It is well known that medical implants cause a reaction of the organism. Drugs are used to influence the immune response. Drug delivery systems (DDS) ensure the delivery of therapeutically effective drug concentrations locally to the target site. They have been shown to be quite promising in facing the major drawbacks of common methods (tablets, intravenous injection) as for instance biodistribution and systemic toxicity . For that reason a wide range of disciplines are concentrating on the development of DDS. The main field is the development of drug eluting cardiovascular stents . But also the fields of glaucoma implants  and implants for the inner ear  provide wide applications for DDS.
It is common to coat the implants with a degradable polymer incorporated with drugs. The use of polymers as drug carriers is not indisputable. Adverse side effects, for example late stent thrombosis, have been associated with polymeric coatings . In the aim to reduce the use of the carrier polymers there has so far been a lot of research work in the field of discrete drug reservoirs on implant surfaces . Promising geometries of reservoirs for DDS, such as drug eluting stents, are presented in . Hsiao et al. show, that micro-sized reservoirs could lead to a significantly increase of drug capacity with an acceptable marginal trade-off in key clinical attributes such as fatigue safety. In this context it was shown before, that inkjet technique can be a suitable tool, to deposit drugs in micro-sized reservoirs .
The presented research work identifies the crucial factors of influence while realizing a drug deposition in laser-drilled micro-sized reservoirs using a drop-on-demand inkjet printhead. The factors of influence are divided into characteristics of the drug solution, operating parameters of the printhead and geometric parameters of the reservoir.
2 Material and methods
2.1 Printhead parameters
For inkjet printing, the drugs which are to be deposited need to be dissolved in appropriate solvents. The printability of the dissolution is also a crucial factor. It is well known that the printability is especially dependent on the viscosity as well as on the surface tension of the fluid to be printed. For drug-loading of micro-reservoirs the evaporation speed also plays a key role. The solvent has to evaporate as fast as possible from the reservoir’s surface so that the next droplet can be placed in. Acetylsalicylic acid (ASA = 99%, Sigma-Aldrich Chemie GmbH, Munich, Germany) is used as a model drug. ASA can be dissolved in various solvents. On the one hand, there is a need of a high saturation limit to deposit a high quantity of drug per single droplet, but on the other hand, a high drug-concentration may affect the printability of the solution. In order to provide a test substance ASA was dissolved in EtOH (≥99,8%, Carl Roth GmbH + Co. KG, Karlsruhe, Germany) by 40 mg/ml. EtOH (ethanol) is well established as a solvent for drugs and is known for its high evaporation rate (compared to other common solvents such as DMSO). The influence of ASA on the critical parameters of viscosity and surface tension is investigated. Viscosity measurements are performed with a rheometer (Haake Mars 2, Thermo Fisher Scientific, Waltham, MA, USA), surface tension measurements are performed with the Pendant Drop Method (OCA 40 Micro, DataPhysics Instruments GmbH, Neuhausen, Germany).
In order to deposit drugs in micro-sized reservoirs on implant surfaces a micro-dispensing device (Nanoplotter 2.1, GeSiM Gesellschaft für Silizium-Mikrosysteme mbH, Großerkmannsdorf, Germany) is used. The printhead (PicoTip J, GeSiM) follows the piezoelectric drop-on-demand principle (DoD), i.e. a drop is released exactly when the piezo ceramic actuator is triggered. In order to realize a precise deposition in micro-sized reservoirs, the major requirement is a reproducible droplet formation, with a suitable volume and trajectory of the droplet. For that reason, the droplet formation of the test substance is investigated under the use of various drive parameters. The major parameters to control the droplet formation of piezo driven printheads are the voltage, pulse width and frequency as well as the nozzle diameter of the printhead. In this study, the printhead is driven with a voltage of 40 V up to 80 V (in 5 V steps). The pulse width (20 μs) and frequency (100 Hz) are set to fix standard values. With reference to the nozzle diameter of the printhead (d = 25 μm), droplets with ∼40 μm in diameter should be expected. The droplet formation is monitored by a stroboscope camera. The resulting image is analyzed with respect to the droplet volume and droplet positioning estimation. A deviation of the droplet trajectory from the vertical line is denoted as angle failure. Five droplets are analyzed per 5 V step using this method. The results will be compared to the printing process of pure EtOH.
2.2 Drug depositing into micro-sized reservoirs
In this study drug depositing is applied to two variants of reservoirs. Variant one has a diameter of ∼100 μm and a depth of ∼20 μm (R100-20); variant two has a higher depth of ∼50 μm (R100-50). The reservoir generation was performed by a picosecond laser (TruMicro 5 × 50, TRUMPF Laser- und Systemtechnik GmbH & Co.KG, Ditzingen, Germany) using a high-precision micromachining system (GL.5, GFH GmbH, Deggendorf, Germany). Picosecond laser provide a high peak power and extremely short interaction time. As result the reservoirs show a sharp contour (see Figure 1).
The micro dosing device uses automatic target recognition to identify the reservoirs. It is assumed that after being printed, the solvent will vaporize and there will be a deposit of drug in the reservoir. Approximately 2 ng of ASA is printed per droplet (∼50 pl). The volume of a reservoir is limited (R100-20 ∼120 pl; R100-50 ∼270 pl). In order to achieve a fill grade of ∼50%, 42 droplets have to be printed in a 100-20 reservoir (calculated with ρASA = 1,35 g/cm3). Because of the limited volume of the reservoirs, there is the need to split the depositing into steps, because the drug-solution needs time to vaporize and overfilling should be avoided. In a first test setup, a reservoir was filled with three droplets per step and 14 steps were performed, with a delay of ∼5 s each. However, such small amounts of fluid are subject to effects like wettability. For that reason two more test setups with a reduced number of droplets per step were performed. In the second test setup, a reservoir was filled with two droplets per step and 21 steps were performed. In the last (3rd) test setup, 42 steps were performed with 1 droplet each. The reservoirs R100-50 were laden with the same test setups. Five reservoirs were filled for each setup.
3 Results and discussion
3.1 Printhead parameters
As expected the droplet formation depends on the applied voltage pulses of the piezo. Increasing the voltage increases the droplet volume and decreases the angle failure (see Figures 2 and 3). If the printhead is driven at low voltage (40 V/45 V), a single droplet is formed with a low droplet volume (Figure 4A). However, there is a significant angle failure as a result of a low droplet speed that is caused by the low application of energy. At higher voltage, the main droplet is followed by a single satellite droplet. As seen in Figure 4B, the satellite is located in the trajectory of the main droplet. If the voltage is increased (70 V) the number of satellite droplets increases as well as the droplet volume. Furthermore, an upper limit of voltage was determined. If the piezo is driven with a voltage of about 75 up to 80 V, the reproducibility of the droplet formation decreases and a sort of spattering can be observed (Figure 4C). Under these conditions a measurement of droplet volume and angle failure was not possible. In comparison to inkjet printing of pure ethanol, ASA seemed to have no significant influence on the droplet formation. However there is a tendency to a lower angle failure. All in all, these results coincide with the expectations derived from surface tension measurements (EtOH: Ø22,5 mN/m, EtOH+ASA: Ø22,4 mN/m, n = 5) and viscosity measurements (EtOH: Ø1,18 mPas, EtOH+ASA: Ø1,23 mPas, n = 3, = 25/s, double-cone geometry 60/1°), where no significant influence of EtOH due to ASA was determined.
For a precise droplet deposition there is a need of a high reproducibility of droplet formation and a low angle failure. Because of that, a piezo driving voltage of 60 V was determined for the drug deposition in micro-sized reservoirs.
3.2 Drug depositing
Figure 5A shows the results of drug depositing in R100-20 by the first test setup. The drug is deposited around the reservoir, instead of inside the reservoir. For test setups no. 2 and 3, the effect decreases significantly. However, drug sedimentations around the reservoir cannot be eliminated (Figure 5B) for this reservoir size. The reason for this could be a wetting of the surface around the reservoir, possibly caused by an overfilling or missing of the reservoirs. In comparison to these results, filling deeper reservoirs (R100-50) results in deposits of the drug inside the reservoir (Figure 5C). This is an indication for valid droplet positioning. However, it can be seen that the deposit of ASA is concentrated at the walls of the reservoir.
This study demonstrates drug deposition into micro-sized reservoirs according to the characteristics of the drug solution, operating parameters of the printhead and geometric parameters of the reservoir. In addition, a feasible manufacturing method for micro-sized reservoirs is shown.
In the shown setup of drug deposition, effects such as wettability may have an important influence, especially when the used solvent shows a high wettability of metal surfaces, as is the case for EtOH. It is indicated that there is the need to figure out an optimal ratio between droplet volume and the volume of the reservoir to be loaded. In further investigations the deposition process will be tested using more variations of reservoirs and solvents.
The authors wish to acknowledge the skillful assistance of Franz Stachitz.
Research funding: We would like to thank the “Bundesministerium für Bildung und Forschung” (BMBF) for financial support for RESPONSE – Partnerschaft für Innovation in der Implantattechnologie. This support is provided with financial resources by the program “Zwanzig20 – Partnerschaft für Innovation”, hosted by BMBF. We would also like to thank the “Ministerium für Wirtschaft, Bau und Tourismus”, Mecklenburg-Vorpommern which provided funding of the Gesim Nanoplotter 2.1 with financial resources via ERDF. Conflict of interest: Authors state no conflict of interest. Material and Methods: Informed consent: Informed consent has been obtained from all individuals included in this study Ethical approval: The research related to human use complies with all the relevant national regulations, institutional policies and was performed in accordance with the tenets of the Helsinki Declaration, and has been approved by the authors’ institutional review board or equivalent committee.
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About the article
Published Online: 2016-09-30
Published in Print: 2016-09-01
Citation Information: Current Directions in Biomedical Engineering, Volume 2, Issue 1, Pages 387–390, ISSN (Online) 2364-5504, DOI: https://doi.org/10.1515/cdbme-2016-0086.
©2016 Robert Mau et al., licensee De Gruyter.. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 License. BY-NC-ND 4.0