Jessica Rowbury explores the role optical modelling software plays in designing medical instruments
It is a long road to getting a medical instrument certified for clinical use, but any device incorporating optics can benefit significantly from optical modelling software in the prototyping stage and during the certification process.
Optical modelling software has been fundamental for the development of many medical instruments available today, commented Rich Pfisterer, CEO of Photon Engineering, an optical software provider. ‘It wouldn’t be possible to develop the kinds of medical systems currently available without optical software. Complex medical systems are simply too expensive and time consuming to build without the guidance afforded by a software model,’ he explained.
The key to developing high performing medical devices, Pfisterer continued, is through accurate end-to-end simulation. This includes modelling the light source, the optical and mechanical components − which can affect the propagation of light through scatter and reflection − and the sample under investigation.
And, other than predicting performance, software is used to identify issues when a device doesn’t perform as expected, due to reasons such as fabrication errors, misalignments, and unexpected light paths. ‘It is much more cost effective to do engineering with accurate software models than trying to debug actual hardware,’ Pfisterer remarked.
New medical instruments with optical and photonic components are being developed for diagnosis and treatment. ‘Non-invasive technology is really needed in the medical industry,’ said Michael Gauvin, vice president of sales and marketing at software company Lambda Research. ‘They [doctors] want to be able to take a measurement in real time, and in several different areas of the body all at once.’
Gauvin noted that the capability of designing optical components into medical instruments means doctors can look inside the body at different wavelengths of light in a fast, non-invasive way. ‘There is a big push in the medical industry for this type of technique, this type of equipment, and for doctors to be able to do this very rapidly,’ he said.
The ability to model light sources in silico as well as illumination structures such as light pipes − physical structures used for transporting light for the purpose of illumination − has been significant in allowing manufacturers to produce such medical devices. Through software, users can determine the best way to integrate a laser light source into a medical system, for example. ‘Virtually all optical modelling software has the ability to read source characterisation data and turn it into a distribution of rays that can be traced through an arbitrary opto-mechanical system. The result is a computer model with traceability back to a known source,’ explained Photon Engineering’s Pfisterer.
‘Interaction between computer-aided-design (CAD) software and optical modelling software has made it possible to seamlessly integrate a light pipe or other waveguide structure that was designed in CAD (possibly by a mechanical designer) into an optical model for analysis,’ Pfisterer continued. ‘This type of “virtual prototyping” allows the designer to perform tolerance or alignment analyses, stray light (including ghost) analyses, and signal-to-noise calculations in software before any costs are incurred for hardware.’
In the optical model displayed on the screen, it is possible to observe how light travels through a system, allowing users to create a design which provides the optimum light output. The software uses standard ray tracing principles to start at the light source, Gauvin explained. It then propagates the light non-sequentially; therefore the software doesn’t know which path the light will take at the outset. ‘Whenever [the light] interacts with a surface of a particular object, then the computer program… calculates how much light will be absorbed, reflected, transmitted and scattered off of that surface.’
The designer can then alter the shape of the light pipe to produce the best efficiency. ‘Changing the shape of the light pipe can either spread the light out, or contain it to give you the light beam that you want,’ Gauvin added. ‘Light pipes work using the etendue principle; if you start narrowing the light pipe’s diameter the angular distribution of the illumination will spread.’
3D printed optics
Speeding up the development time for designing new medical instruments is a big benefit for equipment manufacturers. In March, Lambda Research announced that it had partnered with Dutch additive manufacturing company Luxexcel to produce 3D printed plastic optics. The collaboration means custom optics can be printed in days, instead of months, allowing designers to test their optical prototype more rapidly and reduce time-to-market.
Luxexcel’s technology consists of a custom inkjet printer capable of printing optical, UV-curable PMMA materials. The machines deposit micro drops of acrylic onto a substrate that is then hardened with UV light. The optical structure, specified in a CAD file, is achieved by jetting, flowing and merging the molten droplets before hardening.
Within Lambda’s TracePro software, designers can now select Luxexcel’s materials when prototyping products such as light pipes and the software will display the light path of the 3D printed optic. After design the model can then be exported as a CAD file for Luxexcel to manufacture. ‘In the past, creating a light pipe prototype could take weeks for a quick prototype and months to get optical-grade quality light pipes using plastic injection moulding,’ Gauvin remarked. ‘Now you can send it out to Luxexcel and within a week you could have a functional-grade prototype.’
According to Gauvin, although the technology is already being used by large light-based medical developers such as Cynosure, it is also of interest to start-up companies and students wanting to take advantage of the time and cost reductions. ‘We’ve talked to people at Photonics West just this year, and also at the BiOS conference. This is how they do their work now − they’re expected to bring products to market a lot faster,’ he said.
Another recent collaboration between an optical software company and optics manufacturer is also making it easier for designers to prototype and launch products. ‘OpticStudio recently added a Cost Estimator that quickly obtains prototype cost estimates from Optimax, a leading lens manufacturer,’ commented Akash Arora, product manager at Zemax. ‘This improves the design workflow by allowing prototypes to be quickly quoted and ordered once the design process is complete.’
Zemax also offers a stock lens matching tool within its software, Arora noted, as a means to aid prototyping further: ‘This feature will replace custom designed components with the closest off-the-shelf matches. Using off-the-shelf components reduces cost and time to market,’ he said.
Optical modelling software also helps customers decrease time-to-market by easing the often extensive and time-demanding process of applying for medical approval. Through software, engineers can more or less verify the accuracy and safety of their instruments before anything has actually been built, according to Bjorn Sjodin, vice president of product management at Comsol, a provider of multiphysics software. ‘Simulation plays a more and more important role [in the medical sector]…You still need to run experiments, but you can almost validate your designs with simulation before you start experiments,’ he said, adding that customers regularly submit their simulation calculations when applying for medical approval. ‘This is something that the regulatory authorities are becoming aware of, and they are taking simulation more and more seriously.’
Manufacturers now have two methods of determining if there is a problem with the equipment, added Lambda Research’s Gauvin, which would make authorities such as the FDA more likely to approve a product. ‘For example, with some of the laser energy beams coming out of medical devices, we want to make sure that there is no leakage so that the beam doesn’t go into somebody’s eye. The FDA will want to know how the manufacturer is sure of this and how they tested for it,’ he explained. ‘The manufacturer can show the FDA how the system was designed in the software to ensure that there is no leakage. And then the manufacturer can show the FDA the hardware test that was carried out afterwards that showed the laser light was emitted exactly as shown in the software simulation.’
Software is not only speeding up the design process, but is allowing materials to be developed for functions previously believed to be impossible. One example of this is the development of gold nanorods − synthetic nanoscale particles − which are hailed as a safer way of treating cancer compared to methods such as chemo- and radiotherapy.
The treatment involves inserting gold nanoparticles into the body and then heating them through the skin with a laser. The result is that the cancerous tissue is destroyed but the healthy tissue is left intact. ‘This is something that you obviously don’t want to experiment wildly on human beings,’ explained Sjodin.
Through software, users can determine the optimal geometry and size of the gold nanorod to provide the best heating effect, and therefore the best treatment. Two types of software are used for this application: the Comsol Wave Optics Module, which uses Maxwell’s equations to model the laser beam as an electromagnetic wave in order to determine how the radiation affects the particle; and the Heat Transfer Module for analysing the exact temperature of the nanorod and whether the amount of heat causes any cell damage or death.
According to Sjodin, the fact that modelling software is being used in new areas such as these is promising, and he expects to see the number of applications grow in the future, especially as the cost of lasers and optical components continues to decrease. However, it can create a challenge to software providers as more users want to adopt optical modelling programs when they are not specialists in optics. ‘The number of users in this field is increasing − not all of them can be expected to be experts on our or another company’s software,’ he remarked.
To address this issue, Comsol developed a feature within its software which allows an optics expert within a company to build a simulation ‘app’ which can then be used by other employees who may not be familiar with optical simulation. ‘In our regular software you may have thousands of different options that the user may have to be acquainted with. But in the simulation app, which is a targeted application created by an expert, there could just be two different input fields, maybe a few buttons to click and a graphical display,’ described Sjodin. ‘This is the only thing that is exposed to these end users, and that greatly simplifies the learning curve.’
Optical modelling software is assisting the development of more advanced microscopes. Super resolution microscopy has garnered an increasing amount of attention in recent years, for enabling scientists to observe molecules at resolutions smaller than the diffraction limit of visible light. The 2014 Nobel Prize in Chemistry was awarded to Eric Betzig, Stefan Hell and William Moerner for developing super-resolved fluorescence microscopy.
Classical microscopes are diffraction-limited and so up until recently, the highest resolution that optical microscopes could reach was 200nm. ‘Physical laws determine resolution limits and so depending upon the technical approach, there is only so much that can be done,’ commented Rich Pfisterer, CEO of Photon Engineering. ‘Since microscope systems subtend only a small field of view, classical aberrations are rarely a limiting factor; most of the high-end [traditional] microscope objectives are essentially perfect, i.e. diffraction-limited.’
Super resolution techniques, however, overcome the diffraction limit. ‘Once aberrations are rendered insignificant, the illumination becomes the next critical item,’ Pfisterer continued. ‘For example, by illuminating the sample with partially spatially coherent light, it is possible to resolve structures smaller than would be possible using either coherent or incoherent light.’
In order to model structures smaller than the wavelength of light, explained Akash Arora, product manager at Zemax, ray-tracing needs to be supplemented with a full physical optics treatment. ‘Those programs that support wavefront propagation, like OpticStudio, can model some of the components in these systems. Some of these techniques exploit the fluorescent properties of biological specimens,’ he said.
An important aspect of designing super resolution microscopes is related to an accurate analysis of tolerances, including what often is a complex, multi-staged compensation plan to recover performance loss due to tolerances, according to David Hasenauer, Code V product manager at Synopsys. ‘Synopsys has recently added several new capabilities for optimising as-built (toleranced) performance. Code V’s main tolerancing feature includes a singular value decomposition algorithm that allows the software to determine the most leveraged subset of compensators from a larger list,’ he commented. ‘This can greatly simplify compensation selection and improve the performance of the fabricated system.’
Most microscopes are used in combination with an imaging system and, through software, designers can predict the imaging performance, said Hasenauer. ‘The appearance of the microscope sample as modified by the properties of the optical system can be predicted using Code V’s Image Simulation feature (the sample is represented by a bitmap file, and the software convolves each pixel of the bitmap with the appropriate blurring function to simulate its appearance),’ he said, adding that the software can be used to assist with the alignment of actual fabricated hardware.