Soft lithography is an alternative to silicon-based micromachining that uses replica molding of nontraditional elastomeric materials to fabricate stamps and microfluidic channels. We describe here an extension to the soft lithography paradigm, multilayer soft lithography, with which devices consisting of multiple layers may be fabricated from soft materials. We used this technique to build active microfluidic systems containing on-off valves, switching valves, and pumps entirely out of elastomer. The softness of these materials allows the device areas to be reduced by more than two orders of magnitude compared with silicon-based devices. The other advantages of soft lithography, such as rapid prototyping, ease of fabrication, and biocompatibility, are retained.

http://science.sciencemag.org/content/sci/288/5463/113.full.pdf

Monolithic microfabricated valves and pumps by multilayer soft lithography

High-density microfluidic chips contain plumbing networks with thousands of micromechanical valves and hundreds of individually addressable chambers. These fluidic devices are analogous to electronic integrated circuits fabricated using large scale integration (LSI). A component of these networks is the fluidic multiplexor, which is a combinatorial array of binary valve patterns that exponentially increases the processing power of a network by allowing complex fluid manipulations with a minimal number of inputs. These integrated microfluidic networks can be used to construct a variety of highly complex microfluidic devices, for example the microfluidic analog of a comparator array, and a microfluidic memory storage device resembling electronic random access memories.

https://science.sciencemag.org/content/sci/298/5593/580.full.pdf

Microfluidic large scale integration

Polydimethylsiloxane (PDMS) elastomers are extensively used for soft lithographic replication of microstructures in microfluidic and micro-engineering applications. Elastomeric microstructures are commonly required to fulfil an explicit mechanical role and accordingly their mechanical properties can critically affect device performance. The mechanical properties of elastomers are known to vary with both curing and operational temperatures. However, even for the elastomer most commonly employed in microfluidic applications, Sylgard 184, only a very limited range of data exists regarding the variation in mechanical properties of bulk PDMS with curing temperature. We report an investigation of the variation in the mechanical properties of bulk Sylgard 184 with curing temperature, over the range 25 ◦ C to 200 ◦ C. PDMS samples for tensile and compressive testing were fabricated according to ASTM standards. Data obtained indicates variation in mechanical properties due to curing temperature for Young’s modulus of 1.32‐2.97 MPa, ultimate tensile strength of 3.51‐7.65 MPa, compressive modulus of 117.8‐186.9 MPa and ultimate compressive strength of 28.4‐51.7 GPa in a range up to 40% strain and hardness of 44‐54 ShA.

/pdf/mechanical-characterization-of-bulk-sylgard-184-for-2mkxi71f19.pdf

Mechanical characterization of bulk Sylgard 184 for microfluidics and microengineering

Polydimethylsiloxane (PDMS Sylgard® 184, Dow Corning Corporation) pre-polymer was combined with increasing amounts of cross-linker (5.7, 10.0, 14.3, 21.4, and 42.9 wt.%) and designated PDMS1, PDMS2, PDMS3, PDMS4, and PDMS5, respectively. These materials were processed by spin coating and subjected to common microfabrication, micromachining, and biomedical processes: chemical immersion, oxygen plasma treatment, sterilization, and exposure to tissue culture media. The PDMS formulations were analyzed by gravimetry, goniometry, tensile testing, nanoindentation, scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). Spin coating of PDMS was formulation dependent with film thickness ranging from 308 μm on PDMS1 to 171 μm on PDMS5 at 200 revolutions per minute (rpm). Ultimate tensile stress (UTS) increased from 3.9 MPa (PDMS1) to 10.8 MPa (PDMS3), and then decreased down to 4.0 MPa (PDMS5). Autoclave sterilization (AS) increased the storage modulus (σ) and UTS in all formulations, with the highest increase in UTS exhibited by PDMS5 (218%). PDMS surface hydrophilicity and micro-textures were generally unaffected when exposed to the different chemicals, except for micro-texture changes after immersion in potassium hydroxide and buffered hydrofluoric, nitric, sulfuric, and hydrofluoric acids; and minimal changes in contact angle after immersion in hexane, hydrochloric acid, photoresist developer, and toluene. Oxygen plasma treatment decreased the contact angle of PDMS2 from 109∘ to 60∘. Exposure to tissue culture media resulted in increased PDMS surface element concentrations of nitrogen and oxygen.

/pdf/characterization-of-polydimethylsiloxane-pdms-properties-for-4t0gmb2t3t.pdf

Characterization of polydimethylsiloxane (PDMS) properties for biomedical micro/nanosystems.

Purpose:This paper reviews accelerometry-based activity monitors, including single-site first-generation devices, emerging technologies, and analytical approaches to predict energy expenditure, with suggestions for further research and development.Methods:The physics and measurement principl

The Technology of Accelerometry-Based Activity Monitors: Current and Future

Polydimethylsiloxane (PDMS) is a commercially available physically and chemically stable silicone rubber. It has a unique flexibility with a shear elastic modulus due to one of the lowest glass transition temperatures of any polymer . Further properties of PDMS are a low change in the shear elastic modulus versus temperature , virtually no change in G versus frequency and a high compressibility. Because of its clean room processability, its low curing temperature, its high flexibility, the possibility to change its functional groups and the very low drift of its properties with time and temperature, PDMS is very well suited for micromachined mechanical and chemical sensors, such as accelerometers (as the spring material) and ISFETs (as the ion selective membrane). It can also be used as an adhesive in wafer bonding, as a cover material in tactile sensors and as the mechanical decoupling zone in sensor packagings.

/pdf/the-mechanical-properties-of-the-rubber-elastic-polymer-2jbsjlg7z9.pdf

The mechanical properties of the rubber elastic polymer polydimethylsiloxane for sensor applications

In the medical field triaxial accelerometers are used for the monitoring of movements. Unfortunately, the long-term use of accelerometers is limited by drift of the sensitivities and the offsets. Therefore, a calibration procedure is designed which allows in-use calibration of a triaxial accelerometer. This procedure uses the fact that the modulus of the acceleration vector measured with a triaxial accelerometer equals 1g under quasi-static conditions. The calibration procedure requires no explicit knowledge of the actual orientation of the triaxial accelerometer with respect to gravity and requires only quasi-static random movements. The procedure can be performed within several minutes. Considering practical applications, it is found possible to obtain an average error smaller than 3% (of 1 [g]) with the calibration procedure for inputs caused by ‘standing straight and moving slightly’ when the offsets only are estimated. For the input caused by the sequence that occurs when going to bed, it is found possible to obtain errors smaller than 3% for estimating all parameters.

Procedure for in-use calibration of triaxial accelerometers in medical applications

In this study, the validity and reliability of measurements obtained with an "Activity Monitor" (AM) were examined. The instrument is designed to monitor ambulatory activity by use of accelerometer signals, and it detects several activities associated with mobility (standing, sitting, lying, transitions, movement-related activities). Subiects. Four men with a transtibial amputation and 4 men without a transtibial amputation participated. Methods. The subjects performed normal daily activities, during which accelerations were measured and videotape recordings were made (reference method). Validity was assessed by calculating agreement scores between the AM output and the videotape recordings and by comparing the number of transitions and the duration of activities determined by both methods. Results. The overall agreement between the AM output and the videotape recordings was 90%. Other agreement scores, in addition to the determination of the number of transitions and the duration of
activities, were generally within a range of error of 0% to 10%. Conclusion and Discussion. The reliability and validity of the AM measurements appeared to be good, which supports its potential use in rehabilitation and physical therapy.

/pdf/validity-and-reliability-of-measurements-obtained-with-an-27ysd1p7vf.pdf

Validity and Reliability of Measurements Obtained With an “Activity Monitor” in People With and Without a Transtibial Amputation

The control of a cyclical movement of the lower leg with electrical stimulation of the quadriceps muscles is formulated as an optimal control problem. The time integral of knee torque is taken as the optimisation criterion. As an additional condition, every cycle a certain reference maximum angle should be reached. A model study indicates that one stimulation burst per cycle at the maximum recruitment level is a suboptimal solution to this problem. To compensate for the influence of muscle fatigue, the burst time is adaptively adjusted by a discrete time PID-controller on the basis of the performance in the previous cycles. This strategy appeared to be successful in experimental tests. A considerable time difference (about 0·15 s) was found between the end of the stimulation burst and the tracking of the passive state trajectory, which satisfies the maximum angle condition.

/pdf/control-of-fes-induced-cyclical-movements-of-the-lower-leg-14hzflsth7.pdf

Control of FES-induced cyclical movements of the lower leg.

Polydimethylsiloxane is a silicone rubber. It has a unique flexibility, resulting in one of the lowest glass-transition temperatures of any polymer. Furthermore, it shows a low elasticity change versus temperature, a high thermal stability, chemical inertness, dielectric stability, shear stability and high compressibility. Because of its high flexibility and the very low drift of its properties with time and temperature, polydimethylsiloxane could be well suited for mechanical sensors, such as accelerometers. A novel capacitive accelerometer with polydimethylsiloxane layers as springs has been realized. The obtained measurement results are promising and show a good correspondence with the theoretical values.

/pdf/polydimethylsiloxane-as-an-elastic-material-applied-in-a-52qaxw6tuy.pdf

Petrus H. Veltink

Papers

The mechanical properties of the rubber elastic polymer polydimethylsiloxane for sensor applications

Procedure for in-use calibration of triaxial accelerometers in medical applications

Validity and Reliability of Measurements Obtained With an “Activity Monitor” in People With and Without a Transtibial Amputation

Control of FES-induced cyclical movements of the lower leg.

Polydimethylsiloxane as an elastic material applied in a capacitive accelerometer