e.g., in the form of a B-staged acrylic sheet adhesive, commercially available, e.g., as DuPont PYRALUX FR1500 acrylic sheets) in this embodiment is separate from each layer of structural material in the layup  29  of this embodiment. Accordingly, the adhesive sheet  14  can be laser machined into a pattern differing from any structural layer, and aligned layups  29  of many layers can be made. This capability enables the fabrication of parts with many linkage layers that are perfectly or near-perfectly aligned. 
     After the layers are stacked to form the layup  29 , pressure and heat are applied, typically in a heated platen press to cure/crosslink the adhesive layers. Specifically, the layup  29  can be cured in a heated press, autoclave, or other device that provides the atmosphere (or lack thereof), temperature, and pressure to achieve the bonding conditions required by the adhesive. One embodiment of the curing process uses 50-200 pounds-per-square-inch (psi) clamping pressure, 350° F. (177° C.) temperature, and two-hours cure time (optionally with temperature ramping control) to cure DuPont PYRALUX FR1500 acrylic sheets in a heated press with temperature, pressure, and atmosphere control. 
     Though a single-step lamination process has been demonstrated, a process with two sequential lamination steps may be preferred in various embodiments because it provides a third technique for altering layering composition and because it may ease the problem of chip removal. A separate printed-circuit micro-electromechanical-system (PC-MEMS) structure called a “midplate”  23  can be included to alter the layer stack underneath the PZT plate  24  during initial lamination then removed, as shown in  FIG. 3 , allowing precise accounting for the thickness of the piezoelectric plate  24 . The midplate  23  can be in the form of a simple reusable PC-MEMS laminate of a flat carbon-fiber plate containing alignment holes and a central polyimide boss  33  designed to support the lower PZT plate  24 . This initial lamination results in two sub-laminates  35  and  37 , each with a layered structure including a sequence of carbon  12 , adhesive  14 , polyimide  16 , adhesive  14 , and carbon  12 , as shown in  FIGS. 3 b  and 3 c   . The midplate  23  replaces adhesive layer  14  on top of the lower sub-laminate  37 . The upper sub-laminate  35  also includes the two PZT plates  24 . 
     An adhesive layer  14  is tacked to the lower sub-laminate  37  and micro-machined, while the upper sub-laminate  35  is micro-machined to sever mechanical bridges on the central carbon spacer layer  39 . After chips are removed from the central carbon spacer layer  39 , these two sub-laminates  35  and  37  are stacked and laminated together to produce the laminate structure  31  shown in  FIG. 3   c.    
     The corresponding single-step process utilizes discrete shims underneath the lower PZT plate  24  for support. In addition, machining steps often create unwanted material regions, or “chips,” which must be physically removed. When the spacer layer  39  is micro-machined after initial lamination, all chips from micro-machining can easily be removed from the exposed surface. Post-lamination machining in a single-step process results in trapped chips that must be highly engineered to enable physical removal from the internal spacer layer  39 . 
     4) Post-Lamination Machining 
     The laminate  31  is then machined (e.g., by severing tabs with a laser) to release the device(s)  26  from a surrounding frame structure in the laminate  31 . In some embodiments, additional machining that is not involved with freeing the device  26  from the external frame (circumscribing the device  26  in the laminate  31 ) is reserved for after lamination (e.g., post-lamination machining of a layer that is structurally weak or that, for some other reason, cannot be precisely aligned since the weak layer is better supported after lamination). 
     5) Post-Lamination Treatment 
     A post-lamination treatment can include plating or coating on an exposed layer; and/or the post-lamination treatment can include the addition of a material, such as solder paste, by silk screening or some other method, e.g., for the later joint “locking” step, as shown in  FIG. 10 . Additional components may be attached to the laminate  31  using a pick-and-place methodology. Pick-and-place operations can be used to insert discrete components into layups  29  before press lamination. 
     For example, a stimulus responsive material  24 , such as an electroactive material, can be inserted among the layers to serve as an actuator. In one embodiment, a lead zirconate titanate piezoelectric plate  24  is mounted on a spring clip  25  in the carbon layer  12  (shown in  FIG. 8 ) and has been demonstrated to create a functional bimorph cantilever actuator within a device. A broad range of discrete components can be inserted this way, such as mirrors or other optical components, micro-electro-mechanical systems (MEMS), discrete sensors, etc. These components may alternatively be added earlier—e.g., before lamination at some point in the stack-up process—or they can be added after the subsequent assembly of the device. 
     Press lamination and laser micro-machining can be conducted multiple times. For example, five layers can be laser micro-machined, then press laminated, then laser micro-machined again. Another three layers can be separately laser micro-machined, then press laminated, then laser micro-machined again. These two partial layups can then be press laminated together with a single adhesive layer between them, for a final layup of nine layers. 
     6) Freeing the Assembly Degree of Freedom in Each Part 
     The resulting laminate can then be laser micro-machined and/or scrap materials can be removed from the laminate to “release” functional components in each part. The parts, as laminated, may unfold to have many actuated and passive mechanical degrees of freedom; though, in some embodiments, restraining these non-assembly degrees of freedom during the assembly folding process is advantageous. For example, elements of a flexural linkage can be held in place (i.e., locked)—to prevent the linkages from flexing—by a rigid bar element alongside the elements or by a fixed tab forming an integral bridge between the elements and the surrounding structure. Using a machining process (e.g., punch die or laser cutting), the tabs or other features that restrain the assembly degree of freedom are severed. 
     7) Assembly 
     As fabricated, the mobee can be a flat multi-layer laminate with limited three dimensional structure. Its components undergo a variety of assembly trajectories to realize the final fully three-dimensional topology. A co-fabricated mechanical transmission called an “assembly scaffold” couples all of these assembly trajectories into a single degree of freedom. The mobee emerges from the manufacturing process as a three-degree-of-freedom machine, though internal mechanical connections eliminate these active degrees of freedom during assembly. The resulting mechanism uses 137 folding joints to assume a fully three-dimensional topology in one motion, similar to those created by paper folding in pop-up books. 
     Assembly of the mobee  26  can include two parallel plates  42  and  43  of the assembly scaffold  19 , one constructed from each linkage sub-laminate  35  and  37 , coupled mechanically to form a Sarrus linkage  67 . These plates  42  and  43  surround mobee&#39;s mechanical components and are constrained to a single linear degree of freedom separating the plates  42  and  43  along their normal axes. Interior linkages  44  and  45 , driven by plate separation, are connected to each of mobee&#39;s core components to realize all desired assembly trajectories, as shown in  FIGS. 9 a  and 9 b   . When the top horizontally oriented plate  42  of the scaffold  19  is lifted, interior linkages drive the mobee&#39;s assembly folds. 
     The Sarrus-linkage-assembly scaffold  19  provides a versatile framework to produce diverse assembly motions coupled together into a single degree of freedom. Rotations to a wide range of angles about any axis in a linkage plane can be achieved through an appropriately designed interior linkage. The mobee also incorporates more complex interior linkages to translate the wings  28  and the actuator  24  along three separate arcs without rotation during folding assembly. 
     One plate  42 / 43  of the assembly scaffold  19  is secured to an external jig, which drives six dowel pins  20  through clearance holes in the secured plate to separate the opposing plate  43 / 42 . Separating the scaffold plates  42  and  43  initiates a single-degree-of-freedom folding assembly, causing the mobee&#39;s components to assemble into their final three-dimensional configuration. Various mechanical elements interfere upon completed folding, creating a joint stop. Tabs in one scaffold plate  42 / 43  can be folded manually and inserted into slots in the opposing plate  43 / 42 , creating support pylons that hold the assembly scaffold  19  in its folded state, allowing it to be removed from the external jig. 
     Assembly of the final device  26  (including unfolding of the linkages  44  and  45  into multiple planes) can be performed manually by external actuation, or assembly can happen spontaneously. Where assembly is spontaneous, if one or more of the layers is pre-strained, the relaxation of the pre-strained layers can lead to the assembly of the device as soon as the assembly degree of freedom is freed. The layer that is pre-strained can be, for example, a patterned spring formed of spring steel or another spring-capable material, such as a superelastic nickel titanium alloy (nitinol) or an elastomer material that can survive the lamination conditions without annealing or degradation. The dowel pins and the pin alignment holes in the pre-strained layer can be configured to maintain this tension when the pre-strained layer is in the stack through lamination. The pre-strain can be in the form, for example, of tension or compression, though compression may require consideration of tendencies of linkages to buckle out of plane. 
     In other embodiments, actuators can be built into the laminate to effect assembly. For example, a piezoelectric bending actuator, shape memory layer, or other type of actuator can be laminated into the structure as a pick-and-place component or inserted as an integral part of a layer in the layup  29 ; and the actuator can be actuated, e.g., by supplying electrical current or by changing temperature, to assemble the expanded, three-dimensional structure  26 . In one embodiment, the actuator is a bimorph cantilever including two 127-μm nickel-plated lead zirconate titanate (PZT) piezoelectric plates (PSI-5H4E, Piezo Systems, Inc.) coated with chromium to provide protection during the downstream locking process and bonded to a central carbon-fiber layer. Quasi-kinematic mating features and planar spring clips in the carbon-fiber layer  12  or titanium layer  41  can hold each plate in alignment during lamination. 
     Advantageously, in some embodiments, the assembly of all parts is actuated via a single assembly degree of freedom so that assembly proceeds in parallel for an entire panel, rather than part by part. Assembly can be effected in several ways, depending on the design and complexity of the part. For example, a human operator can actuate the assembly degree of freedom manually or semi-automatically. In one embodiment, the assembly degree of freedom is in the form of a plate connected to a Sarrus linkage  67  that is pulled up or pushed down, as shown in  FIGS. 9 a  and 9 b   . Spherical joints or four-bar mechanisms can be attached to the Sarrus linkage, raising and folding other components into their three-dimensional position. Note that by having multiple rigid-flex planar layers and selective adhesion, complex mechanisms and collections of mechanisms can be released in the assembly step. 
     8) Joint Locking of Assembled Part 
     After assembly into a final three-dimensional structure, structural members can be bonded together in a fixed configuration (i.e., locked, fixed or frozen). In one embodiment, adhesive can be manually applied to structural members and/or joints, though this approach may not be ideal if many parts are being made. Alternatively, adjacent members that have come together to form a locked joint can be automatically laser welded. If adjacent members  45  and  46  have metal pads  17  (e.g., formed of brass) on them, then wave or dip soldering can form strong filleted bonds  48  between the members. Alternatively, solder paste can be applied, for example, by screen printing before assembly to the laminate; and then, after assembly, a re-flow step in a hot oven creates the bonds. Other variations include the use of two-part adhesives, etc. 
     In one embodiment, the mobee  26  includes 52 brass pads  47  distributed across outer surfaces of its linkage sub-laminates, as shown in  FIG. 10 a    and shown with arrows in  FIG. 10 a   . After folding, pads on disparate links align into 24 “bond points,” in the form of either two pads  47  meeting at right angles, as shown in  FIGS. 10 c  and 10 d   , or three pads forming the corner of a cube. The structure, held in its folded state, is submerged in a water soluble flux (e.g., Superior Supersafe No. 30) and then pre-heated in an oven at 100° C. for 10 minutes. It is then submerged in 260° C. tin-lead eutectic solder for approximately 1 second. Finally, the structure is ultrasonically cleaned in distilled de-ionized water to remove the water-soluble flux residue. The result of this soldering process is the formation of solder fillets  48  at all bond points, as shown in  FIG. 10 d   , eliminating the assembly degree of freedom and locking all disparate machine components together. 
     9) Freeing the Non-Assembly Degrees of Freedom 
     Any non-assembly degrees of freedom in the part  26  can be unlocked by removing any features (e.g., connected tabs) that restrain them via, e.g., laser machining. 
     10) Separating Parts from the Scrap Frame 
     Now that the individual parts are fully assembled and ready for operation, the parts  26  can be separated from the scrap frame (e.g., an outer frame to which the parts are connected by bridges of material) of the scaffold  19  by laser machining, punching, etc. 
     Another monolithic “robotic bee,” “robobee” or “mobee”  26  with a PARITy drivetrain fabricated with this technique using the 15 layers, described above, is illustrated before pop-up assembly (i.e., lying flat) in  FIG. 11 ; a US penny is placed adjacent to the robotic bee  26  to provide a size reference. The robotic bee  26  is shown after pop-up in  FIG. 12 . 
     Perspective views of another embodiment of the mobee  26  are shown in the photographic images of  FIGS. 13-20 , where the mobee  26  is mounted in the central region with a surrounding assembly frame  19  (including a hexagonal base plate  50  and a smaller inset plate  51  raised above it) after unfolding from the flat configuration (as in  FIG. 11 ) in which it is manufactured after bonding and lamination. The resulting liberated mobee is illustrated in  FIG. 20  against a US quarter for size comparison. As seen in the  FIG. 20 , the mobee  26  includes an airframe  27 , a plurality of linkages separated by joints that form a drivetrain, a pair of wings  28  for generating flight, a piezoelectric bimorph cantilever actuator  14 , and an input platform and transmission  49 , coupling the actuator  24  with the remainder of the drivetrain. 
     Raised planes in the mobee  26  are formed, e.g., by making three intersecting orthogonal cuts in multiple layers to form the mechanical vias that can be folded 90° to extend vertically and by bonding layers at the ends of the mechanical vias to maintain connections between the layers after the three-dimensional assembly. The rectangular voids that are evident around the mobee  26  (shown in  FIGS. 13-19 ) are left when the mechanical vias are folded out of that base plane. The mobee  26  will be laser cut out of the frame after the joints are locked. This version incorporates biomimetic wing spars  52  manufactured out of titanium as well as brass pads with a thickness, e.g., of 12.5 μm at folding locations to lock the joints when the device is dip soldered. The titanium can be clamped flat and stress relieved at 550° C. for one hour prior to assembly to reduce or eliminate curvature induced by micro-machining. The wing membranes will be added at a later step in the process. The mobee  26  includes carbon fiber components  12  for high stiffness, lightweight structural components, polyimide film  16  for resilient flexures, grade  25  titanium alloy  41  for robust and complex wing spars  52 , and half hard brass for solder pads  47  for automated joint-locking. 
     In another embodiment of the monolithic bee  26 , several additional concepts are implemented, namely, layer sharing, post-release wing membrane molding, and dip solder pads. 
     This device has 10 structural layers and 8 adhesive layers. The layup is sequentially stacked as follows: 
     Dip Solder Pads to Facilitate Joint Locking:
         1) half hard brass, 25.4 um thick   2) glue       

     Linkage  1 :
         3) carbon fiber   4) glue   5) kapton   6) glue   7) carbon fiber   8) glue       

     Lone Carbon Layer for Spacing:
         9) carbon fiber   10) glue       

     Linkage  2 :
         11) carbon fiber   12) glue   13) kapton   14) glue   15) carbon fiber   16) glue       

     Dip Solder Pads to Facilitate Joint Locking:
           17   a ) half hard brass, 25.4 μm thick       

     Wing Spars:
           17   b ) grade  9  or 25 titanium alloy, 50.8-μm thick       

     Layers  1 - 13  of the earlier design appear in this embodiment of the monolithic bee as layers  3 - 15 . In this new embodiment, layers  1  and  2  allow solder pads to be placed on the bottom of the device, whereas layers  16  and  17   a  allow solder pads to be placed on top of the device. 
     The hexagonal scaffold plate  50  and the smaller plate  51  suspended parallel above it and within which the bee  26  is suspended via linkages (seen in  FIGS. 13-19 ) form a Sarrus linkage  67  and are constrained by connecting linkages to have one degree of freedom; the smaller plate  51  can linearly translate upwards from the hexagonal plate  50 . This is the assembly degree of freedom. All components of the bee  26  are connected by linkages to these two plates; and a laser can later to be used to cut these linkages and release the bee  26 . As the Sarrus linkage  67  is actuated, each individual component of the bee  26  is folded up into its desired final configuration. 
     Layer Sharing: 
     There is no adhesive layer between the topmost brass  40  and titanium  41  layers (layers  17   a  and  17   b , respectively) in the above outline. These two sub-layers can be thought of as sharing the same layer because they are non-overlapping and both engage with the adhesive layer  14  (layer  16  in the above outline), e.g., glue, to bond with the carbon fiber layer  12  (layer  15  in the above outline). 
     Two ways to accomplish layer sharing are described, as follows. In the first, multiple layers occupy non-overlapping areas in the x-y plane. For example, four alignment pins  26  can be used. The brass layer  40  ( 17   a ) can cover half of the full area of the device, while the titanium layer  41  ( 17   b ) can cover the other half. The brass  40  can be used to form solder pads  47 , while the titanium  41  can be used to form the wing spars  52 . Each sub-layer ( 17   a  and  17   b ) can engage with just two out of the four alignment pins  20  [i.e., two pins can engage with the brass sub-layer ( 17   a ), while the two other pins can engage with the titanium sub-layer ( 17   b )]. Taken to the extreme, the layer can be split into many sub-layers if each sub-layer is engaged with enough alignment pins. For example, a single layer with six sub-layers can look like a map of New England, with each state made out of a different material, and with two alignment pins per state. 
     A second way of achieving layer sharing (implemented in this embodiment of the monolithic bee) is by applying pressure to layers that are unsupported from below to bend the layers into the space below. Basically, if a large hole is cut in a thin layer, the application of pressure to the layer immediately above it (or below it) can be designed to warp and bend that adjacent layer around the edge of the hole, filling in the hole. In this embodiment, the brass layer  40  ( 17   a ) covers the entire laminate area, but is machined with a large wing-shaped hole (shown via the white outline  54  in  FIG. 21 ). The titanium layer ( 17   b ) is stacked on top of the brass layer  40  ( 17   a ). The wing spar pattern  52  is machined into the titanium  41 , with a bridge connecting the wing spar  52  to the bulk titanium sheet. Nominally, this wing spar pattern  52  is suspended above the hole cut out of the brass layer  40 . The titanium layer  41 , however, is designed such that it will bend slightly when lamination pressure is applied, allowing the wing to engage with adhesive layer ( 16 ) through the hole in the brass layer  40  when pressure is applied. 
     Dip Solder Pads: 
     Brass pads  47  for dip soldering are evident in the images of the monolithic bee shown in  FIGS. 12-19  (particularly in  FIG. 18 ). The brass pads  47  are gold in color (contrasted with the bright yellow polyimide film forming joints) and many rectangular pads  47  can be seen in the vicinity of the base of the actuator  24 . The device is submerged in flux and then in solder, causing solder fillets  48  to form on all of the brass pads  47  in close proximity, locking the assembly degrees of freedom in the mobee  26 . Once this locking has taken place, the interior mobee  26  is cut out of the large surrounding assembly mechanism. 
     Wing Membrane Laminating: 
     After initial lamination, pop-up assembly, and joint locking, there are no membranes on the wing spars  52  of the monolithic bee  26  due to the difficulty of any membrane  53  surviving the dip soldering process. The mobee  26  is released from the surrounding Sarrus hinge assembly mechanism  67 , but its active degrees of freedom are not yet released. The mobee  26  is then taken through a second lamination step where the wing spars  52  are sandwiched between two layers of 1.5 um polyester film (a thermoplastic). Heat and pressure (e.g., 120° C., 340 kPa for 15 minutes) cause the two films to bond to each other, sealing them to the spars  52 —and thereby forming the membrane  53 . The device is then placed into the laser to cut the wing outline  54 . Only then are the active degrees of freedom of the bee  26  released. The resulting wing  28  is shown in  FIG. 22 . 
     11) Operation 
     After the manual attachment of three wires to the piezoelectric actuator  24 , the mobee  26  is ready to operate. Applying an oscillating voltage to the piezoelectric actuator causes reciprocating flapping motion of the wings  28  ( FIG. 19 ), e.g., at 100 Hz. As a completed machine, the mobee  26  can be constitutively similar to the earlier Harvard microrobotic fly (HMF); the mobee  26  distinguishes itself by the precision and scalability of the manufacturing process used to produce it. 
     B) Additional Embodiments 
     Flexure mechanisms and assembly folds result from the patterning and lamination of alternating rigid and compliant layers, similar to rigid-flex circuit board construction. We extend these methods with the concept of a superplanar topology; adhesive layers are patterned with a laser to allow selective mechanical connection between multiple rigid-flex planar layers. These “mechanical vias” enable the creation of complex multi-layer mechanisms, such as Sarrus linkages, which can actuate normal to the working plane. Device components may now reside on separate planar layers, reducing interference during folding and allowing greater complexity than is possible with a single flat pattern. Provided, below, is a detailed explanation of the fabrication method and present three example parts to illustrate its potential. The last device demonstrated achieves self-assembly by introducing a pre-strained layer into the part laminate. 
     Laminate Fabrication 
     The process begins with the production of multilayer laminates. Each layer is first bulk machined to define part geometry. Layers (post-machining) remain contiguous to preserve the structural integrity of each layer and to provide a connection from each device component to the alignment pins. Most features can be machined, leaving small tabs or bridges connecting parts to the surrounding bulk material, similar to break-off tabs in panelized circuit boards or the part panels found in plastic model kits. At a later step, a second round of machining frees the individual parts. Any method of machining that is compatible with the layer materials and meets the accuracy requirements of a particular application can be used, such as deep reactive ion etching (DRIE), photo-chemical machining and electroforming (for metals), laser machining, and punching. 
     For our research purposes, we used laser micromachining for its mask-less nature and for its compatibility with a wide range of materials. We employed an Nd:YVO4 DPSS laser, q-switched and frequency tripled to 355 nm. The maximum average power of the laser was approximately 1.5 Watts, which we found sufficient for machining layers in the 1 to 150 micron thickness range. The beam was focused to a spot approximately 8 microns in diameter using a telecentric objective lens. Full-range accuracy and repeatability of beam/part positioning was 2 microns or better. A laser of this type easily machines most materials, excepting glass and others that are highly transmissive of 355-nm radiation. 
     After each layer is machined, optional steps, such as electropolishing, ultrasonic cleaning and plasma treatment, may be performed to prepare each layer for lamination. In rigid printed-circuit-board (PCB) fabrication, circuit layers are usually bonded by interleaving pre-impregnated fiberglass-epoxy composite sheets (prepregs) or adhesive bondply layers. For this work, we used acrylic sheet adhesives. These adhesives are most commonly used to coat polyimide (or other polymer) film to form bondply (on two sides) or coverlay (on one side) laminates, but are also available in free-standing sheet form. PCB sheet adhesives are highly engineered materials with tailored thermal expansion properties and with very little flow during the bonding cycle. We used DuPont FR1500 acrylic sheet adhesive, 0.0005 inches (12.5 microns) thick. First, the acrylic sheet adhesive was machined with alignment holes. Second, the acrylic sheet adhesive was added as a free-standing layer to the stack or, alternatively, tack-bonded to an adjacent layer. For either technique, laser machining is used to pattern the adhesive. Other adhesives or methods of adhesion cab be used, but we find the combination of properties present in this type of adhesive to be well suited for MEMS-scale microfabrication. 
       FIG. 23  illustrates the laminate stack and alignment tooling used for this work. The part layers are 25 mm on a side. Each outer layer  66  represents possible pressure-distribution, conformal and release layers. Between the outer layers  66 , the following layers are stacked in sequence bottom-to-top: carbon-fiber layer  12 , adhesive layer  14 , polyimide film  16 , adhesive layer  14 , pre-strained spring steel layer  68 , adhesive layer  14 , polyimide film  16 , adhesive layer  14 , and carbon-fiber layer  12 . The carbon-fiber layer  12  is our standard 0-90-0 three-layer laminate with thickness of approximately 100 μm. The adhesive layer  14  is DuPont PYRALUX FR1500 with 12.5 μm thickness. The polyimide layer  16  is 7.6 μm-think KAPTON polyimide film. Finally, the spring steel layer  68  is 0.003-inches (76 μm) thick and laser machined with planar springs. The alignment holes are placed in positions such that this layer  68  must be stretched during stacking, extending the planar springs and storing energy in the laminate. These springs drive pop-up self-assembly once the device is completed. 
     Multiple part layups  29  may be bonded simultaneously, using rigid separator plates between stacks. Alignment accuracy is determined by several factors, such as the accuracy of the alignment holes  22  and pins  20 , coefficients of thermal expansion (CTEs) of the layer materials and the size of the laminate. For alignment, we used standard precision dowel pins ( 1/16 inch) and alignment holes typically undersized by a few microns to benefit from elastic averaging. In practice, post-lamination alignment is better than 5 microns. The exact numbers were difficult to measure, as this accuracy approaches the material-uniformity and edge-roughness limits of our materials and machining process. 
     We first demonstrated these methods by making a fairly complex part from a very simple layup; with just two rigid layers  12  separated by a single adhesive layer  14 , we can make a linked chain  56 .  FIGS. 24 and 25  illustrate the process with a simple two-link version. Essentially, the outline  57  of two interlocked rings is machined into each rigid layer. However, where they overlap, one rigid layer  12  (top or bottom) continues, and the other has a gap; for the other intersection, the reverse. The adhesive layer  14  is machined to prevent bonding between the two rigid layers  12  at both overlap regions. Selective adhesion is an enabler of this part. 
     After laser machining, the layers are aligned using pins  20 , and bonded. Note that in this application, the adhesive layer  14  is free-standing. We found that PCB-type acrylic sheet adhesives (only 12.5 microns thick) have sufficient strength and stability to support themselves and maintain accurate alignment in the layup. The chain is “singulated” after bonding by completing the outline cut. A chain  56  so-fabricated with 549 links is shown in the  FIGS. 26 and 27 . The rigid layers are carbon fiber composite, each 95 microns thick. They were cut from a pre-cured 0-90-0 laminate of unidirectional carbon fiber (33 grams/m 2  per ply) impregnated with cyanate ester resin. This material is very strong, stiff and light; and it laser machines easily and has a low coefficient of thermal expansion. After lamination and singulation, the chain  56  is simply lifted out of the scaffold frame  19 . 
     Structure by Folding 
     The chain  56  provides a good example of the complexity possible when selective adhesion is used in laminated construction. Increasing the number of layers does allow parts of greater complexity; however, this 3D printing approach runs into several limitations: as part thickness grows, it becomes increasingly difficult to make singulation cuts deep in the part; excess supporting material typically must be removed; and structural elements aligned normal to the working plane are weakened by interleaved adhesive layers. As described, herein, we have explored folding as an alternative approach for making 3D structures. 
     There are many examples of folding, including origami, sheet metal construction and rigid-flex PCBs. A flat pattern is folded at creases, at score lines, or at flexible hinges. To form flat patterns in our process, we machine “links” out of a rigid material, separated by narrow gaps spanned by a compliant material. These flexures serve as assembly folds or mechanism joints. Structures of incredible complexity are possible through origami folding and modern algorithms can yield crease patterns directly from a 3D model. 
     A downside to folding is typically the challenge of assembly. When working with a single rigid-flex planar layer, forming complex shapes typically requires many sequential folds and thus many assembly degrees of freedom. If the goal is batch production, assembly ideally will occur using only a single degree of freedom. A motivating example is a pop-up book, where a single rotation results in the folding and assembly of many interconnected components. Unlike origami, pop-up book scenes—when closed and unfolded—include multiple folding layers. Using the laminated construction process described herein, similar structures can be created. 
     A model of the “Wright” flyer  58  is shown in  FIGS. 28-30 . A schematic of the folding mechanism for the Wright flyer  58  is shown in  FIG. 28 , while a photographic image of the model  58  (before folding/pop-up) is shown in  FIG. 29 . Finally, a perspective view of a first embodiment of the Wright Flyer model  58  is shown on a U.S. quarter-dollar coin in  FIG. 30  after folding (after pop-up). 
     Another embodiment, that of a spring-actuated eight-sided hexagonal enclosure  59  is shown in  FIGS. 31-36 . Specifically, perspective views of the structure  95  in collapsed (before pop-up) and actuated (after pop-up) states are respectively provided in  FIGS. 31 and 32 . Top and side schematic views of the collapsed structure  59  are respectively provided in  FIGS. 33 and 34 , while a side schematic view of the structure  59  popping up with actuation of a spring  60  in a spring steel layer  68  under tension is provided in  FIG. 35 . Finally, a top view of the spring mechanism  60  is shown in  FIG. 36 . 
     Monolithic Icosahedron 
     Assembly of a PC-MEMS pop-up icosahedron  62  is shown in  FIGS. 37-48 , which show a pop-up assembly process at approximately three-second increments. The collapsed structure contained in and by a scaffold including two flat plates  32  is shown in  FIG. 37 . The icosahedron  62  comprises 20 substantially identical triangular faces  34  and can be folded into a nearly spherical form with 30 edges and 12 vertices. Dowel pins supporting the top disk  32  are raised in  FIGS. 38-42  to increase the separation between the plates  32 , wherein an actual still capture is shown in the bottom of each figure, with a corresponding computer-generated image above. 
     Mounted to the top plate  32 ′ are three planks  36 , each of which is pivotably mounted to the top plate  32 ′ via a fold and fixed to a respective triangular face  34  adjacent to the top-most triangular face  34 ′, as can be better seen in  FIGS. 43-48 , which show the pop-up assembly with the scaffold plates  32  removed for clarity of illustration. Different prime values for the triangular faces (i.e., faces  34 ,  34 ′, and  34 ) are provided for ease of illustration and characterization, though references herein to “face  34 ” can refer to any of these faces. 
     As the top plate  32 ′ rises relative to the bottom plate  32 ″, the planks  36  pivot downward and pull the respective triangular faces  34  to which they are attached upward, along with the other triangular faces  34  to which each is interconnected about its edges. 
     At its base, the icosahedron  62  is mounted to an inner rotatable disk  38  (see, e.g.,  FIG. 41 ), which includes a plurality of outward-extending tabs  63  that extend under slots  64  on the lower scaffold plate  32 ″, acting as a plane linear bearing, for reciprocal axial rotation therein. The underside of the lower scaffold plate  32 ″ showing the tabs  63  against respective slots  64  in the lower scaffold plate  32 ″ is shown in  FIG. 50 , though the tab-slot structure serves as a backup here, since the linkages driving the rotatable disk, in theory, are enough to keep the disk rotating in plane without any other connections. The rotatable disk  38  is turned by the pull of linkages  65  extending down via a pivot point from the top plate  32 ′. As each linkage  65  is raised, it drags the respective tab  63  to which each is coupled underneath the position at which the linkage  65  is joined to the top plate  32 ′, thereby causing rotation of the disk  38  relative to the plates  32 . Additional folding mechanical vias  44  are provided to support the top plate  32 ′ over the bottom plate  32 ″. 
     Here, many of the folds are at an angle of about 45° when the icosahedron is assembled; and the inner disk  38  is ultimately rotated about 60° in plane relative to the plates  32  as the icosahedron  62  is unfolded. The resulting structure can then be locked in position, e.g., via a soldering technique, as described above. A side view of the folded icosahedron in the expanded scaffold is shown in  FIG. 49 . 
     The resulting icosahedron  62  can be used in a variety of applications, such as, for example, in camera-shutter optics or in balloon-angioplasty. In other embodiments, a sensor (e.g., a camera) can be provided on each face  34  to provide insect-like multi-directional vision and awareness. In still other embodiments, a mirror, circuit board, and/or a communication transmitter or receiver can be provided on each triangular face. Likewise, a variety of other complex shapes can be likewise formed via a combination of unfolding and twisting by changing the configuration of folds, linkages, planks, faces, mechanical vias, etc. 
     In describing embodiments of the invention, specific terminology is used for the sake of clarity. For the purpose of description, specific terms are intended to at least include technical and functional equivalents that operate in a similar manner to accomplish a similar result. Additionally, in some instances where a particular embodiment of the invention includes a plurality of system elements or method steps, those elements or steps may be replaced with a single element or step; likewise, a single element or step may be replaced with a plurality of elements or steps that serve the same purpose. Further, where parameters for various properties or other values are specified herein for embodiments of the invention, those parameters or values can be adjusted up or down by 1/100 th , 1/50 th , 1/20 th , 1/10 th , ⅕ th , ⅓ rd , ½, ⅔ rd , ¾ th , ⅘ th , 9/10 th , 19/20 th , 49/50 th , 99/100 th , etc. (or up by a factor of 1, 2, 3, 4, 5, 6, 8, 10, 20, 50, 100, etc.), or by rounded-off approximations thereof, unless otherwise specified. Moreover, while this invention has been shown and described with references to particular embodiments thereof, those skilled in the art will understand that various substitutions and alterations in form and details may be made therein without departing from the scope of the invention. Further still, other aspects, functions and advantages are also within the scope of the invention; and all embodiments of the invention need not necessarily achieve all of the advantages or possess all of the characteristics described above. Additionally, steps, elements and features discussed herein in connection with one embodiment can likewise be used in conjunction with other embodiments. The contents of references, including reference texts, journal articles, patents, patent applications, etc., cited throughout the text are hereby incorporated by reference in their entirety; and appropriate components, steps, and characterizations from these references may or may not be included in embodiments of this invention. Still further, the components and steps identified in the Background section are integral to this disclosure and can be used in conjunction with or substituted for components and steps described elsewhere in the disclosure within the scope of the invention. In method claims, where stages are recited in a particular order—with or without sequenced prefacing characters added for ease of reference—the stages are not to be interpreted as being temporally limited to the order in which they are recited unless otherwise specified or implied by the terms and phrasing.