Materials Science and Engineering C 33 (2013) 3309–3318

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Materials Science and Engineering C

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Print head design and control for electrohydrodynamic printing of silk fibroin

S.H. Hashimdeen, M. Miodownik, M.J. Edirisinghe ⁎Department of Mechanical Engineering, University College London, Torrington Place, London WC1E 7JE, UK

⁎ Corresponding author. Tel.: +44 20 76793920; fax:E-mail address: m.edirisinghe@ucl.ac.uk (M.J. Edirisi

0928-4931/$ – see front matter © 2013 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.msec.2013.04.020

a b s t r a c t

a r t i c l e i n f o

Article history:Received 27 March 2013Accepted 6 April 2013Available online 11 April 2013

Keywords:Silk fibroinElectrohydrodynamicPrintingPrint headResolution

This study investigates the effect of print head design on the electrohydrodynamic printed resolution of silkfibroin. Needles with large orifices measuring at 800 μm were used to build five different print heads. Theprint heads were manufactured, tested, and optimized using four different silk fibroin solution concentra-tions of 10 wt.%, 15 wt.%, 20 wt.%, and 22 wt.% at applied voltages that ranged from 10 to 20 kV with twodifferent flow rates of 1.5 μl/min and 2.0 μl/min. Each print head design behaved in a unique manner interms of printed line characteristics as the flow rate, voltage and concentration were varied. The highestprinted resolution of the order of 1 μm was achieved using the pinhole reservoir print head. Possible expla-nations for each of the observed behaviors and design criteria for future print heads are discussed.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Ink-jet printing is a powerful technique that affords the user theability to precisely deposit pre-determined amounts of functionalizedmaterials on a variety of substrates to create either two dimensionalsurface features or three dimensional structures. Thermal, electrostaticor acoustic mechanisms are used to dispense droplets that are largerthan the nozzle size [1–3]. The technique is able to handle picolitersof samples efficiently and without wastage and is thus used widely inthe fields of combinatorial chemistry and biology. The low risk of con-tamination coupled with gentle working conditions has led researchersto investigate its use in tissue engineering, in particular organ and cellprinting [4–7]. This efficient handling of minute biological samplesalso allows the technique to print enzymes, micro-organisms andanti-bodies to create a variety of biological devices and functional bio-sensors, bioMEMs being the most popular one. It is also used in highthroughput experimentation to prepare material arrays for screeningand systematic studies for subsequent selection of appropriate mate-rials in a waste efficient and cost effective manner [8].

The limitations of the technology depend and vary on the mecha-nism of droplet generation. The resolution, range of shear forces with-in the nozzle and the rheological properties of the ink used are someof the key limitations that are relevant to the current study. In con-ventional ink-jet printing, low viscosity ink is used to allow for stableflow through the nozzles without the risk of clogging. Similarly thenozzle dimensions need to be sufficiently large enough to allow forparticles, macromolecules and isolated cells used in the ink to flowthrough the aperture without accumulating at the orifice [9,10].

+44 2073880180.nghe).

rights reserved.

Electrohydrodynamic printing is a new and versatile non-contactdirect fabrication technology that is capable of printing at a muchhigher resolution when compared to conventional ink jet printing[9]. This ismade possible by virtue of a large ratio of jet diameter to noz-zle diameter which is achievable when an electric field is used to pullout thin continuous micrometer sized jets from a meniscus suspendedfrom the end of the nozzle in the cone-jet mode [11–13]. The twomain process control parameters of electrohydrodynamic printing arethe applied voltage which is directly proportional to the electric fieldand the flow rate of the material to be printed, a solution or a suspen-sion of it. The flexibility and processing speed of the technique allow itto compete on cost with conventional lithographic patterning tech-niques with deposited resolutions reaching the submicrometer andnano-scale dimensions [9].

Ironically the technique's main strength is also its main drawbackin terms of jet stability. Varicose and whipping i.e., axisymmetric andnon-axisymmetric instabilities and capillary break-up limit the useand effectiveness of the technique. Significant gains have been maderecently by Korkut et al. [14] in controlling capillary jets of glyceroland polyethylene oxide (PEO) through the partial neutralization ofthe surface charges on the jets brought about by the ionization of thegas in the immediate vicinity of the cone-jet. They also reported thatthe ions produced had a contribution to the final measured currentvalues at the counter electrode [14–17].

Most of the currentwork are aimed at achieving controlled deposi-tion of various materials with nano-scale dimensions using nozzleswith small orifices. Successes have been reported by quite a few re-search groups with Park et al. [18] generating line widths reachingclose to 700 nm using a nozzle with a 1 μm internal diameter. Theypredicted that improved nano-scale resolutions would be achievableif nozzle dimensions were further reduced. Park et al. also speculatedthat the electric field concentration brought about by the sharp tips of

Table 1Physical properties of silk solutions used in the experiments. All % refer to weight.

Solution Density(kgm−3)

Viscosity(mPa s)

Electrical conductivity(mS m−1)

Surface tension(mN m−1)

Formic acid 1220 1.44 ± 0.2 30 ± 2 35 ± 0.3SF 10% 1310 10 ± 0.2 598 ± 5 40 ± 0.6SF 15% 1330 18.6 ± 0.2 745 ± 3 42 ± 0.2SF 20% 1320 29.5 ± 0.1 859 ± 6 44 ± 0.8SF 22% 1340 39.2 ± 0.2 916 ± 2 45 ± 0.7

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the nozzles could help reduce lateral distribution of the jet deploy-ment provided that stand-off heights and applied voltages were keptlow. Schrimer et al. [19] were successful in depositing nano dotswith diameters close to 200 nm. They also went onto deposit thesedots in close succession to produce lines with nanometer widths.This was done using nozzles with sub-micrometer dimensions.

Ferraro et al. [20] employed the use of a pyroelectric effect wherebya lithium niobate crystal substrate is placed near a target substrate andis heated using a focused infrared laser to produce high voltages in thekV range. These voltages elicit an electrohydrodynamic response from areservoir of solution placed near the target substrate thus drawing outattoliter droplets in the form of a fine jet. They were able to depositmany advanced materials with resolutions reaching 100 nm in linewidths. The technique overcomesmany of the issues relating nozzle di-mensions to the ability to print at nano-scale resolutions. The creationof a virtual nozzle not only makes nano-scale patterning a realistic pos-sibility but also eliminates the need for specialized capillaries or nozzlesthat are more often than not fragile and prone to clogging up [20–23].

Apart from reducing nozzle dimensions or using specialized equip-ment there is very little work reported on investigating nozzle design.Li et al. [24] performed a systematic study into the effect of needle tipgeometry on the electrohydrodynamic spraying process and reportedthat the size distribution and liquid relic size significantly reducedwith a reduction in needle tip angle.

Wang et al. [25] controlled the jetting frequency by designing anovel retractable needle that was able to deposit both dots and“beads on a string” structures. They varied the flow rate, the concen-tration of polymer and the frequency of needle oscillation during theprinting and established that the jetting frequency shared a close re-lationship with the retractable frequency of the needle.

Complex protein macromolecules such as silk fibroin are knownfor their mechanical properties and biocompatibility. The material of-fers many opportunities in the fields of controlled drug delivery andscaffolds and a recent study conducted by Bayram et al. [26] highlight-ed the potential for depositing silk fibroin using electrohydrodynamicprinting. They successfully printed a variety of intricate patterns atresolutions that varied between 3 and 40 μmdepending on the chosenset of parameters. The solution viscosity was identified to be the most

Fig. 1. Schematic diagram illus

important parameter when it came to printing with a flatbed printhead [26].

The impracticality of use and the cost of building micrometer andsub-micrometer sized nozzles warrant an investigation into alterna-tive routes to printing with high resolution using nozzles that arecheap to produce, easy to handle and consistent in delivery. The enor-mous capillary pressures that would have to be overcome in order touse highly viscous materials may also limit the types of feed thatcould be used in high resolution printing [19]. Thus, the work de-scribed in this study is focused on improving print head design inan attempt to achieve high resolution electrohydrodynamic print pat-terns using silk fibroin.

2. Experimental details

2.1. Materials and silk solution preparation

100 g of de-gummed bombyx silk produced from silk worms of theBombyxMori mothwas purchased fromWorld ofWool, Huddersfield,UK. 15 g of de-gummed silkwas used to prepare 60 ml of silk solution.Formic acid reagent ≥98.0 vol.% and lithium bromide ≥99 wt.% wereobtained from Sigma-Aldrich (Dorset, UK).

Aweighed amount (15 g) of de-gummed silk fibroinwas dissolvedin 9.3 M concentration of lithium bromide solution at 70 to 80 °C. Thesilk solutions were then poured into 30 ml slide-A-Lyzer DialysisCassettes (Fisher Scientific Ltd, Loughborough, UK) which suited amolecular weight of up to 3.5 kDa, and dialysed against deionisedwater for two days to remove the lithium bromide salts in the silk so-lution. The dialysed silk fibroin solution had a concentration of 8 wt.%.The solutions were then cast into petri dishes and heat dried in anoven at 60 °C until thin, transparent silk fibroin films were produced.The heat dried silk fibroin films were then cut up and weighed beforebeing dissolved in formic acid to produce four different concentrations(10 wt.%, 15 wt.%, 20 wt.%, and 22wt.%) for printing.

2.2. Characterisation of silk solutions

Solution characteristics (density, surface tension, electrical con-ductivity and viscosity) that are vital to stable electrohydrodynamicjetting were measured. The characterization equipment was initiallycalibrated before measurements were taken with the silk solutions(Table 1) at the ambient temperature (25 °C), pressure (101.2 kPa)and humidity of (50–55%). The viscosity was measured using Brook-field DV-III Ultra Rheometer for small volumes with a SC4-18 spindle(Brookfield Viscometers Ltd, Harlow, UK). Surface tension was mea-sured using a Kruss tensiometer (Standard Wilhelmy's Plate Method)and electrical conductivity readings were taken using a Jenway 3540pH/conductivity meter (Bibby Scientific Limited, Stone, UK).

trating the printing setup.

Table 2Orifice dimensions of needles in each print head. In each case the needle supplying so-lution and generating the electric field was a shafted needle with an 800 μm internaldiameter.

Print head design Orifice

Reservoir 3 mmPinhole reservoir 300 μmInsulated shafted 800 μmShafted 800 μmCalligraphy 800 μm needle leading to an open

ended broad tip calligraphy nib

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2.3. Electrohydrodynamic printing

The equipment used in this investigation is shown in (Fig. 1). Apurpose built electrohydrodynamic printer (Fig. 2) was the key fea-ture. More details about the printer have been published previously[27]. The x and y axis are driven by servo motors while the z axis iscontrolled by a stepper motor. When the axes are reset back to theirhome-positions both the print head and lower electrode are in linewith each other (Fig. 2c). A printing stage made of Perspex is usedto hold the substrate onto which the patterns are printed. 1 μm linearencoders and endstop sensors on all three axes feed back informationto the program COM-puCAM to control the precise motion of theprinting stage [27].

The 2D print pattern usedwaswritten by specifying the x and y co-ordinates of the print route using Motion Planner software. The pat-tern produced in this study consists of a series of parallel linear linesseparated by a 250 μm spacing. The printing program was uploadedto a programmable motion-controller PC unit. The print speed wasset and maintained at 10 mm/s for all the prints produced.

A Harvard syringe perfusor pump was used to deliver the silksolutions to the print head (needle in Fig. 1) via silicone tubing of300 μm diameter. The applied voltage was regulated via a high volt-age power supply (Glassman Europe Ltd, Tadley, UK). The patternswere printed on standard microscope glass slides at the ambient con-ditions specified earlier.

2.4. Print heads

The dimensions of the needles of the print heads used in this pro-ject are summarized in Table 2. The format and geometry of the dif-ferent needles are described below.

Fig. 2. The electrohydrodynamic printer that was used to test the print heads: (a) the x, y anhead assembly (scale bar 100 mm) and (c) the positions of the print head relative to the lo

2.4.1. ReservoirConsists of an 800 μm shafted needle embedded within a plastic

shell with a conical tapering end (Fig. 3). The needle feeds the silk fi-broin solution into the conical end of the plastic shell and also gener-ates the electrical field when connected to the high voltage supply.The needle hovers over a small reservoir of silk fibroin solution thatis maintained at a constant level by virtue of a regular flow rate.

2.4.2. Pinhole reservoirThis print head (Fig. 4) is built to the same specifications as the

reservoir needle but the difference lies in the size of the orifice. Inthe reservoir needle the orifice has a diameter of 3 mm while it is300 μm in the pinhole reservoir.

2.4.3. Insulated shaftedThis design (Fig. 5) consists of an 800 μm shafted needle partially

insulated with silicone tubing. The sharpest point on the needle is po-sitioned 780 μm from the rim of the silicon tubing.

d z axis printing stages which can move independently of the print head; (b) the printwer electrode (scale bar 100 mm).

Fig. 3. Reservoir print head: (a) in action, and (b, c) design features.

Fig. 4. Pinhole reservoir print head: (a) in action, and (b, c) design features.

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2.4.4. Shafted needleIn this design (Fig. 6), the needle is slanted and positioned at 45°

to the horizontal printing surface.

2.4.5. Calligraphy needleIn this design (Fig. 7) a broad tip calligraphy nib with a slit down

the middle is positioned at 45° to the horizontal and an 800 μm

Fig. 5. Insulated shafted print head: (a)

shafted needle is used to feed the solution along the slit to the tip ofthe nib.

2.5. Assessment of printed patterns

Images of the printed patterns were studied using a Nikon EclipseME600 optical microscope (Nikon Company, Tokyo, Japan). A set of

in action, and (b) design features.

Fig. 6. Shafted print head: (a) in action, and (b) design features.

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10 line width measurements was made for each pattern and the cor-responding mean values and standard deviations were calculated.

3. Results and discussion

The printed patterns generated by each print head are comparedin Fig. 8. Fig. 9a–e provide a detail assessment of the printed linewidth generated by each print head with respect to the applied volt-age. A gradual increase in the line width was observed as the voltageapplied was increased from 10 to 20 kV for all silk concentrations andflow rates. As the voltage increased there was an increase in the ap-plied electric stresses that results in more material being pulled out.At low voltages (10 kV, and 12 kV) small amounts of solution arepinched-off from the large meniscus that is suspended from the tipsof all the print head designs. Each print head design responded wellto change in low flow rates and was able to print its highest resolvedlines at1.5 μl/min with the exception of the calligraphy needle. No

Fig. 7. Calligraphy print head: (a) in

obvious trend was noted as the concentration of the solutions wasvaried from 10 wt.% to 22 wt.% but it produced different results foreach print head design.

Short electrode separation distances of 5 mm were used and theeffect of four different concentrations with each print head and theresulting line widths were studied. The effect of silk concentrationon electrical conductivity and viscosity was different for each solution(Table 1). The higher viscosity solutions produced the most stable jetsbut tended to thicken the jets produced. Solutions with a higher vis-cosity have a higher electrical conductivity but the effect on printingis diminished due to reduced charge mobility.

The lowworking distance helps reduce the chances of varicose andwhipping instabilities from arising in the jet as well as in maintainingthe rheological properties of the solution by limiting solvent evapora-tion [28]. The concentrated electrical field that is created over a shortworking distance of 5 mm brings about the enhanced ionization ofthe gas around the tip of the needle and this produces electrons that

action, and (b) design features.

i)

ii)

iii)

iv)

v)

vi)

i)

ii)

iii)

iv)

v)

vi)

Reservoir Insulated shafted

Pinhole reservoir Shafted Calligraphy

No patterns at 10kV No patterns at 10kV

Fig. 8. A comparison of best print patterns produced by each print head as the applied voltage is increased from i) 10 kV to vi) 20 kV in increments of 2 kV.

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partially neutralize the charges on the surface of the jet, hencerestricting the growth of whipping disturbances [17]. As a result, anyirregularities observed on the optical micrographs of the printedpatterns can be attributed to instabilities brought about by buckling.Controlling mechanical buckling instabilities triggered by competingaxial compression and lateral bending of the electrified jet proved tobe very difficult. These instabilities arise as a result of compressiveand bending forces being exerted onto a straight electrified jet as itmakes contact with the firm glass substrate on which printing takesplace thus forcing the jet to bend and curve during deposition [29].

The geometries of the set of print heads were designed to allow in-vestigation of the different ways to control resolution and jet stability.

It was found that differences in designs forced mechanical bucklingunder a variety of conditions and as a result for the same set of para-metric conditions different buckling patterns were produced wheneach of the print head designs was used.

The excessive coiling and looping that is observed in patternsprinted using the reservoir print head (Fig. 8) can be attributed to buck-ling instabilities [29]. This effect is most pronounced at low voltages(10 kV) and the frequency with which the looping occurs increases asthe voltage is gradually increased to 20 kV. For this particular printhead design, the use of low flow rates and low concentrations producedvery thin jets that bent and looped when they impinged on the sub-strate. The opposite was true for higher concentrations and flow rates

Fig. 9. Printed mean line width variation with applied voltage and silk concentration for both flow rates (1.5 μl/min and 2.0 μl/min). The key in each graph indicates: silk concen-tration in wt.%, flow rate in μl/min. a) reservoir, b) pinhole reservoir, c) insulated shafted, d) calligraphy and e) shafted.

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where the more viscous, heavier deposits were able to absorb some ofthe impact energy and required a much higher applied bending stressto cause mechanical buckling. The optical micrographs (Fig. 8) also re-vealed that as the frequency of buckling intensified the coiled depositswould coalesce to form thicker lines at higher applied voltages. Thehighest resolution for the reservoir print head was achieved at 10 kVwith a 10 wt.% silk concentration with the flow rate set at 1.5 μl/min(Fig. 9a).

The insulated shafted print head produced a mixture of resultswith patterns containing excessive coiling, looping and zig-zag foldingdue to buckling instabilities in addition to patterns with good linearcoherence. Unlike the reservoir design, the insulated shafted print

head design forced high radial electrostatic stresses directly into themeniscus that enveloped the tip of the needle. As a result it can beexpected that the electric field strength is at maximum near the tipof the electrohydrodynamic cone generating the jets thus bringingabout a higher jetting velocity. Hence, the buckling instabilities wereapparent at high concentrations and high flow rates. The high viscos-ities encountered in the jetting of themore concentrated silk solutions(20 wt.%, and 22wt.%) would have resulted in very high compressiveforces being applied on the jet as it impinged on the substrate duringthe printing process. Conversely, low concentrations and low flowrates produced the best results for the insulated shafted print headin terms of resolution and pattern coherence with the highest

Fig. 10. Transition in the intensity of mechanical buckling as concentration of the silk solution was increased resulting in (a) straight printed lines becoming (b) non-linear. Printhead used was insulated shafted, the flow rate was 1.5 μl/min and the applied voltage was set to 12 kV. The silk concentration (wt.%) was 15 in (a) and 22 in (b).

Fig. 11. Print head designs tested in the work, (a) comparison of optimized silk concentration, flow rate, applied voltage and mean line width and (b) comparison of highestresolution.

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resolution of 1.87 μm being achieved at a 15% silk concentration withthe flow rate set at 1.5 μl/min. The intensity of mechanical buckling(Fig. 10) depended on the silk concentration and therefore the vis-cosity. This supports our aforementioned hypothesis that as the vis-cosity of the silk solutions increased, the compressive and bendingstresses applied on the jet also increase. Better results were obtainedat 15 wt.% because under the printing conditions (1.5 μl/min and12–20 kV) no competition between axial compression and lateralbending of the electrified jet was triggered; hence smooth continuousprint depositions were obtained.

The pinhole reservoir print head was by far the best design interms of jet stability and resolution because it was able to deposit pat-terns in a controlled fashion for all applied voltages, flow rates andconcentrations tested, with no visible whipping or buckling instabil-ities (Figs. 8 and 9c). The highest resolution printed with this designwas 1.05 μmwhich was obtained at 12 kV using a solution concentra-tion of 20 wt.% with the flow rate set at 1.5 μl/min (Fig. 9c). The onlydifference between the pinhole reservoir and the reservoir design isthe size of orifice (compare Figs. 3b with 4b). The smaller orifice(300 μm) on the pinhole reservoir offered more control over the sup-ply of solution to the meniscus in addition to narrowing the radialcomponent of the electrostatic field lines. This coupled with the factthat the electrostatic pressure was applied from a distance ensuredsteady jet deployment.

Images of print heads in action in Figs. 3–7 indicate that the shapeand volume of the conical jets rarely changed or fluctuated as thevoltage was varied from 10 to 20 kV. Unstable modes such as the pul-satile and the intermittent cone-jet mode were not observed. Thepresence of a reservoir ensures a constant and steady supply of solu-tion to the meniscus thus preventing the meniscus from oscillating.Jet oscillations are usually brought about by competition betweensupply of feed from the syringe and withdrawal of material causedby the applied electric field [29,30]. It also serves to eliminate the ef-fects of flow regimes such as turbulent and laminar flow on jet stabil-ity. The reservoir is more or less stagnant and is only replenished at asteady rate that is specified by the flow rate. Thus the solution simplyoozes out of the orifice.

The shafted and calligraphy print heads were designed to enhancethe lateral components of the applied electric field as opposed to the ra-dial components. In each case, this was achieved structurally in slightlydifferentwayswith the calligraphy design having a narrow slit down itsmiddle and the slanted shafted print head having an 800 μmorifice thattapers to a sharp needle point (compare Figs. 6b with 7b).

Both these designs behaved in a similar manner, with printed pat-tern resolutions (2–4 μm) achieved for all applied voltages and flowrates at 22 wt.%, 20 wt.% and 15 wt.% silk concentrations. The largestline widths were recorded in both designs for a 10 wt.% concentra-tion. This silk concentration has lower viscosity and surface tensionproperties (Table 1) thus creating and maintaining a stable jet wasdifficult without the appropriate stabilizing viscoelastic stresses. En-hancing the electric field strength in the lateral plane had the effectof throwing the jet off course during the printing process. This canbe seen in Fig. 8 which indicated a significant degree of scatter, anddiscontinuous and irregular printing. The performances of the printheads are compared in Fig. 11.

4. Conclusions

Five different print head designs were used to print silk patternsthat had an average line width which varied between 1 and 10 μm.The results show that modifying the print head design has a signifi-cant effect on resolution of the silk patterns produced. Thus, usingneedle orifices that were much larger than those quoted in the liter-ature, printed lines with widths that were of the order of 1 μm wereobtained. This study indicates potential and highlights an alternative

route to achieving high resolution in electrohydrodynamic printingwithout the need to reduce the needle diameter.

The investigation of insulated shafted, reservoir and pinhole reser-voir designs highlighted some key features that need to be incorpo-rated in future print head development. The use of sharp tip needlesto introduce intense electrostatic stresses directly into the meniscuswill produce stable prints only under specific conditions. This draw-back can be overcome by pulling the needle back to a distance ofabout 5 mm from the meniscus and applying a concentrated radialelectric field. The use of a reservoir ensures a steady and stable supplyof solution to the needle print head. This is critical to the stability andcongruency of the shape and volume of the region where the jetleaves the tip. This can be further regulated by virtue of a small orificeat the base of the reservoir which could also serve to narrow the elec-tric field lines, but applying an electric field with a tilted configurationproved to be detrimental to stable printing.

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Shaikh Hafeez Hashimdeen holds anMEng inMaterials Sci-ence from the Queen Mary University of London. Currently heis a PhD student in theMechanical Engineering Department ofthe University College London (UCL) working in the Biomate-rials Processing and Forming Laboratory. His research focuseson high resolution electrohydrodynamic printing of biopoly-mers.

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Mark Miodownik is a Professor of Materials & Society inthe UCL Mechanical Engineering Department. He receivedhis PhD in turbine jet engine alloys from Oxford Universityin 1996, and his current research areas are biologically in-spired materials, innovative manufacturing, and psycho-physical materials. He is the Director of the Institute ofMaking which is a multidisciplinary research club forthose interested in the made world. Prof Miodownik is abroadcaster and writer: he regularly presents BBC tv pro-grams on engineering, and has written a book called StuffMatters to promote materials science to the public.

Mohan Edirisinghe DSc holds the Bonfield Chair of Bioma-terials in the Department of Mechanical Engineering at theUniversity College London. He has published over 300 jour-nal papers and his most recent research is on advanced jettechniques for the preparation of novel biostructures in-cluding microbubbles and drug delivery capsules. He hasbeen awarded many prizes for his research including the2012 UK Biomaterials Society President's prize.