A Review on Stereolithography and its Biomedical Applications

ABSTRACT

Until recently, conventional method of fabrication of prototypes by skilled model makers using two- dimensional drawings was the only method available. Advent of rapid prototype technology has opened up new perspectives for designing and production of prototypes. Stereolithography is widely used rapid manufacturing and rapid prototyping technology. 3D models are created with highest fabrication accuracy and excellent surface finish. Its limitations include high cost of the equipment, need of complicated machinery and special expertise to run the machinery during production. The expense could be justified in terms of its many benefits. In this article, stereolithography and its various medical and dental applications in day to day life are reviewed.

Medical imaging has been used to provide information for diagnostic and therapeutic purposes. Ability to convert this information into physical models provides additional benefits in these fields. With the advent of solid free form fabrication technique, ability to produce structures with precise geometries using CAD and data from medical imaging systems has significantly improved 1. Prototypes may now be rapidly produced from 3D computer models that are being used in various applications and in industries that ranges from dentistry and medicine to Hollywood film making 2, 3, 4, 5.
Rapid prototyping refers to automatic construction of mechanic models from graphical computer data. RP term embraces a range of new technologies which produces accurate parts directly from CAD models in a few hours, without need for human intervention 5. It is one of the types of computer-aided manufacturing (CAM).To date, there are 2 strains:

• Additive(solid free form fabrication technique)
• Subtractive

Difference lies in the processing protocols, material used and their respective accuracy. Additive manufacturing methods can produce large work pieces with significant surface variations and accuracy. That is why they are considered ideal for facial prosthesis. Subtractive may methods provide more homogeneous objects with acceptable accuracy that may be more suitable for the production of intraoral prosthesis where it is subjected to high occlusal forces 6.
Additive technology (SFF) is the one which is widely used. According to Kruth, the material accretion technologies may be divided by the state of the prototype material before beginning of the process (figure 1). It has following types 7

Selective laser sintering

• Fused deposition modelling
• Multi jet modelling
• Stereo lithography(SL)( figure 2 )

At present, SFF technique is the common method of rapid prototyping in the automobile industry, jewellery making and for designing other appliances 1.

Amongst all these, Stereolithography is the first commercially available SFF technique developed by 3D systems. The term ‘Stereolithography’ was first coined by Charles (chuck) W. Hull in 1986. He patented it as a method and apparatus for making 3-dimensional solid objects using Ultraviolet curable material (photopolymer) and a concentrated beam of light 1. It is most utilized of all types of rapid prototyping methods, being the most accurate with finest surface finish. It gives one the ability to print out a CAD/CAM computer model in three dimensions and to print out a drawing in 2D to a 3D object. It can be used for surgical simulation to assure predictable results and to diminish operation time.
Stereo lithography is 90% of all rapid prototyping. It is an excellent choice where a close approximation to the finished product is desired. It is also referred as 3D printing, photo solidification, optical fabrication, solid imaging or solid free-form (SFF) fabrication.

Fig 1. Classification of rapid prototyping methods.

Fig 2. A Stereolithography Machine.

Device which is used for fabricating a SL model has following parts 7:

1. A container filled with liquid resin (acrylic/epoxy).
2. A movable elevator platform inside the container.
3. An ultraviolet laser with beam focussing options on top.
4. A deflecting mirror system to control the laser beam.

Step by step procedure of fabrication of a SL model is 7:

1. CAD model: - process begins with the fabrication of CAD model.
2. Translator: - CAD file goes from CAD to rapid prototyping translator in which CAD data is input in the tessellated SL format. In this process boundary surfaces of the object are represented as numerous triangles similar to the facets of a cut jewel.
3. Supports:-Their role is to ensure that the sweeper bar will not strike the platform upon which the part is being built and to provide a simple means of removing the part from the platform upon completion.
4. Slice:-CAD model is sliced horizontally into cross sections of 0.002-0.006 inch and each layer is laid down just like the floors of a tall building.
5. Merge: Computer representations of the support and parts are merged.
6. Prepare: In this step certain operational parameters are decided such as number of sweeps of sweeper bar/ recoater blade per layer known as the sweep period and required z-wait which is the time in seconds that the system has to wait or pause after recoating.

Procedure for fabrication of a SL model (figure 3)

Fig 3. Procedure for fabrication of a SLA model .

Fig 4. Schemes of two types of stereolithography setups. Left : a bottom-up system with scanning laser.Right : atop down set-up with digital light projection .

Two types of stereolithography setups are available at present based on the method of illumination and build orientation. In this one is a bottom –up system with scanning laser and second is top-down setup with digital laser projection 1. Till date most SLA setups in use resemble bottom up procedure (figure 4) 8, 9, 10,11

To create an SL rapid prototype using bottom-up procedure, first a three dimensional CAD part is ‘sliced’ horizontally into 0.002-0.006 inch thick cross-sections. These slices are then fed to the stereolithography apparatus. Usually the data slices are said to be in x-y plane and part is built in z direction. Inside the chamber of the apparatus an ultraviolet ( helium, cadmium or argon ion) laser traces the first layer of the part on a metal platen submerged just below the surface of a vat of photo-sensitive polymer, stimulating its polymerisation thus solidification5. The solidification starts at first at the object’s borders then the internal parts. Once the layer is traced, the platen sinks the thickness of a layer below the level of the liquid. Thus, the hardened layer is covered with a new liquid layer and drawing continues. A sweeper bar moves across the surface of the last layer, making sure there is exact amount of resin on top. The next layer is then built upon this layer. In this manner the entire part is built from the bottom up, with the completed section of the part remaining submerged. In this way the model is built layer by layer.
During the process support must be provided to prevent sagging of isolated and overhanging parts. One of the important problems is automatic support generation and part orientation. Part orientation determines the final prototype build time and surface finish of the critical areas. The number and position of the supports also adversely affect the build time and surface finish of the prototype 12, 13, 14.
In the top- down approach light is projected on a transparent, non-adhering plate from underneath and build platform is dipped into the resin from above. This has several advantages like recoating of the structure is not required, the illuminated surface is always smooth, amount of resin required is small and oxygen inhibition is prevented as the surface is not exposed to the environment1.
The accuracy of any additive technology is dependent on layer thickness and width of beam for more accuracy. Layers should be thin and curing beam should be narrower. Increasing the number of layers and decreasing the diameter of beam will exponentially increase the fabrication time15, 16, 17.

FINISHING

Once the completed prototype is ready to be taken out, supports are manipulated and must be removed manually. Along with this, it involves lite sanding and bead blasting. Excess liquid resin is drained or removed using solvent like alcohol and final curing of the object surfaces is done under U.V floodlights. This gives the parts a matte or frosted finish.
Additional finishing options are available but require a custom quote. Special finishings are also available like:

• Painting and clear coats
• Custom finishing
• Chrome plating, etc.

Most SL prototyping can be completed in as little as 1-2 days, with complex objects typically completed in less than 5 days. The size of the largest part that can be created with most of the SL machine is 20”x20”x24”.

RESINS USED IN STEREOLITHOGRAPHY

Numbers of resins which are commercially available for use in stereolithography are very limited and is one of the main limitation of this technique. Requirements of the resins that can be used in SL are that they should be in the liquid form that should solidify rapidly on exposure to light.
Initially used resin were low molecular weight polyacrylate or epoxy monomers which form glassy network upon photo-initiated polymerisation and cross linking 1. Few other resins includes polyphenyl sulfone (PPSF), polypropylene-like, polycarbonate –like, etc. Several new resins are being developed which are used for manufacturing not only prototypes but functional parts also 18. But the technique is still limited to the use of single resin at a time. Use of multiple resin in a single construct is possible in stereolithography but it requires complex sequential polymer and rinsing steps for each layer 19, 20, 21-26 .
Along with resins, polymer –ceramic composite object can also be fabricated. For this maximum ceramic content upto 53wt% has been reported and the ceramic particles size should be smaller than the individual layer thickness 20, 21, 22, 23. All ceramic objects are also being made by first fabricating a complete structure by stereolithography and then burning out the polymer and sintering the ceramic particles 24, 25, 26.
Trade names of few are ACCURA -25, ACCURA -60, WATER CLEAR ULTRA 10122, PROTOGEN 18420, SL 5530HT (PROTOCAM), SOMOS 11120 which simulate ABS plastic.
Other materials which are used are paper, metal foil, etc.

BIO-MEDICAL APPLICATONS OF STEREOLITHOGRAPHY

This is one area of RP that is most commonly used especially in the field of medicine and dentistry. Development of quality control, production precision and simple fabrication protocol by industry has inspired professionals in the medical and dental fields to adopt and modify these technologies 27, 28, 29, 30, 31. The status of this methodology in clinical and surgical medicine is still at the prototype stage. It can aid in the following fields:-

• Maxillofacial prosthodontics:-This technique is used to create contact-free reproduction of facial features, mirror image of anatomic parts and can compensate for patient growth and material distortions 32. It has following uses:-
• Production of auricular prosthesis32
• Production of Obturators
• Duplication of existing maxillary/mandibular prosthesis especially crucial when an accurate fit to existing teeth or an implant is needed.
• To fabricate surgical stents for patients with large tumours and are scheduled for excision.
• For the production of lead shields to protect healthy tissue surrounding the lesion during radiotherapy. It allows contact free data acquisition and therefore, avoids tissue tension, flow of excess impression material in undercuts and any reaction possible by mechanical contact 33.
• In case of burn patients, non contact digitised impression can be made for the fabrication of burn stents without subjecting sensitive burn tissues to impression making procedures.

Range of accuracy of facial prosthesis pattern fabricated by SL is 0.1-0.4 mm 33, 34.

• Implant prosthodontics:-Stereolithography surgical templates are being used for surgical and prosthodontic implant planning and placement. With the help of this technology, the surgeon and prosthodontist can now simulate ideal implant placement on the reformatted CT images and treatment plan the exact dimensions of the implants along with the ideal depth and angulation &at the same time can transfer the exact location to the computer generated stereo lithographic surgical template35.Thus 3D planning resulted in a better planned implant position with relation to bone quality and quantity, biomechanics and esthetics.(figure 5)

Fig 5. Stereolithographic surgical template.

Using this technique minimally invasive surgery is performed without a flap, and the prosthesis is delivered achieving immediate functional loading to the implants 36, 37, 38, 39. Thus its role in reducing complications in the field of implants is indisputable. It allows precise evaluation of anatomic points such as the size or the maxillary sinus and the location of inferior alveolar nerve in the mandible36.
The accuracy of intraoral implant placement with the use of stereolithographic template has been evaluated to be in the range of 0.4-2.0 mm and angulations in the range of 2-5o40,41,42. for extraoral implants this accuracy is 1.5mm43. A study proved that SL guides leads to more accurate implant placement closer to the planned position than conventional surgical guides44.

• Medicine/Biomedical engineering:- Data available from MRI or CT is particularly useful in this field. The information obtained is useful for surgeons in conducting surgeries by creating 3D physical models to view affected part of the body. This helps to reduce both time taken and risk involved. While preparing the computer file, the area of anatomical interest (bone, tumour, soft tissue or others) are identified. The model is then created by applying a different colour to the different affected areas which can then be clearly defined.

It is also used for surgical reconstruction following severe trauma, tumours, gun-shot wounds, birth defects or revision of previous surgery. Mostly it is used to fabricate moulds for the preparation of anatomically shaped implants for craniofacial reconstruction, neurosurgery and orthopaedic surgery 1.
Use of SL in the fabrication of moulds for ear-shaped implants, aorta, etc is now routinely done. Other tailored parts which have specific functions and are placed in direct contact with the body are also manufactured for e.g hearing aids. RP is being used to produce custom-fit marks for burn victims that reduces scarring. For this non-contact optical scanning is done and the data is used for the production of an RP model of a mask that shows perfect fitting to the patient’s face 1.

• Implantable devices:- For these devices, mechanical properties and biocompatibility of the material is of utmost importance. Manufacturing and implantation of these types of devices using SL has been reported only in few cases. Popov et al. prepared polyacrylate and hydroxyapatite composite parts and implanted them into the femur of rats for 8 weeks with no adverse effects 21.

Other medical implants like bone fracture fixation devices, artificial hip or knee parts, etc can also be manufactured using SL1.

• Tissue engineering:- SL is becoming increasingly used in the fabrication of precisely defined tissue engineering scaffold. Resorbable scaffold in combination with cells and/or biologically active compounds induce regeneration of tissues both in vitro or in vivo 1.

• Forensic:-3D models can be generated for use as exhibits in trial.Defective products or equipments can be scanned accurately to create 3D animations depicting the events surrounding a product failure or other situations.

• Others: - It can be used as a master pattern for use in injection molding core , thermoforming, blow melting and various metal casting processes.