Conor O’Mahony

Characterization of micromechanical structures using white-light interferometry

As microelectromechanical systems (MEMS) move rapidly towards commercialization, the issue of mechanical characterization has emerged as a major consideration in device design and fabrication. It is now common to include a set of test structures on a MEMS wafer for extraction of thin film material properties (in particular, residual stress, stress gradient and Youngs modulus), and for process and device monitoring. These structures usually consist of micromachined beams and strain gauges. Measurement techniques include tensile testing, scanning electron microscopy (SEM) imaging, atomic force microscopy (AFM) analysis, surface profiling and Raman spectroscopy. However, these tests are often destructive and may be difficult to carry out at the wafer scale. Instead of these methods, this paper uses white-light interferometry surface profiling for material characterization and device inspection. Interferometry is quick, non-destructive, non-contact, and can offer a high density lateral resolution with extremely high sensitivities to the surface in the z-direction—all essential requirements for high volume manufacturing. A range of devices is employed to illustrate the capabilities of white-light interferometry as a measurement and process characterization tool.It is shown that residual stress may be determined by using electrostatic actuation to pull fixed–fixed beams towards the substrate, and interferometry to record the beam deflection profile. Finite-element simulation software is employed to model this deflection, and to estimate the material properties which minimize the difference between the measured and simulated profiles. The results agree well with blanket film measurements.

Determination of parameters for successful spray coating of silicon microneedle arrays

Coated microneedle patches have demonstrated potential for effective, minimally invasive, drug and vaccine delivery. To facilitate cost-effective, industrial-scale production of coated microneedle patches, a continuous coating method which utilises conventional pharmaceutical processes is an attractive prospect. Here, the potential of spray-coating silicon microneedle patches using a conventional film-coating process was evaluated and the key process parameters which impact on coating coalescence and weight were identified by employing a fractional factorial design to coat flat silicon patches. Processing parameters analysed included concentration of coating material, liquid input rate, duration of spraying, atomisation air pressure, gun-to-surface distance and air cap setting. Two film-coating materials were investigated; hydroxypropylmethylcellulose (HPMC) and carboxymethylcellulose (CMC). HPMC readily formed a film-coat on silicon when suitable spray coating parameter settings were determined. CMC films required the inclusion of a surfactant (1%, w/w Tween 80) to facilitate coalescence of the sprayed droplets on the silicon surface. Spray coating parameters identified by experimental design, successfully coated 280μm silicon microneedle arrays, producing an intact film-coat, which follows the contours of the microneedle array without occlusion of the microneedle shape. This study demonstrates a novel method of coating microneedle arrays with biocompatible polymers using a conventional film-coating process. It is the first study to indicate the thickness and roughness of coatings applied to microneedle arrays. The study also highlights the importance of identifying suitable processing parameters when film coating substrates of micron dimensions. The ability of a fractional factorial design to identify these critical parameters is also demonstrated. The polymer coatings applied in this study can potentially be drug loaded for intradermal drug and vaccine delivery.

Hydrogel-forming microneedle arrays exhibit antimicrobial properties: potential for enhanced patient safety.

We describe, for the first time, the microbial characterisation of hydrogel-forming polymeric microneedle arrays and the potential for passage of microorganisms into skin following microneedle penetration. Uniquely, we also present insights into the storage stability of these hydroscopic formulations, from physical and microbiological viewpoints, and examine clinical performance and safety in human volunteers. Experiments employing excised porcine skin and radiolabelled microorganisms showed that microorganisms can penetrate skin beyond the stratum corneum following microneedle puncture. Indeed, the numbers of microorganisms crossing the stratum corneum following microneedle puncture were greater than 10⁵ cfu in each case. However, no microorganisms crossed the epidermal skin. When using a 21G hypodermic needle, more than 10⁴ microorganisms penetrated into the viable tissue and 10⁶ cfu of Candida albicans and Staphylococcus epidermidis completely crossed the epidermal skin in 24 h. The hydrogel-forming materials contained no microorganisms following de-moulding and exhibited no microbial growth during storage, while also maintaining their mechanical strength, apart from when stored at relative humidities of 86%. No microbial penetration through the swelling microneedles was detectable, while human volunteer studies confirmed that skin or systemic infection is highly unlikely when polymeric microneedles are used for transdermal drug delivery. Since no pharmacopoeial standards currently exist for microneedle-based products, the exact requirements for a proprietary product based on hydrogel-forming microneedles are at present unclear. However, we are currently working towards a comprehensive specification set for this microneedle system that may inform future developments in this regard.

Production of dissolvable microneedles using an atomised spray process: Effect of microneedle composition on skin penetration

Dissolvable microneedles offer an attractive delivery system for transdermal drug and vaccine delivery. They are most commonly formed by filling a microneedle mold with liquid formulation using vacuum or centrifugation to overcome the constraints of surface tension and solution viscosity. Here, we demonstrate a novel microneedle fabrication method employing an atomised spray technique that minimises the effects of the liquid surface tension and viscosity when filling molds. This spray method was successfully used to fabricate dissolvable microneedles (DMN) from a wide range of sugars (trehalose, fructose and raffinose) and polymeric materials (polyvinyl alcohol, polyvinylpyrrolidone, carboxymethylcellulose, hydroxypropylmethylcellulose and sodium alginate). Fabrication by spraying produced microneedles with amorphous content using single sugar compositions. These microneedles displayed sharp tips and had complete fidelity to the master silicon template. Using a method to quantify the consistency of DMN penetration into different skin layers, we demonstrate that the material of construction significantly influenced the extent of skin penetration. We demonstrate that this spraying method can be adapted to produce novel laminate-layered as well as horizontally-layered DMN arrays. To our knowledge, this is the first report documenting the use of an atomising spray, at ambient, mild processing conditions, to create dissolvable microneedle arrays that can possess novel, laminate layering.

Structural characterization and in-vivo reliability evaluation of silicon microneedles

This work presents an analysis of the failure mechanisms, structural properties and reliability of wet-etched silicon microneedles, which have wide-ranging applications in transdermal delivery, sensing and diagnostics. For the first time, in-vivo skin insertion forces are measured and the structural properties of individual silicon microneedles are assessed using both compression and shear tests. Compressive failure of this particular microneedle design does not occur because of buckling, but instead is predominantly due to progressive fracture along the relatively weak {111} crystal plane. Compressive and shear failure strengths are experimentally determined to be (2.9 +/− 0.3) GPa and (9.2 +/− 0.2) MPa, respectively. It is also shown that basic mechanical tests that are commonly used in the field of microneedle development may significantly underestimate safety factors for this type of needle due to the unrepresentative nature of the interaction of a rigid surface with the needle tips. Therefore, a new figure-of-merit for the reliability of such microneedles is proposed, which is based on the ratio of material failure strength to peak stress during skin insertion. The distribution of forces over the sharp, conical needle tip during skin penetration leads to a very large safety margin of over 700, and a correspondingly high degree of reliability when applied to in-vivo human tissue.

Performance and reliability of post-CMOS metal/oxide MEMS for RF application

This paper describes work carried out to assess the elastic performance of microelectromechanical (MEMS) test structures in a post-CMOS (complementary metal oxide semiconductor) metal oxide process. Electrostatic pull-in measurements are used to extract the residual stress, elastic modulus and stress-gradient for the process. Results are presented for metal structures and for composite metal/oxide structures. The extracted parameters are compared with values obtained from blanket film measurements and from analysis based on bulk material properties. Test structures have also been tested for cycling repeatability. A drift is observed in successive cycles of electrostatic actuation and this drift is attributed to charge trapping in the nitride passivation of the underlying CMOS process. A charge-balance model is used to estimate the trapped charge and the effect of actuation voltage polarity is also discussed. The results indicate that satisfactory elastic performance of mechanical structures is dependent on process conditions but can be achieved. The stability of electrical operating characteristics is dominated by nitride charge trapping effects. This effect can be quantified using a basic model. To minimize problems due to charge trapping dielectric properties must be investigated and operating characteristics modified.

Dissolvable microneedle fabrication using piezoelectric dispensing technology

Dissolvable microneedle (DMN) patches are novel dosage forms for the percutaneous delivery of vaccines. DMN are routinely fabricated by dispensing liquid formulations into microneedle-shaped moulds. The liquid formulation within the mould is then dried to create dissolvable vaccine-loaded microneedles. The precision of the dispensing process is critical to the control of formulation volume loaded into each dissolvable microneedle structure. The dispensing process employed must maintain vaccine integrity. Wetting of mould surfaces by the dispensed formulation is also an important consideration for the fabrication of sharp-tipped DMN. Sharp-tipped DMN are essential for ease of percutaneous administration. In this paper, we demonstrate the ability of a piezoelectric dispensing system to dispense picolitre formulation volumes into PDMS moulds enabling the fabrication of bilayer DMN. The influence of formulation components (trehalose and polyvinyl alcohol (PVA) content) and piezoelectric actuation parameters (voltage, frequency and back pressure) on drop formation is described. The biological integrity of a seasonal influenza vaccine following dispensing was investigated and maintained voltage settings of 30 V but undermined at higher settings, 50 and 80 V. The results demonstrate the capability of piezoelectric dispensing technology to precisely fabricate bilayer DMN. They also highlight the importance of identifying formulation and actuation parameters to ensure controlled droplet formulation and vaccine stabilisation.

Modelling and performance evaluation of a MEMS dc/dc converter

Microelectromechanical (MEMS) structures for dc voltage conversion on a silicon substrate are presented. The boost conversion replaces the normal inductor for energy storage in a magnetic field by energy storage in a mechanical system. The mechanical design of the MEMS voltage converter is presented with good agreement between analytical and finite-element modelling (FEM) methods. A discrete analytical model for the converter output voltage at each MEMS actuation cycle is developed and results are compared with electrical device simulation in the PSpice environment. The model takes account of parasitic components in the system implementation, in particular in the blocking diode/switch. Typical MEMS and diode capacitance values are investigated and the voltage gain in these conditions is found to be negligible. The use of a MEMS switch in the blocking diode role can minimize these problems at the cost of more expensive control circuitry and increased power consumption. A suitable application in an RF receiver with intermittent electrostatic actuation of an array of MEMS resonators requiring high transient voltages is described. The efficiency of operation is less than 1.4% but total power consumption is around 1 mJ and this may be acceptable in this intermittent mode of operation.

A study of capacitance-voltage curve narrowing effect in capacitive microelectromechanical switches

In this letter, we report on the capacitance-voltage (C-V) curve narrowing effect, which occurs in the oxide-based microelectromechanical switches that are subjected to dc bias stress for a prolonged period of time. The narrowing effect for the noncontact dc bias stress condition is shown, which proves that membrane-to-dielectric contact is not needed for narrowing to occur. It is also shown that neither mechanical degradation nor charge trapping due to dielectric conduction or air ionization is solely responsible for the C-V instabilities reported in the literature.

Reliability assessment of MEMS switches for space applications: laboratory and launch testing

A novel combination of ground-based and flight tests was employed to examine the reliability of capacitive radio-frequency microelectromechanical switches for use in space applications. Laboratory tests were initially conducted to examine the thermomechanical effects of packaging and space-like thermal stresses on the pull-in voltage of the devices; during this process it was observed that operational stability is highly dependent on the geometrical design of the switch and this must be taken in to account during the design stage. To further expose the switches to acceleration levels experienced during a space mission, they were launched on board a sounding rocket and then subjected to free-fall from a height of over 1.3 km with a resulting impact of over 3500g. Post launch analysis indicates that the switches are remarkably resilient to high levels of acceleration. Some evidence is also present to indicate that time-dependent strain relaxation in die attach epoxy materials may contribute to minor variations in device shape and performance.