The Scale

We chose an Acculab 210.4 scale because it was accurate enough (.0001 gram resolution, which is about a tenth of a microliter for room temperature water) and because it has a serial interface that we can call from Visual Basic to easily collect data and put into a spreadsheet. The data stream is limited to about 8Hz, but it was fast enough for our purposes.

Scales with more resolution often come with damping mechanisms to keep them stable. Although the glass cover keeps the air movement down, this one didn’t have a means to damp vibrations, so we needed to isolate the scale ourselves. We used a small table with some rubber feet as the foundation. Then we used one of those heavy engineering textbooks we have so many of on some thick foam and put the scale on top. Considerate use of flexible beams, soft materials, and masses meant the scale was rock-steady without any software averaging required.

The Fluid Connection

To get the fluid onto the scale, we attached tubing with a luer-lock fitting to a long needle inserted into the scale cover through a hole in the top plate and held in place with a thumb-screw. By using a needle instead of just inserting the tubing into the container, we eliminated any contact with the container or scale. we found that any such contact threw the scale out of whack, especially when the fluid flowed through the tubing with momentum.

The question came up as to whether the needle should be above the water-level or below it. If the needle is below the water-level, surface tension on the needle can reduce the weight of the water. However, if the needle is above the water-level, a drop can form at the tip instead of depositing the small volume onto the scale. We needed fine-resolution flow-rate data, so we opted to ignore the surface tension, which we found to be below the resolution of the scale.

Evaporation of the Water

Over the duration of a test, water will evaporate. The rate is dependent upon temperature, relative humidity, and the surface area of the container. Many suggest adding mineral oil to the water to create a barrier layer to supposedly eliminate evaporation of the water. Data showed that the oil layer did reduce the rate of evaporation (from 0.13 g/hr to 0.08 g/hr), although the oil ruined the plastic connections of the tubing by making them brittle and causing them to leak and break. Instead, we drilled a 1/4” hole into the cap of the container for the needle to fit through, which dropped the rate of evaporation by two orders of magnitude. We collected data for various configurations of the water, oil, and lid and plotted them below. So, without using any oil, we were able to drop the rate of evaporation from 0.127 g/hr to 0.002 g/hr.

Volume and Flow Rate

So, how does a measure of weight relate to volume and flow-rate? Well, the density of water is pretty well defined based on temperature (and weight / density = volume). Accounting for the error of this density value, as well as errors in the scale resolution and time resolution provided excellent measurement results by which to evaluate the devices.

Read more about the symbiosis of modeling and prototyping (PDF) for designing microscale parts in an article I wrote that was published in MICROmanufacturing Magazine this month, page 33 (Jan/Feb 2010, Volume 3, Issue 1).

You can read the article in the Summer 2009 printed publication or catch it in the online version, Meeting the challenges of micropart design.

The traditional way to make a product cheaper has always been subtraction – in essence, minimizing the size and complexity of a device without sacrificing its performance.  Size and complexity reductions can drive down costs on materials, packaging, and shipping, while also favoring higher-throughput production and the use of disposable parts – an increasingly important consideration in biomedical applications.  With that said, the simpler and smaller approach is not without limitations, and these limitations are being tested now by “hugely small” applications.

In the case of micro-electromechanical systems (MEMS), microfluidic chips, nano-sensing technology, and numerous other scale-intensive fields, reduced size is actually a profound contributor to increased complexity.  And while these innovative fields show tremendous promise for the future, they currently pose costly manufacturing hurdles as a consequence.  The cost of prototyping and manufacturing micro-parts should be carefully weighed when considering whether or not to pursue an otherwise-avoidable micro-approach.  As of now, these costs can quickly consume the benefits of implementing questionable technology since this often requires several iterations of low-volume custom components.  Lab-on-a-chip devices are a good example prone to this paradoxical limbo.  Even a relatively straightforward microfluidic component can require robust interfaces and innovative prototyping and assembly processes to ensure proper functionality.  Before long, the microfluidic system isn’t so “micro” anymore – in size or cost.

So what can designers and our manufacturing comrades do to advance the cost effectiveness of these emerging technologies?  For starters, let’s abandon subtraction and opt for addition;  additional measures to define and achieve design tolerances, additional manufacturing techniques for creating repeatable micron and sub-micron parts, additional design features for ease of alignment during assembly, additional quality assurance measures to assess as-built dimensions, and – most importantly – additional communication between manufacturers and designers for continued success on the field of dreams we now find ourselves playing.

To me, “rapid prototyping” refers to any fabrication method that can approximate the production part while shaving weeks or months off the delivery time. Three-dimensional printing processes like SLA, Polyjet, and SLS are just part of the product designer’s toolbox. I’ve used processes such as cast urethane, wood tooling (thermoforming), or even machined plastic to quickly create parts that I can hold in my hands, check the fit, and even test depending on the circumstance. In most instances, prototyped parts cannot be subjected to the full spectrum of real world conditions, such as drop-tests or thermal cycles, but it can happen. I’d consider any of these processes to be rapid compared to the production process.

I was recently interviewed for an article regarding the state of rapid prototyping as it pertains to micro-scale manufacturing and product development. From what I’ve seen, the prototyping processes are just not down to the micro-scale, yet. Granted, the micromanufacturing industry is still pretty young, and it’s been growing so quickly in size and capabilities that I expect to see more rapid prototyping solutions soon.

You can find the article and my comments online at MicroManufacturing Magazine.

Further advances in microfluidics technology development will educe the most profound breakthroughs in medical diagnostic and therapeutic devices — and ultimately improve patient care. Microfluidics chips enable miniaturization of common macro-scale diagnostic devices down to microliter-level hand-held “lab-on-a-chip” devices. Smaller devices enable use at the point of care, and in certain cases, at home with the patient.

The technical advantages of lab-on-a-chip devices, as commonly known, include smaller sample size, higher throughput, faster analysis, and improved accuracy. Certainly, microfluidics diagnostic devices exist on the market today, but there still is significant untapped potential. For example, recent advances in micro fabrication techniques will enable micro pumps and valves to be located directly on the microfluidic chip, instead of requiring macro-scale components to drive the microfluidic flow.

The challenge for microfluidics is bridging the complex gap between R&D and production. Aside from the basic science employed to monitor the analyte, such as ultrasound or advanced optics, the primary challenge is miniaturizing the surrounding electronics and fluid controls, then integrating them seamlessly with the backbone of the device, the microchip.

For microfluidics devices to be successful, it is imperative for design teams to incorporate experts at all points along the value chain, from concept to design to manufacturing, such that the common mishaps associated with transitioning a design from the micro chip level to the macro world are overcome.