What do you get when you combine a motivated student leader, enthusiastic classmates, a worldwide online community, and the latest 3D Printing technology from Stratasys? You give children around the world a cool way to hold things again. That is what happened when high school student Rahul Jayaraman of Basis Chandler decided to take part in a project called Enabling The Future. They describe themselves as “A global network of passionate volunteers using 3D Printing to give the world a ‘helping hand'” by designing a wide variety of prosthetic hands for kids that can be printed and assembled by volunteers.
Local news station, KSAZ FOX 10 Phoenix stopped by PADT while we were printing three hands in our Stratasys FORTUS 450 to interview Rahul and talk to us about the project. It gives a great summary:
And Channel 3, KTVK, came to the assembly event at Basis Chandler:>
3D Printing is a fantastic technology for one simple reason, it enables almost anyone to manufacture parts. All you need is a good design. And that is where the people at Enabling the Future come in. Check out their website to see some great examples of how their volunteer work changes so many lives. Have a box of tissue handy if you watch the videos…
This is how the project works. A leader like Rahul takes the initiative to sign up for the project. He then chooses which of the many designs he wants to make. For this first go around, he picked a general design from Thingiverse called the Raptor Reloaded. Next they needed the hardware you could not 3D Print – screws springs, velcro, and bits and pieces that hold the design together. For this they needed to raise $25 per hand so Rahul was given the opportunity to learn how to raise money, a very useful skill.
PADT’s Dhruv Bhate and the rest of our 3D Printing team worked with Rahul to get the design just right and then 3D Print the hands. That will be done this week and this weekend the next phase will take place. Rahul and a large number of his classmates from Basis Chandler will get together at the school this weekend to put thirty or so hands together. They will then box them up and another volunteer group, www.HandChallenge.com, will ship them to kids in the developing world that need them.
Here is a video from Tom Fergus from Fox10 showing a closeup of the hand in action:
We at PADT love projects like this because it is win-win-win. The students get a chance to run a complicated project by themselves, learning the skills they will need later in life to organize, manage, and finish a project. PADT wins because we can contribute to our chosen area of charity, STEM education, in a way that benefits others beyond a given school. And the big winners are the kids around the world that receive a new and cool way to grab hold of life.
We will have sample hands at our open house next Thursday: Nerdtoberfest as well as an update when we get feedback from the distribution of the hands.
Noticed an interesting email in my inbox the other day with the subject line:
“Oktoberfest Time: 3D Print a Beer Stein in Beer Filament”
Marketing gold, you have my attention!
After reading the reviews from the filament manufacturer, I dove in and got some of the hoppy, malty filament on order from 3D Fuel. I was very excited when it came in and couldn’t wait to print PADT’s own beer stein for our upcoming Nerdtoberfest event. Meanwhile I found a nice starting point with a file from GrabCad and added my own additions and alterations.
I quickly went to load the beer filament into one of our 3D printers, when I noticed that the roll size was not compatible with the spool holder on the printer. It was this disconnect that would have previously stopped this experiment in it’s track, however, the future is NOW!
I popped onto the Thingiverse, and alas, I was not alone in having this issue and a plethora of solution were populated before me. I was about to 3D print and adapter to allow my 3D printer to accept a new roll size that was found to be incompatible just moments before. Disaster averted, I was now cooking with gas, er, beer.
The printing process was uneventful and the beer filament printed well. We now have a beer mug printed out of beer filament for PADT’s annual Nerdtoberfest!
In this post, I discuss six challenges that make the modeling of 3D printed cellular structures (such as honeycombs and lattices) a non-trivial matter. In a following post, I will present how some of these problems have been addressed with different approaches.
At the outset, I need to clarify that by modeling I mean the analytical representation of material behavior, primarily for use in predictive analysis (simulation). Here are some reasons why this is a challenging endeavor for 3D printed cellular solids – some of these reasons are unique to 3D printing, others are a result of aspects that are specific to cellular solids, independent of how they are manufactured. I show examples with honeycombs since that is the majority of the work we have data for, but I expect that these ideas apply to foams and lattices as well, just with varying degrees of sensitivity.
1. Complex Geometry with Non-Uniform Local Conditions
I state the most well-appreciated challenge with cellular structures first: they are NOT fully-dense solid materials that have relatively predictable responses governed by straightforward analytical expressions. Consider a dogbone-shaped specimen of solid material under tension: it’s stress-strain response can be described fairly well using continuum expressions that do not account for geometrical features beyond the size of the dogbone (area and length for stress and strain computations respectively). However, as shown in Figure 1, such is not the case for cellular structures, where local stress and strain distributions are non-uniform. Further, they may have variable distributions of bending, stretching and shear in the connecting members that constitute the structure. So the first question becomes: how does one represent such complex geometry – both analytically and numerically?
2. Size Effects
A size effect is said to be significant when an observed behavior varies as a function of the size of the sample whose response is being characterized even after normalization (dividing force by area to get stress, for example). Here I limit myself to size effects that are purely a mathematical artifact of the cellular geometry itself, independent of the manufacturing process used to make them – in other words this effect would persist even if the material in the cellular structure was a mathematically precise, homogeneous and isotropic material.
It is common in the field of cellular structure modeling to extract an “effective” property – a property that represents a homogenized behavior without explicitly modeling the cellular detail. This is an elegant concept but introduces some practical challenges in implementation – inherent in the assumption is that this property, modulus for example, is equivalent to a continuum property valid at every material point. The reality is the extraction of this property is strongly dependent on the number of cells involved in the experimental characterization process. Consider experimental work done by us at PADT, and shown in Figure 2 below, where we varied both the number of axial and longitudinal cells (see inset for definition) when testing hexagonal honeycomb samples made of ULTEM-9085 with FDM. The predicted effective modulus increases with increasing number of cells in the axial direction, but reduces (at a lower rate) for increasing number of cells in the longitudinal direction.
This is a significant challenge and deserves a full form post to do justice (and is forthcoming), but the key to remember is that testing a particular cellular structure does not suffice in the extraction of effective properties. So the second question here becomes: what is the correct specimen design for characterizing cellular properties?
3. Contact Effects
In the compression test shown in the inset in Figure 2, there is physical contact between the platen and the specimen that creates a local effect at the top and bottom that is different from the experience of the cells closer the center. This is tied to the size effect discussed above – if you have large enough cells in the axial direction, the contribution of this effect should reduce – but I have called it out as a separate effect here for two reasons: Firstly, it raises the question of how best to design the interface for the specimen: should the top and bottom cells terminate in a flat plate, or should the cells extend to the surface of contact (the latter is the case in the above image). Secondly, it raises the question of how best to model the interface, especially if one is seeking to match simulation results to experimentally observed behavior. Both these ideas are shown in Figure 3 below. This also has implications for product design – how do we characterize and model the lattice-skin interface? As such, independent of addressing size effects, there is a need to account for contact behavior in characterization, modeling and analysis.
4. Macrostructure Effects
Another consideration related to specimen design is demonstrated in an exaggerated manner in the slowed down video below, showing a specimen flying off the platens under compression – the point being that for certain dimensions of the specimen being characterized (typically very tall aspect ratios), deformation in the macrostructure can influence what is perceived as cellular behavior. In the video below, there is some induced bending on a macro-level.
5. Dimensional Errors
While all manufacturing processes introduce some error in dimensional tolerances, the error can have a very significant effect for cellular structures – a typical industrial 3D printing process has tolerances within 75 microns (0.003″) – cellular structures (micro-lattices in particular) very often are 250-750 microns in thickness, meaning the tolerances on dimensional error can be in the 10% and higher error range for thickness of these members. This was our finding when working with Fused Deposition Modeling (FDM), where on a 0.006″ thick wall we saw about a 10% larger true measurement when we scanned the samples optically, as shown in Figure 4. Such large errors in thickness can yield a significant error in measured behavior such as elastic modulus, which often goes by some power to the thickness, amplifying the error. This drives the need for some independent measurement of the manufactured cellular structure – made challenging itself by the need to penetrate the structure for internal measurements. X-ray scanning is a popular, if expensive approach. But the modeler than has the challenge of devising an average thickness for analytical calculations and furthermore, the challenge of representation of geometry in simulation software for efficient analysis.
6. Mesostructural Effects
The layerwise nature of Additive Manufacturing introduces a set of challenges that are somewhat unique to 3D Printed parts. Chief among these is the resulting sensitivity to orientation, as shown for the laser-based powder bed fusion process in Figure 5 with standard materials and parameter sets. Overhang surfaces (unsupported) tend to have down-facing surfaces with different morphology compared to up-facing ones. In the context of cellular structures, this is likely to result in different thickness effects depending on direction measured.
For the FDM process, in addition to orientation, the toolpaths that effectively determine the internal meso-structure of the part (discussed in a previous blog post in greater detail) have a very strong influence on observed stiffness behavior, as shown in Figure 6. Thus orientation and process parameters are variables that need to be comprehended in the modeling of cellular structures – or set as constants for the range of applicability of the model parameters that are derived from a certain set of process conditions.
Modeling cellular structures has the above mentioned challenges – most have practical implications in determining what is the correct specimen design – it is our mission over the next 18 months to address some of these challenges to a satisfactory level through an America Makes grant we have been awarded. While these ideas have been explored in other manufacturing contexts, much remains to be done for the AM community, where cellular structures have a singular potential in application.
In future posts, I will discuss some of these challenges in detail and also discuss different approaches to modeling 3D printed cellular structures – they do not always address all the challenges here satisfactorily but each has its pros and cons. Until then, feel free to send us an email at email@example.com citing this blog post, or connect with me on LinkedIn so you get notified whenever I write a post on this, or similar subjects in Additive Manufacturing (1-2 times/month).
As 3D Printing makes it’s long awaited move from being dominated by prototyping to manufacturing production parts, companies need to consider a few key issues. In “Five Unique Considerations for 3D Printing Production Parts” I share what we have learned at PADT as we have helped customers make this transition.
Updated (8/30/2016): Two corrections made following suggestions by Gilbert Peters: the first corrects the use of honeycomb structures in radiator grille applications as being for flow conditioning, the second corrects the use of the Maxwell stability criterion, replacing the space frame example with an octet truss.
Within the design element, the first step in implementing cellular structures in Additive Manufacturing (AM) is selecting the appropriate unit cell(s). The unit cell is selected based on the performance desired of it as well as the manufacturability of the cells. In this post, I wish to delve deeper into the different types of cellular structures and why the classification is important. This will set the stage for defining criteria for why certain unit cell designs are preferable over others, which I will attempt in future posts. This post will also explain in greater detail what a “lattice” structure, a term that is often erroneously used to describe all cellular solids, truly is.
Honeycombs are prismatic, 2-dimensional cellular designs extruded in the 3rd dimension, like the well-known hexagonal honeycomb shown in Figure 1. All cross-sections through the 3rd dimension are thus identical, making honeycombs somewhat easy to model. Though the hexagonal honeycomb is most well known, the term applies to all designs that have this prismatic property, including square and triangular honeycombs. Honeycombs have a strong anisotropy in the 3rd dimension – in fact, the modulus of regular hexagonal and triangular honeycombs is transversely isotropic – equal in all directions in the plane but very different out-of-plane.
1.2 Design Implications The 2D nature of honeycomb structures means that their use is beneficial when the environmental conditions are predictable and the honeycomb design can be oriented in such a way to extract maximum benefit. One such example is the crash structure in Figure 2 as well as a range of sandwich panels. Several automotive radiator grilles are also of a honeycomb design to condition the flow of air. In both cases, the direction of the environmental stimulus is known – in the former, the impact load, in the latter, airflow.
2. Open-Cell Foam
Freeing up the prismatic requirement on the honeycomb brings us to a fully 3-dimensionalopen-cell foam design as shown in one representation of a unit cell in Figure 3. Typically, open-cell foams are bending-dominated, distinguishing them from stretch-dominated lattices, which are discussed in more detail in a following section on lattices.
2.2 Design Implications Unlike the honeycomb, open cell foam designs are more useful when the environmental stimulus (stress, flow, heat) is not as predictable and unidirectional. The bending dominated mechanism of deformation make open-cell foams ideal for energy absorption – stretch dominated structures tend to be stiffer. As a result of this, applications that require energy absorption such as mattresses and crumple zones in complex structures. The interconnectivity of open-cell foams also makes them a candidate for applications requiring fluid flow through the structure.
3. Closed-Cell Foam
3.1 Definition As the name suggests, closed cell foams are open-cell foams with enclosed cells, such as the representation shown in Figure 6. This typically involves a membrane like structure that may be of varying thickness from the strut-like structures, though this is not necessary. Closed-cell foams arise from a lot of natural processes and are commonly found in nature. In man-made entities, they are commonly found in the food industry (bread, chocolate) and in engineering applications where the enclosed cell is filled with some fluid (like air in bubble wrap, foam for bicycle helmets and fragile packaging).
3.2 Design Implications
The primary benefit of closed cell foams is the ability to encapsulate a fluid of different properties for compressive resilience. From a structural standpoint, while the membrane is a load-bearing part of the structure under certain loads, the additional material and manufacturing burden can be hard to justify. Within the AM context, this is a key area of interest for those exploring 3D printing food products in particular but may also have value for biomimetic applications.
Lattices are in appearance very similar to open cell foams but differ in that lattice member deformation is stretch-dominated, as opposed to bending*. This is important since for the same material allocation, structures tend to be stiffer in tension and/or compression compared to bending – by contrast, bending dominated structures typically absorb more energy and are more compliant.
So the question is – when does an open cell foam become stretch dominated and therefore, a lattice? Fortunately, there is an app equation for that.
Maxwell’s Stability Criterion
Maxwell’s stability criterion involves the computation of a metric M for a lattice-like structure with b struts and j joints as follows:
In 2D structures: M = b – 2j + 3
In 3D structures: M = b – 3j + 6
Per Maxwell’s criterion, for our purposes here where the joints are locked (and not pinned), if M < 0, we get a structure that is bending dominated. If M >= 0, the structure is stretch dominated. The former constitutes an open-cell foam, the latter a lattice.
There are several approaches to establishing the appropriateness of a lattice design for a structural applications (connectivity, static and kinematic determinism etc.) and how they are applied to periodic structures and space frames. It is easy for one (including for this author) to confuse these ideas and their applicability. For the purposes of AM, Maxwell’s Stability Criterion for 3D structures is a sufficient condition for static determinancy. Further, for a periodic structure to be truly space-filling (as we need for AM applications), there is no simple rigid polyhedron that fits the bill – we need a combination of polyhedra (such as an octahedron and tetrahedron in the octet truss shown in the video below) to generate true space filling, and rigid structures. The 2001 papers by Deshpande, Ashby and Fleck illustrate these ideas in greater detail and are referenced at the end of this post.
Video: The octet truss is a classic stretch-dominated structure, with b = 36 struts, j = 14 joints and M = 0 [Attr. Lawrence Livermore National Labs]
4.2 Design Implications Lattices are the most common cellular solid studied in AM – this is primarily on account of their strong structural performance in applications where high stiffness-to-weight ratio is desired (such as aerospace), or where stiffness modulation is important (such as in medical implants). However, it is important to realize that there are other cellular representations that have a range of other benefits that lattice designs cannot provide.
Conclusion: Why this matters
It is a fair question to ask why this matters – is this all just semantics? I would like to argue that the above classification is vital since it represents the first stage of selecting a unit cell for a particular function. Generally speaking, the following guidelines apply:
Honeycomb structures for predictable, unidirectional loading or flow
Open cell foams where energy absorption and compliance is important
Closed cell foams for fluid-filled and hydrostatic applications
Lattice structures where stiffness and resistance to bending is critical
Finally, another reason it is important to retain the bigger picture on all cellular solids is it ensures that the discussion of what we can do with AM and cellular solids includes all the possibilities and is not limited to only stiffness driven lattice designs.
Note: This blog post is part of a series on “Additive Manufacturing of Cellular Solids” that I am writing over the coming year, diving deep into the fundamentals of this exciting and fast evolving topic. To ensure you get each post (~2 a month) or to give me feedback for improvement, please connect with me on LinkedIn.
 Ashby, “Materials Selection in Mechanical Design,” Fourth Edition, 2011
 Gibson & Ashby, “Cellular Solids: Structure & Properties,” Second Edition, 1997
 Gibson, Ashby & Harley, “Cellular Materials in Nature & Medicine,” First Edition, 2010
 Ashby, Evans, Fleck, Gibson, Hutchinson, Wadley, “Metal Foams: A Design Guide,” First Edition, 2000
 Deshpande, Ashby, Fleck, “Foam Topology Bending versus Stretching Dominated Architectures,” Acta Materialia 49, 2001
 Deshpande, Fleck, Ashby, “Effective properties of the octet-truss lattice material,” Journal of the Mechanics and Physics of Solids, 49, 2001
* We defer to reference  in distinguishing lattice structures as separate from foams – this is NOT the approach used in  and  where lattices are treated implicitly as a subset of open-cell foams. The distinction is useful from a structural perspective and as such is retained here.
I am writing this post after visiting the 27th SFF Symposium, a 3-day Additive Manufacturing (AM) conference held annually at the University of Texas at Austin. The SFF Symposium stands apart from other 3D printing conferences held in the US (such as AMUG, RAPID and Inside3D) in the fact that about 90% of the attendees and presenters are from academia. This year had 339 talks in 8 concurrent tracks and 54 posters, with an estimated 470 attendees from 20 countries – an overall 50% increase over the past year.
As one would expect from a predominantly academic conference, the talks were deeper in their content and tracks were more specialized. The track I presented in (Lattice Structures) had a total of 15 talks – 300 minutes of lattice talk, which pretty much made the conference for me!
In this post, I wish to summarize the research landscape in AM cellular solids at a high level: this classification dawned on me as I was listening to the talks over two days and taking in all the different work going on across several universities. My attempt in this post is to wrap my arms around the big picture and show how all these elements are needed to make cellular solids a routine design feature in production AM parts.
Classification of Cellular Solids
First, I feel the need to clarify a technicality that bothered me a wee bit at the conference: I prefer the term “cellular solids” to “lattices” since it is more inclusive of honeycomb and all foam-like structures, following Gibson and Ashby’s 1997 seminal text of the same name. Lattices are generally associated with “open-cell foam” type structures only – but there is a lot of room for honeycomb structures and close-cell foams, each having different advantages and behaviors, which get excluded when we use the term “lattice”.
The AM Cellular Solids Research Landscape
The 15 papers at the symposium, and indeed all my prior literature reviews and conference visits, suggested to me that all of the work in this space falls into one or more of four categories shown in Figure 2. For each of the four categories (design, analysis, manufacturing & implementation), I have listed below the current list of capabilities (not comprehensive), many of which were discussed in the talks at SFF. Further down I list the current challenges from my point of view, based on what I have learned studying this area over the past year.
Over the coming weeks I plan to publish a post with more detail on each of the four areas above, summarizing the commercial and academic research that is ongoing (to the best of my knowledge) in each area. For now, I provide below a brief elaboration of each area and highlight some important research questions.
1. Representation (Design)
This deals with how we incorporate cellular structures into our designs for all downstream activities. This involves two aspects: the selection of the specific cellular design (honeycomb or octet truss, for example) and its implementation in the CAD framework. For the former, a key question is: what is the optimum unit cell to select relative to performance requirements, manufacturability and other constraints? The second set of challenges arises from the CAD implementation: how does one allow for rapid iteration with minimal computational expense, how do cellular structures cover the space and merge with the external skin geometry seamlessly?
2. Optimization (Analysis)
Having tools to incorporate cellular designs is not enough – the next question is how to arrange these structures for optimum performance relative to specified requirements? The two most significant challenges in this area are performing the analysis at reasonable computational expense and the development of material models that accurately represent behavior at the cellular structure level, which may be significantly different from the bulk.
3. Realization (Manufacturing)
Manufacturing cellular structures is non-trivial, primarily due to the small size of the connecting members (struts, walls). The dimensions required are often in the order of a few hundred microns and lower, which tends to push the capabilities of the AM equipment under consideration. Additionally, in most cases, the cellular structure needs to be self-supporting and specifically for powder bed fusion, must allow for removal of trapped powder after completion of the build. One way to address this is to develop a map that identifies acceptable sizes of both the connecting members and the pores they enclose. For this, we need robust ways of monitoring quality of AM cellular solids by using in-situ and Non-Destructive techniques to guard against voids and other defects.
4. Application (Implementation)
Cellular solids have a range of potential applications. The well established ones include increasing stiffness-to-weight ratios, energy absorption and thermal performance. More recent applications include improving bone integration for implants and modulating stiffness to match biological distributions of material (biomimicry), as well as a host of ideas involving meta-materials. The key questions here include how do we ensure long term reliability of cellular structures in their use condition? How do we accurately identify and validate these conditions? How do we monitor quality in the field? And how do we ensure the entire life cycle of the product is cost-effective?
I wrote this post for two reasons: I love to classify information and couldn’t help myself after 5 hours of hearing and thinking about this area. But secondly, I hope it helps give all of us working in this space context to engage and communicate more seamlessly and see how our own work fits in the bigger picture.
A lot of us have a singular passion for the overlapping zone of AM and cellular solids and I can imagine in a few years we may well have a conference, an online journal or a forum of some sort just dedicated to this field – in fact, I’d love to assess interest in such an effort or an equivalent collaborative exercise. If this idea resonates with you, please connect with me on LinkedIn and drop me a note, or send us an email (firstname.lastname@example.org) and cite this blog post so it finds its way to me.
Is PolyJet MED610 truly biocompatible? And what does that mean anyway?
A couple of months ago, our product development team contacted me to see if I could 3D print them a small bio-compatible masking device that was needed for temporary attachment to an invasive device prior to insertion for surgery. That led me to investigate all the different bio-compatible materials we did have access to at PADT on our FDM (Fused Deposition Modeling) and PolyJet machines. Given the tiny size and high detail required in the part, I decided to opt for PolyJet, which does offer the MED610 material that is claimed to be biocompatible. As it so happens, we have an Objet Eden 260V PolyJet machine that has been dedicated to running MED610 exclusively since it’s installation a year ago.
We printed the mask, followed all the post-processing instructions per supplier recommendations (more on that later) and delivered the parts for further testing. And that is when I asked myself the questions at the top of this post.
I set off on a quest to see what I could find. My first stop was the RAPID conference in (May 2016), where the supplier (Stratasys Inc.) had a well-staffed booth – but no one there knew much about MED610 apart from the fact that some orthodontists were using it. I did pick up one interesting insight: one of the engineers there hypothesized that MED610 was not very popular because it was cost-prohibitive since its proper use required machine dedication. I then went to the Stratasys Direct Manufacturing (a service bureau owned by Stratasys) booth, but it turned out they don’t even offer MED610 as a material option for service jobs – presumably because of the low demand for this material, consistent with our own observations.
So I took a step back and began searching for all I could find in the public domain on MED610 – and while it wasn’t much, here is the summary of my findings that I hope help anyone interested in this. I categorize it in three sources of information: claims made by the supplier, published work on in vitro studies and finally, some in vivo animal trials. But first, we must ask…
What does it mean for a Material to be Biocompatible?
A definition by Williams (The Williams Dictionary of Biomaterials, 1999) is in order: “Biocompatibility is the ability of a material to perform with an appropriate host response in a specific application.” So if PolyJet MED610 is to be called biocompatible, we must ask – what application do we have in mind? Fortunately, the supplier has a recommendation.
MED610 was launched by Objet in 2011 (Objet was acquired by Stratasys in 2012) as a biocompatible material, ideal for “applications requiring prolonged skin contact of more than 30 days and short-term mucosal-membrane contact of up to 24 hours“. Stratasys claims that parts printed according to Objet MED610 Use and Maintenance Terms were evaluated for biocompatibility in accordance with standard “DIN EN ISO 10993-1: 2009, Biological Evaluation of Medical Devices-Part 1: Evaluation and testing within a risk management process. This addresses cytotoxicity, genotoxicity, delayed hypersensitivity, and USP plastic Class VI, which includes the test for irritation, acute systemic toxicity and implantation”. Unfortunately, the actual data from the biocompatibility study conducted by Objet have not been made publicly available.
It is important to remember that Stratasys publishes a “Use and Maintenance Terms” document that details the steps needed not just to clean the part after printing, but also on the proper setup of the machine for ensuring best chances of meeting biocompatibility requirements. These are published online at this link and include a 3 hour soak in a 1-percent NaOH solution, a 30 min soak in IPA and multiple water jet rinses, among other steps. In other words, the claimed biocompatibility of MED610 is only valid if these instructions are followed. These steps are primarily driven by the need to completely remove supports and any support-residue, but it is not clear if this is needed if a part can be printed without supports. Given such strong process dependencies, it is only to be expected that Stratasys provide a disclaimer at the end of the document clarifying that the users of their machines are responsible for independently validating biocompatibility of any device they make with MED610.
The next question is: have there been any relevant published, independent studies that have used MED610? In my search, I could only find two instances, which I discuss below.
Primary Human Cells Response (In Vitro)
In a recent (January 2016) study published in the Journal of Medical and Biological Engineering, Schmelzer et al. studied the response of primary human cells to four 3D printed materials in vitro: ABS, PC, PLA and MED610 – the only such study I could find. All samples instead went through a 100% ethanol brief rinse and were washed 5 times with de-mineralized water – this seems like a less stringent process than what the supplier recommends (3 hour 1-percent NaOH solution soak, 30 minutes IPA soak and 10 times waterjet blasting) but was designed to be identical across all the materials tested.
There were some very interesting findings:
Different cells had different responses:
MED610 had the most negative impact on cell viability for keratinocytes (epidermal cells that produce keratin) – and the only material that showed statistically significant difference from the control.
For bone marrow mesenchymal (stem) cells, a different effect was observed: direct culture on ABS and PC showed significant growth (7X compared to control) but MED610 and PLA showed no significant effect
Surface Roughness influences cell attachment and proliferation:
In agreement with other work, the authors showed that while rougher surfaces promote initial cell attachment, subsequent cell proliferation and overall cell numbers are higher on smoother surfaces. The MED610 samples had rougher surfaces than the FDM samples (possibly due to the use of the “matte” finish option) and could be one of the contributors to the observed negative effects on cell viability, along with the leached contents from the specimen.
Glaucoma Drainage Device (In Vivo, Rabbit studies)
The devices were printed on a Connex 350 PolyJet machine, after which the supports were removed from the devices with a water jet and “were repeatedly washed and inspected for consistency and integrity.” Tubes were attached with Silicone adhesive and the entire assembly was then “washed and sterilized with a hospital-grade hydrogen peroxide system before use”. The researchers did not examine the cellular and extracellular reactions in great detail, but did conclude that the reactions were similar between the MED610 device and the more standard polypropylene injection-molded device.
A short video recorded by some of the researchers as part of a Bioprinting course also provides some details into the 3D printing aspects of the work done.
In conclusion, the question I posed at the start of this post (Is PolyJet MED610 truly biocompatible?) is too simplistic. A process and a material together are not sufficient – there are procedures that need to be defined and controlled and further and more importantly, biocompatibility itself has to be viewed in the context of the application and the specific toxicity and interaction demands of that application. And that brings us to our key takeaways:
MED610 is only recommended at best for applications requiring prolonged skin contactof more than 30 days and short-term mucosal-membrane contact of up to 24 hours and there is no data to dispute the suppliers claim that it is biocompatible in this context once all recommended procedures are implemented
The work done by Australian researchers in using PolyJet MED610 for devoloping their Glaucoma Drainage Device in animal trials is perhaps the best example of how this material and the technology can be pushed further for evaluating designs and hypothesis in vivo when really fine features are needed. Stratasys’s FDM PC-ISO or ABS M30i materials, or other FDM extrusion capable materials like PLA, PCL and PLGA may be better options when the resolution allows – but this is a topic for a follow-on blog post.
More in vitro work needs to be done to extend the work done by Schmelzer et al., which suggests that MED610 potentially has leachables that do impact cell viability negatively. Specifically, effects of surface finish (“matte” vs “gloss”) and sterilization on cell viability is a worthwhile follow-on step. In the interim, MED610 is expected to perform well for mucosal membrane contact under 24 hours (and why this is a great technology for dental guides and other temporary in-mouth placement).
If you have any thoughts on this matter or would like to collaborate with us and take advantage of our access to a PolyJet printer that is dedicated to MED610 or other bio-compatible FDM materials, as well as our extensive post-processing and design & analysis facilities, please connect with me on LinkedIn or send us a note at email@example.com and cite this blog post.
Schmelzer, E., Over, P., Gridelli, B., & Gerlach, J. (2016). Response of Primary Human Bone Marrow Mesenchymal Stromal Cells and Dermal Keratinocytes to Thermal Printer Materials In Vitro. Journal of Medical and Biological Engineering, 36, 153-167.
Ross C, Pandav S, Li Y, et al. Determination of Bleb Capsule Porosity With an Experimental Glaucoma Drainage Device and Measurement System. JAMA Ophthalmol.2015;133(5):549-554. doi:10.1001/jamaophthalmol.2015.30.
Our loop around the Southwest with the new Stratasys J750 Full Color 3D Printer finished strong with a well attended gathering at ASU’s Skysong in Scottsdale. The event was so popular, Channel 10 did a story on it. Over 130 people signed up to learn more about this fantastic device, get caught up on latest industry trends, and talk with other users of Additive Manufacturing.
This event had a great mix of users from multiple industries as well as students and people wanting to just learn more about the technology.
The presentations were a big hit from every seminar, and Dhruv’s was especially popular in Arizona. You can download the presentations here:
During the breaks and after the presentations, we had a chance to interact one-on-one with customers, show off parts, and answer questions. If you have any questions, please feel free to contact us at 480.813.4884 or firstname.lastname@example.org.
Lastly, we were visited by local TV channel 10, KSAZ who did a short but really informative segment on the show and the Stratasys J750:
According to some, the novelty of 3D printing has been wearing off — its mentioned in daily conversations, used on Grey’s Anatomy episodes, incorporated in high school and college classes. Most iPhone-wielding millennials know what it is and how it works. It’s not a “new thing” anymore, right?
Coming to Denver, Salt Lake City, and Phoenix — Phoenix Analysis & Design Technologies (PADT) invites you to be one of the first to meet the Stratasys J750 3D Printer: the latest introduction in the portfolio of PolyJet 3D Printers. The Stratasys J750 is the first-ever full-color, multi-material system, which finally addresses the frustration of designers who want realistic models but have to contend with inconsistent color results and rough finishes from current technology.
Unlike other 3D printers currently in existence, the Stratasys J750 can operate with five different colors: cyan, magenta, yellow, black and white — all of the primary colors in the CMYK color process, just like day-to-day 2D full-color printers. The Stratasys J750 also achieves very fine layer thicknesses, enabling high surface quality and the creation of models and parts with very fine, delicate details, where current 3D printers usually result in relatively rough surface finishes.
What does this mean for those who use 3D printing? The Stratasys J750 not only delivers incredible realism but it’s also the most versatile 3D printer available. Designers and producers can say goodbye to the days of adopting multiple 3D-printing technologies and still resorting afterwards to extensive post-processing, such as sanding, painting and bonding.
Before the Stratasys J750, no single 3D printer could deliver full color, smooth surfaces and multiple materials. Now, however, you can print realistic prototypes, presentation models, Digital ABS injection molds, jigs, fixtures, educational and promotional pieces, production parts – or all of the above, with one system.
The Stratasys J750 even goes one step past versatile, simultaneously being the fastest, simplest, and easiest 3D printer to use. The printer includes several user-requested upgrades, such as server functionality, six-material capacity, and even three print modes that are suitable for different priorities: high speed, high mix and high quality. Additionally, where some 3D printing processes must run in a dedicated facility due to the possible hazard of the materials, chemicals and post-processing steps involved, the Stratasys J750 3D Printer uses a clean, easy process, with no hazardous chemicals to handle.
The Stratasys J750 is one choice among an ever-growing array of 3D printers in the marketplace. But its capabilities and versatility make it more than just a 3D printer; It’s a solution-maker.
In other words, Stratasys has just invented 3D printing. Again. PADT’s 3D Printing team can help you pick the best printer for your job and provide you with one-on-one engineering and prototype support.
If you’re at all interested in technology, you won’t want to miss this printer’s big coming-out day.
A few months ago, I did a post on the Technology Trends in Laser-based Metal Additive Manufacturing where I identified 5 key directions that technology was moving in. In this post, I want to do the same, but for a different technology that we also use on a regular basis at PADT: Fused Deposition Modeling (FDM).
1. New Materials with Improved Properties
Many companies have released and are continuously developing composite materials for FDM. Most involve carbon fibers and are discussed in this review. Arevo Labs and Mark Forged are two of many companies that offer composite materials for higher performance, the table below lists their current offerings (CF = Carbon Fiber, CNT = Carbon Nano Tubes). Virtual Foundry are also working on developing a metal rich filament (with about 89% metal, 11% binder polymer), which they claim can be used to make mostly-metal parts for non-functional purposes using existing FDM printers and a heat treatment to vaporize the binder. In short, while ABS and PLA dominate the market, there is a wide range of materials commercially available and this list is growing each year.
2. Improved Properties through Process Enhancements
Even with newer materials, a fundamental problem in FDM is the anisotropy of the parts and the fact that the build direction introduces weak interfaces. However, there are several efforts underway to improve the mechanical properties of FDM parts and this is an exciting space to follow with many approaches to this being taken. Some of these involve explicitly improving the interfacial strength: one of the ways this can be achieved is by pre-heating the base layer (as being investigated by Prof. Keng Hsu at the Arizona State University using lasers and presented at the RAPID 2016 conference). Another approach is being developed by a company called Essentium who combine microwave heating and CNT coated filaments as shown in the video below.
Taking a very different approach, Arevo labs has developed a 6-axis robotic FDM process that allows for conformal deposition of carbon fiber composites and uses an FEA solver to generate optimized toolpaths for improved properties.
3. Faster & Bigger
A lot of press has centered around FDM printers that make bigger parts and at higher deposition rates: one article discusses 4 of these companies that showcased their technologies at an Amsterdam trade show. Among the companies that showcased their technologies at RAPID was 3D Platform, that showed a $27,000 3D printer for FDM with a 1m x 1m x 0.5m printing platform. Some of the key questions for large form factor printers is if and how they deal with geometries needing supports and enabling higher temperature materials. Also, while FDM is well suited among the additive technologies for high throughput, large size prints, it does have competition in this space: Massivit is one company that in the video below shows the printing of a structure 5.6 feet tall in a mere 5 hours using what they call “Gel Dispensed Printing” that reduces the need for supports.
4. Bioprinting Applications
Micro-extrusion through syringes or specialized nozzles is one of the key ways bioprinting systems operate – but this is technically not “fused” deposition in that it may not involve thermal modification of the material during deposition. However, FDM technology is being used for making scaffolds for bio-printing with synthetic, biodegradable or bio-compatible polymers such as PCL and PLGA. The idea is these scaffolds then form the structure for seeding cells (or in some cases the cells are bioprinted as well onto the scaffold). This technology is growing fast and something we are also investigating at PADT – watch this space for more updates.
5. Material Modeling Improvements
Modeling FDM is an important part of being able to use simulation/analysis to design better processes and parts for functional use. This may not get a lot of press compared to the items above, but is a particular interest of mine and I believe is a critical piece of the puzzle going to true part production with FDM. I have written a few blog posts on the challenges, approaches and a micromechanics view of FDM printed structures and materials. The idea behind all of these is to represent FDM structures mathematically with valid and accurate models so that their behavior can be predicted and designs truly optimized. This space is also growing fast, the most recent paper I have come across in this space is from the University of Wisconsin-Madison that was published May 12, 2016.
Judging by media hype, metal 3D printing and 3D bioprinting are currently dominating the media spotlight – and for good reasons. But FDM has many things going for it: low cost of entry and manufacturing, user-friendliness and high market penetration. And the technology growth has no sign of abating: the most recent, 2016 Wohlers report assesses that there are over 300 manufacturers of FDM printers, though rumor on the street has it that there are over a thousand manufacturers coming up – in China alone. And as the 5 trends above show, FDM has a lot more to offer the world beyond being just the most rapidly scaling technology – and there are people working worldwide on these opportunities. When a process is as simple and elegant as extruding material from a hot nozzle, usable innovations will naturally follow.
PADT was recently asked to do an interview with Composites Weekly to talk about what is new in the world of Additive Manufacturing. Host Jonathan Taylor asked some great questions and we covered a lot of important advances and what to look for in the near future. Listen here.
The Committee aims to meet once a month, our second meeting occurs Monday, July 11 2016 at the ASU Polytechnic Campus and is open to anyone in Arizona that works in Additive Manufacturing and has an interest in promoting its growth statewide through collaboration. For more info, connect with me on LinkedIn or send a note to email@example.com and cite this blog post.
PADT’s Norman Stucker joins Nathan Morimitsu from Manufacturer’s Edge to discuss the current and future state of manufacturing in Colorado. Norman speaks to the impact of 3D Printing and how it is changing manufacturing. It is a great discussion that looks beyond the hype and shares where Additive Manufacturing is today and how it is being applied in the real world.
Have you ever looked at the mechanical properties in an FDM material datasheet (one example shown below for Stratasys ULTEM-9085) and wondered why properties were prescribed in the non-traditional manner of XZ and ZX orientation? You may also have wondered, as I did, whatever happened to the XY orientation and why its values were not reported? The short (and unfortunate) answer is you may as well ignore the numbers in the datasheet. The longer answer follows in this blog post.
Mesostructure has a First Order Effect on FDM Properties
In the context of FDM, mesostructure is the term used to describe structural detail at the level of individual filaments. And as we show below, it is the most dominant effect in properties.
Consider this simple experiment we did a few months ago: we re-created the geometry used in the tensile test specimens reported in the datasheets and printed them on our Fortus 400mc 3D printer with ULTEM-9085. While we kept layer thickness identical throughout the experiment (0.010″), we modified the number of contours: from the default 1-contour to 10-contours, in 4 steps shown in the curves below. We used a 0.020″ value for both contour and raster widths. Each of these samples was tested mechanically on an INSTRON 8801 under tension at a displacement rate of 5mm/min.
As the figure below shows, the identical geometry had significantly different load-displacement response – as the number of contours grew, the sample grew stiffer. The calculated modulii were in the range of 180-240 kpsi. These values are lower than those reported in datasheets, but closer to published values in work done by Bagsik et al (211-303 kpsi); datasheets do not specify the meso-structure used to construct the part (number of contours, contour and raster widths etc.). Further, it is possible to modify process parameters to optimize for a certain outcome: for example, as suggested by the graph below, an all-contour design is likely to have the highest stiffness when loaded in tension.
Can we Borrow Ideas from Micromechanics Theory?
The above result is not surprising – the more interesting question is, could we have predicted it? While this is not a composite material, I wondered if I could, in my model, separate the contours that run along the boundary from the raster, and identify each as it’s own “material” with unique properties (Er and Ec). Doing this allows us to apply the Rule of Mixtures and derive an effective property. For the figure below, the effective modulus Eeff becomes:
Eeff = f.Ec + (1-f).Er
where f represents the cross-sectional area fraction of the contours.
With four data points in the curve above, I was able to use two of those data points to solve the above equation simultaneously and derive Er and Ec as follows:
Er = 182596 psi Ec = 305776 psi
Now the question became: how predictive are these values of experimentally observed stiffness for other combinations of raster and contours? In a preliminary evaluation for two other cases, the results look promising.
So What About the Orientation in Datasheets?
Below is a typical image showing the different orientations data are typically attributed to. From our micromechanics argument above, the orientation is not the correct way to look at this data. The more pertinent question is: what is the mesostructure of the load-bearing cross-section? And the answer to the question I posed at the start, as to why the XY values are not typically reported, is apparent if you look at the image below closely and imagine the XZ and XY samples being tested under tension. You will see that from the perspective of the load-bearing cross-section, XY and XZ effectively have the similar (not the same) mesostructure at the load-bearing cross-sectional area, but with a different distribution of contours and rasters – these are NOT different orientations in the conventional X-Y-Z sense that we as users of 3D printers are familiar with.
The point of this preliminary work is not to propose a new way to model FDM structures using the Rule of Mixtures, but to emphasize the significance of the role of the mesostructure on mechanical properties. FDM mesostructure determines properties, and is not just an annoying second order effect. While property numbers from datasheets may serve as useful insights for qualitative, comparative purposes, the numbers are not extendable beyond the specific process conditions and geometry used in the testing. As such, any attempts to model FDM structure that do not account for the mesostructure are not valid, and unlikely to be accurate. To be fair to the creators of FDM datasheets, it is worth noting that the disclaimers at the bottom of these datasheets typically do inform the user that these numbers “should not be used for design specifications or quality control purposes.”
If you would like to learn more and discuss this, and other ideas in the modeling of FDM, tune in to my webinar on June 28, 2016 at 11am Eastern using the link here, or read more of my posts on this subject below. If you are reading this post after that date, drop us a line at firstname.lastname@example.org and cite this post, or connect with me directly on LinkedIn.