Similar fashion. This process is continued until the final

Similar to inkjet
printing, layers of photopolymer resin are jetted on to the build platform and
are simultaneously cured using UV light source. Unlike inkjet process, multiple
types of materials can be jetted simultaneously and cured. This gives us the
ability to fabricate a complex multi-material object. Due to these
capabilities, polyjet is widely used in medical field to fabricate anatomical
models for surgical planning and pre-operative simulations. High resolution
objects with varied modular strengths can be 3D printed with high dimensional
accuracy using polyjet technique. Since the UV source is right next to the
jetting nozzle and cures the resin instantaneously, post-processing of the
construct will not be necessitated. This technology is relatively new to the
additive manufacturing field. Many types of photopolymers such as ABS like,
Veroclear, Verodent and Fullcure are commercially available for use in polyjet
printing. Table 7 shows some of the photopolymers used in medical applications.

This process is similar to SLS,
instead of fusing the powder bed with laser or electron beam, binding liquid is
selectively dropped on to the powdered bed to bind the materials in a
layer-by-layer fashion. This process is continued until the final object is
formed. Thermal and piezoelectric print head are two types of printing heads
used in this technique. In thermal print head systems, an electric heating unit
is present at the deposition head which evaporates the binding material to form
a vapor bubble. This vapor bubble expands due to pressure and comes out of the
print head as a droplet. Whereas in piezoelectric print head systems, the
voltage pulse in the print head induces a volumetric change (change in pressure
and velocity) in the binder liquid resulting in the formation of a droplet.

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These printers are used due to their speed, accuracy and inexpensive nature.

            Water,
phosphoric acid, citric acid, PVA, PDLLA are some of the commonly used binding
materials for inkjet 3D printing. A wide range of powdered substances including
polymers and composites are used for medical and tissue engineering
applications. Final 3D printed objects are often post-processed to enhance the
mechanical properties. Wang et al., have used phosphoric acid and PVA as
binding liquids to bind HA/?-TCP powders for bone tissue regeneration
applications. The accuracy and mechanical strength of constructs printed using
phosphoric acid were higher than constructs printed using PVA 24. Sanler et al., has fabricated precise and
personalized dosage forms using concentrated solutions of paracetamol,
theophylline and caffeine 25. Uddin et al., has surface coated metallic
transdermal needles with chemotherapeutic agents using soluplus, a copolymer of
PVC-PVA-PEG, for transdermal drug delivery 26. Table 6 shows the types of binding liquids and
respective powder materials used for inkjet printing.