3D printing is an additive technology in which objects are built up in layers in a process that often takes several hours. The first commercial 3D printer was based on a technique called stereolithography. This was invented by Charles Hull in 1984. Stereolithographic 3D printers (known as SLAs or stereolithography apparatus) position a perforated platform just below the surface of a vat of liquid photocurable polymer. A UV laser beam then traces the first slice of an object on the surface of this liquid, causing a very thin layer of photopolymer to harden. The perforated platform is then lowered very slightly and another slice is traced out and hardened by the laser. Another slice is then created, and then another, until a complete object has been printed and can be removed from the vat of photopolymer, drained of excess liquid, and cured. Stereolithographic printers remain one of the most accurate types of hardware for fabricating 3D output, with a minimum build layer thickness of only 0.06mm (0.0025 of an inch).

Another common commercial 3D printing technology is fused deposition modelling (FDM). Here a semi-liquid material -- and most usually a hot thermoplastic -- is extruded from a temperature-controlled print head to produce fairly robust objects to a high degree of accuracy. A key benefit of this technique is that objects can be made of out of exactly the same thermoplastics used in traditional injection moulding. Most FMD 3D printers can print with both ABS (acrylonitrile butadiene styrene), as well as a biodegradable bioplastic called PLA (polylactic acid) that is produced from organic alternatives to oil. Within a decade developments in synthetic biology are likely to make the direct production of PLA from a range of biomass materials quite common, hence allowing 3D printing supplies to be grown in many a back yard.

In addition to being used to output plastic objects, FDM printers have also been developed that can output other semi-liquid materials. The applications are already quite diverse, and range from food printers that can print in cake icing, cheese or chocolate, to concrete printers that may in future allow entire buildings (or large parts thereof) to be 3D printed.

As an alternative to FDM, a company called Objet has developed a process called Polyjet Matrix. This jets two liquid photocurable polymers from a 96 nozzle print head. Each object layer is cured by a UV light immediately it has been printed. One of the key benefits of this process is that is allows printing to take place in multiple materials simply by varying the combination of the photocurable polymers jetted from the print head. You can learn more about this very impressive technology in this video.

A fourth and very well established 3D printing technology is selective laser sintering (SLS). This builds objects by laying down a fine layer of powder and then using a laser to selectively fuse some of its granules together. At present, SLS 3D printers can output objects using a wide range of powdered materials. These include wax, polystyrene, nylon, glass, ceramics, stainless steel, titanium, aluminium and various alloys including cobalt chrome. During printing, non-bonded powder granules support the object as it is constructed. Once printing is complete, almost all excess power is able to be recycled.

When SLS is used to directly produce metal objects the process is also called direct metal laser sintering (DMLS). Metal objects created by a DMLS 3D printer are about 99.99 per cent dense, and hence can be used in place of traditional metal parts in the vast majority of applications. (There are some good pages on DMLS here).

While DMLS 3D prints metal objects directly, it is also common to use laser sintering to produce wax objects that are then sacrificed in a traditional lost-wax casting process. Here, once the wax object has been 3D printed a plaster mould is poured around it. When heated, the wax then melts and is poured away, after which a liquid metal can be poured in. Once this cools the plaster is removed, leaving a metal object that -- in some senses -- began its life on a 3D printer.

A closely related 3D printing technique to SLS is known as selective laser melting (SLM). This uses a laser to fully melt the powder granules that form a final object, rather then just heating them enough to fuse them together. As yet another variant, a technique called selective heat sintering (SHS) uses a thermal print head -- rather than a laser -- to apply heat to successive layers of a thermoplastic powder, and as explained in depth here.

A final 3D printing technology in very common application is multi-jet modelling (MJM) -- although this process goes under a wide number of names. This again builds up objects from successive layers of powder, with an inkjet print head used to spray on a binder solution that selectively glues only the required granules together. Some MJM printers -- such as the ZPrinter 650 from ZCorp -- can spray on four different colours of binder solution, so permitting them to create full-colour 3D objects at up to 600x540dpi. You can watch a great promotional video for the ZCorp 650 here.

A wide range of commercial 3D printers for industrial application are now available from companies including 3D Systems (which works with most technologies and is rapidly acquiring many smaller manufacturers), Stratasys (which initially developed FDM), Fortus and Dimension Printing (which use the Stratasys FDM process), Solid Scape (which uses its own FDM process but is now owned by Stratasys), Objet (which developed the Polyjet Matrix process) and ZCorp (which specializes in MJM as noted above, and is now also owned by 3D Systems). Another major player is envisionTEC whose Perfactory 3D printers projecting voxel datasets into a photopolymer. (A video that shows how this voxelization process differs to more "traditional" layered technologies can be found here).

All of the above producers of 3D printers have built their growing businesses from scratch in the rapidly expanding 3D printing marketplace. However, traditional 2D printer manufacturers are now also starting to dip their toes into 3D waters. For example, Hewlet Packard (HP) now sells its FDM-based HP Designjet 3D printer series having done a deal with Stratasys.

Prices for most commercial/industrial 3D printers tend to start in the ten-to-twenty thousand dollar bracket and spiral upwards. Although some desktop models are on the market, most commercial 3D printers are usually fairly bulky and often floor-standing. It is, however, already possible for lone designers and private individuals to obtain high quality 3D printouts from commercial 3D printing hardware by using an online bureau. For example, Sculpteo, Shapeways, 3D ProParts and iMaterialize allow anybody to have their designs 3D printed and marketed online. The first two of these firms even offer a facility to upload two photographs of yourself from which a full-colour figurine will be created! Online 3D printing services can also be obtained from these suppliers. The following video shows how I used the iMaterialize online service to 3D print a metal object:

For home enthusiasts and lone inventors who want to start 3D printing themselves, a growing range of personal 3D printing initiatives, kits and printers are also now available. For those seriously into real DIY, there are two open source 3D printing initiatives. Called RepRap -- which stands for the Replicating Rapid-prototyper -- and Fab@Home. These initiatives are both based around online communities that use the web to share all designs and other information required to build a printer.

For those not keen or able to scratch build, several companies now sell 3D printer kits, often based on the above RepRap designs. For example, BitsfromBytes.com sells its RapMan 3.1 3D printer kit from £795, and its pre-assembled BFB-3000 3D printer starting at under £2,000. RepRap Central is also a great supplier of 3D printer kits costing from £599 upwards. In addition to several RepRap models, these include the MakerBot Thing-O-Matic.

For those who do not want to build their own 3D printer, pre-assembled personal hardware is now also available. These include the BotMill Glider 3.0 (which is also available as a kit), and the excellent UP! Personal Portable 3D Printer. The latter is a truly plug-and-play FDM 3D printer that can be up-and-running 15 minutes after being unpacked, and does not require any calibration by its user. There is a fantastic video about the Up! 3D printer here. Most recently, the Cube Personal 3D Printer has been launched as part of Cubify. This sells for $1,299 and can print about 10 models from each $49.99 plug in cartridge of ABS plastic (10 colours are available!).

Most current 3D printers are not used to create final consumer products. Rather, they are generally employed for rapid product prototyping, or to produce moulds or mould masters that will in turn allow the production of final items. Such printing of 3D objects already enables engineers to check the fit of different parts long before they commit to costly production, architects to show detailed and relatively low-cost scale models to their clients, and medical professionals or archaeologists to handle full-size, 3D copies of bones printed from 3D scan data. There are also a wide range of educational uses.

The range of products that have employed 3D printers in their design process or to produce final moulds or mould masters is constantly growing. To date such products include automobiles, trainers, jewellery, plastic toys, coffee makers, and all sorts of plastic bottles, packaging and containers. You can even now purchase a 3D printed Sad Keanu Reeves. More usefully, some dental labs have for some years been using 3D printers to help create appliances, with envisionTEC selling its Perfactory Digital Dental Printer for use in the creation of crowns, bridges and temporaries by dental technicians. Using this technology, even long-term temporaries can now be created, meaning that 3D printers can quite literally already print you new teeth! envisionTEC 3D printers are now also widely used by many major hearing aid manufacturers to produce ear moulds and shells for final consumer use.

Whilst most 3D printers are currently used for prototyping and in pre-production mould making processes, the use of 3D printing to manufacture end-use parts is also now occurring. This is becoming known as direct digital manufacturing (DDM). As Fortus explain, for low-volume manufacturing DDM is more cost-effective and simpler than having to pay and wait for machining or tooling, with on-the-fly design changes and just-in-time inventory being possible. For example their customer Klock Werks Kustom Cycles has built one-of-a-kind motorcycles using a Fortus 3D printer to directly digitally manufacture some of the required custom parts.

Another company using 3D printing to create final products is Freedom of Creation (and which was recently acquired by 3D Systems). Their range of incredible, designer 3D printed products includes lighting, furniture, trays, bags and jewellery. In a similar vein, a company called Make Eyewear is now manufacturing customiziable and designer sunglasses using 3D printers.

Many believe that 3D printers have a great future in the creation of fashion items including jewellery and shoes. For example, Forebes recently reported on a Shoes from a 3D Printer that Smell Like Bubblegum, with injection moulding set to give way to 3D printing to allow maufacture-on-demand and higher levels of customization. You can see even more 3D printed shoes here.

It is also already not just a few specialist plastic items that are being made using a 3D printer. For example, engineers at the University of Southampton recently 3D printed a flyable aircraft (well, aside from its electric motor). Rolls Royce is also currently running a project called MERLIN with the goal of using 3D printing in the manufacture of civil aircraft engines. A driveable prototype of a new electric car called the Urbee has also been 3D printed. Mainstream automobile makes are also already in on the DDM act, with Audi now 3D printing parts of its cars using Objet Polyjet 3D printers.

Some artists are now also using DDM to create their masterpieces. For example, sculptor Bathsheba Grossman already uses 3D printers to create her works. In the future, museums could also print out exhibits as required from their own digital collection -- something that the Smithsonian is already working on -- or indeed from a global archive of artworks scanned from long-lost or too-delicate-to-display originals. You can find out more about 3D artworks, including a great video, on the Singularity Hub.

Whether or not they arrive en-mass in the home, 3D printers have many promising areas of potential future application. They may, for example, be used to output spare parts for all manner of products, and which could not possibly be stocked as part of the inventory of even the best physical store. Hence, rather than throwing away a broken item (something unlikely to be justified a decade or two hence due to resource depletion and enforced recycling), faulty goods will be able to be taken to a local facility that will call up the appropriate spare parts online and simply print them out. NASA has already tested a 3D printer on the International Space Station, and recently announced its requirement for a high resolution 3D printer to produce spacecraft parts during deep space missions. The US Army has also experimented with a truck-mounted 3D printer capable of outputting spare tank and other vehicle components in the battlefield.

As noted above, 3D printers may also be used to make future buildings. To this end, a team at Loughborough University is working on a 3D concrete printing project that could allow large building components to be 3D printed on-site to any design, and with improved thermal properties.

Another possible future application is in the use of 3D printers to create replacement organs for the human body. This is known as bioprinting, and is an area of rapid development. You can learn more on the bioprinting page, or see more in my bioprinting video or the Future Visions gallery.

In an age in which the news, books, music, video and even our communities are all the subjects of digital dematerialization, the development and application of 3D printing reminds us that human beings have both a physical and a psychological need to keep at least one foot in the real world. 3D printing has a bright future, not least in rapid prototyping (where its impact is already highly significant), but also in the manufacture of many kinds of plastic and metal objects, in medicine, in the arts, and in outer space. Desktop 3D printers for the home are already a reality, and should cost no more than a few hundred dollars by 2015. 3D printers capable of outputting in colour and multiple materials also exist and will continue to improve to a point where functional products will be able to be output. As devices that will provide a solid bridge between cyberspace and the physical world -- and as an important manifestation of the Second Digital Revolution -- 3D printing is therefore likely to play some part in all of our futures.

For a fascinating glimpse at a wide range of amazing and unusual printers -- including concrete printers, glass printers, bioprinters, and printers that print on toast! -- click here. You may also find interesting the great websites 3DPrinter.net, 3DPrinting.com and 3ders.org.

Finally(!), please see my 3D Printing Directory for a whole host of information on 3D printer manufacturers, suppliers, bureau, pioneers and other stuff! More information on 3D printing can also be found in my book 25 Things You Need to Know About the Future. The 3D printing web references from this book can also be found here.