Study of the fabrication of AISI 316L microneedle arrays

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ScienceDirect Procedia Manufacturing 00 (2017) 000–000

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* Paulo Afonso. Tel.: +351 253 510 761; fax: +351 253 604 741 E-mail address: [email protected]

2351-9789 © 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the scientific committee of the Manufacturing Engineering Society International Conference 2017.

Manufacturing Engineering Society International Conference 2017, MESIC 2017, 28-30 June 2017, Vigo (Pontevedra), Spain

Costing models for capacity optimization in Industry 4.0: Trade-off between used capacity and operational efficiency

A. Santanaa, P. Afonsoa,*, A. Zaninb, R. Wernkeb

a University of Minho, 4800-058 Guimarães, Portugal bUnochapecó, 89809-000 Chapecó, SC, Brazil

Abstract

Under the concept of "Industry 4.0", production processes will be pushed to be increasingly interconnected, information based on a real time basis and, necessarily, much more efficient. In this context, capacity optimization goes beyond the traditional aim of capacity maximization, contributing also for organization’s profitability and value. Indeed, lean management and continuous improvement approaches suggest capacity optimization instead of maximization. The study of capacity optimization and costing models is an important research topic that deserves contributions from both the practical and theoretical perspectives. This paper presents and discusses a mathematical model for capacity management based on different costing models (ABC and TDABC). A generic model has been developed and it was used to analyze idle capacity and to design strategies towards the maximization of organization’s value. The trade-off capacity maximization vs operational efficiency is highlighted and it is shown that capacity optimization might hide operational inefficiency. © 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the scientific committee of the Manufacturing Engineering Society International Conference 2017.

Keywords: Cost Models; ABC; TDABC; Capacity Management; Idle Capacity; Operational Efficiency

1. Introduction

The cost of idle capacity is a fundamental information for companies and their management of extreme importance in modern production systems. In general, it is defined as unused capacity or production potential and can be measured in several ways: tons of production, available hours of manufacturing, etc. The management of the idle capacity

Procedia Manufacturing 26 (2018) 117–124

2351-9789 © 2018 The Authors. Published by Elsevier B.V.Peer-review under responsibility of the scientific committee of the 46th SME North American Manufacturing Research Conference.10.1016/j.promfg.2018.07.014

10.1016/j.promfg.2018.07.014 2351-9789

© 2018 The Authors. Published by Elsevier B.V.Peer-review under responsibility of the scientific committee of the 46th SME North American Manufacturing Research Conference.

Available online at www.sciencedirect.com



2351-9789 © 2018 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the scientific committee of NAMRI/SME.

46th SME North American Manufacturing Research Conference, NAMRC 46, Texas, USA

Study of the fabrication of AISI 316L microneedle arrays

Erika García-Lópeza, Héctor R. Sillerb, Ciro A. Rodrígueza* a. Tecnológico de Monterrey, Ave. Eugenio Garza Sada 2501 Sur, N.L., 64849México

b. Department of Engineering Technology, University of North Texas, Denton, TX 76207, USA

* Corresponding author. Tel.:+52 (81) 83582000; E-mail address: [email protected]

Abstract

Microneedles are an alternative to deliver chemical compounds through the skin for the treatment of several health conditions. In this work, a micro milling process was used to manufacture AISI 316L stainless steel needles arrays with a conical geometry (i.e., a base diameter and height of 1 mm by 1 mm, respectively). An analysis of tool wear on ball nose micro end milling tools with a diameter of ~200 𝜇𝜇𝜇𝜇 was performed to evaluate the use of lubricant in the manufacture of those conical surfaces. A tool diameter reduction of 6% and 8% was obtained in dry cutting and wet cutting, after the manufacture of 9 and 54 needles respectively. Geometrical features on the needle tip resulted more affected in dry cutting regime compared with the wet treatment. A maximum needle tip diameter of 188 μm was reported on dry cutting versus values lower than 30 μm using wet regime. The results indicate that surface roughness increases after machining 36 and 9 needles in wet cutting regime and dry cutting treatment, respectively. However tool breakage occurs after milling 12 needles in dry cutting. A cost analysis shows that the micro milling approach is a competitive method for prototyping of microneedle arrays. © 2018 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the scientific committee of NAMRI/SME.

Keywords: Micro milling, microneedles, tool wear, surface roughness

1. Introduction

According to the World Health Organization (WHO), 16 billion of therapeutic and prophylactic injections are administered annually in the world [1]. Microneedles are an alternative to deliver

pharmaceutical compounds through the stratum corneum (outermost layer of the epidermis) in a painless minimally invasive, and with self-administration approach [2,3]. The stratum corneum, is a dead tissue (i.e., the outer 10 µm -15 µm of skin) that forms the primary barrier to drug transport [4].

Available online at www.sciencedirect.com



2351-9789 © 2018 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the scientific committee of NAMRI/SME.

46th SME North American Manufacturing Research Conference, NAMRC 46, Texas, USA

Study of the fabrication of AISI 316L microneedle arrays

Erika García-Lópeza, Héctor R. Sillerb, Ciro A. Rodrígueza* a. Tecnológico de Monterrey, Ave. Eugenio Garza Sada 2501 Sur, N.L., 64849México

b. Department of Engineering Technology, University of North Texas, Denton, TX 76207, USA

* Corresponding author. Tel.:+52 (81) 83582000; E-mail address: [email protected]

Abstract

Microneedles are an alternative to deliver chemical compounds through the skin for the treatment of several health conditions. In this work, a micro milling process was used to manufacture AISI 316L stainless steel needles arrays with a conical geometry (i.e., a base diameter and height of 1 mm by 1 mm, respectively). An analysis of tool wear on ball nose micro end milling tools with a diameter of ~200 𝜇𝜇𝜇𝜇 was performed to evaluate the use of lubricant in the manufacture of those conical surfaces. A tool diameter reduction of 6% and 8% was obtained in dry cutting and wet cutting, after the manufacture of 9 and 54 needles respectively. Geometrical features on the needle tip resulted more affected in dry cutting regime compared with the wet treatment. A maximum needle tip diameter of 188 μm was reported on dry cutting versus values lower than 30 μm using wet regime. The results indicate that surface roughness increases after machining 36 and 9 needles in wet cutting regime and dry cutting treatment, respectively. However tool breakage occurs after milling 12 needles in dry cutting. A cost analysis shows that the micro milling approach is a competitive method for prototyping of microneedle arrays. © 2018 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the scientific committee of NAMRI/SME.

Keywords: Micro milling, microneedles, tool wear, surface roughness

1. Introduction

According to the World Health Organization (WHO), 16 billion of therapeutic and prophylactic injections are administered annually in the world [1]. Microneedles are an alternative to deliver

pharmaceutical compounds through the stratum corneum (outermost layer of the epidermis) in a painless minimally invasive, and with self-administration approach [2,3]. The stratum corneum, is a dead tissue (i.e., the outer 10 µm -15 µm of skin) that forms the primary barrier to drug transport [4].

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118 Erika García-López et al. / Procedia Manufacturing 26 (2018) 117–1242 E. García-López/ Procedia Manufacturing 00 (2018) 000–000

The transportation of the compounds through the skin is limited by their inability of some drugs to cross this barrier [5]. Some studies have analyzed the influence of the microneedle geometry and quality features on the minimization of pain and the possible risks of infection from blood borne pathogens [6]. These geometrical features (i.e., height, length, top diameter, surface roughness) are achieved through the manufacturing process according to their functional attributes. Table 1 presents a review of the geometrical needle features and the corresponding manufacturing processes used in microneedle fabrication classified by manufacturing principle. Table 1. Literature review on microneedle manufacturing.

Ref. Manufacturing process (Material)

Geometry; (height (H), Base (B), Tip (T)), Construction: [P]: Positive, [N]: Negative

Principle: Chip formation process [7] CNC micro milling

(PMMA) Pyramidal [P], Conical [P] and dwarf conical [P].

Principle: Dissolution [8] Reactive Ion Etching

(Silicon) Cantilevered hollow [P] (H: 60 µm, B: 80µm)

[9] Dry etching (Polyamide and Silicon)

Conical [P]

[10] Photolithography (Silicon and PLA)

Planar tip with hole [P] (H: 100µm, B: 230 µm, T: 10 µm)

Principles: Deposition and lamination. [11] LIGA and hot embossing

(PMMA) Pyramidal [P] (H: 700 µm -900 μm; B: 200 μm – 470 μm)

Principles: Dissolution and chip formation [2] CNC micro milling and

isotropic wet etching (Aluminum)

Pyramidal [N] (H: 500 μm; T:10 μm]

[3] CNC micro milling and lithography (Aluminum)

Pyramidal with beveled edge [N] (H: 645 μm, 950 μm, 510 μm)

Principles: Dissolution and Vaporization [12] Laser micromachining

and LIGA (Glass) Conical [N]

[13] Laser ablation and etching (silicon)

Conical [P] (H: 400 μm)

Principles: Solidification and Vaporization [14] Laser micromachining

and micro-injection molding (Stainless steel 420)

Pyramidal beveled [N] (H: 500 μm, B: 200 μm)

In addition, Table 1 shows the construction type (i.e. a) “negative” when a master mold is used to replicate the needles and b) “positive” when the needles are manufactured directly on the final material). Although there several manufacturing methods have been reported, CNC micro milling process stands out as a viable alternative for the production of complex geometries with a good smoothness, sharp exterior

corners and very vertical sidewalls [15]. When needles are manufactured through mechanical processes, the tool path strategy must be selected carefully due to the choice of the milling strategies programmed in the CAM (Computer aided manufacturing) software which would result in breakage or sharp tips due to the micro-scale [7]. As an alternative, irregularities on surface can be removed through postprocessing techniques such as mechanical or chemical etching in order to clean the surface from debris, slag or burrs. [7,13]. However, ideally the combination of milling strategies and cutting parameters must be properly selected to accomplish the quality features required for the application. There are many studies focused on tool wear and chip formation in micro milling with square end mills geometries. However, only a few studies have investigated the effect of tool wear on ball nose tooling with the purpose of manufacturing sculptured surfaces in the micro-size regimen [16-19]. For example, the cutting tool deterioration was studied for the machining of micro-cavities for die and mold industry with the aim of performing a process calibration avoiding the tool wear which reduces the cutting tool diameter, dulls the cutting edge, decreases the rake angle and diminishes the surface quality of the machined surface [16]. Also, tool flank wear was analyzed in order to compare the MQL (Minimum Quantity Lubrication) cutting with dry cutting resulting in a 60% of tool wear reduction with MQL cutting [20]. This paper presents a study of the feasibility to directly manufacture microneedle arrays on AISI 316L stainless steel using micro milling process. Experimental tests were performed to evaluate tool wear, and its effect on surface finish and the geometrical features on the needles patches.

2. Materials and methods.

2.1. Microneedles design and CAM strategies

A base setup with cylinders was modelled on NX software (Siemens, USA). Separately, a conical needle was modelled with the same software. Two operations were programmed on NX software: The CAM strategy selected for the base setup was a planar mill setup with a path setting of mill finish and for the needles manufacture a streamline strategy was selected with the same path setting and a drive method in spiral. The dimensions of the stock cylinders are 1 mm in diameter by 1 mm in height. The arrays consisted of 9 cylinders separated by 2.5 mm between centers to

Erika García-López et al. / Procedia Manufacturing 26 (2018) 117–124 119 Author name / Procedia Manufacturing 00 (2018) 000–000 3

integrate a needle patch. According to our review presented in Table 1. The typically arrangement is based in arrays with 9 [3], 105 [5], 25 [7] and 66 [12] needles which depends of the final application. Also, the material selection is related to the needle structure (i.e., hollow or solid), therefore commercial microneedles [21,22] are manufactured on polymers with pharmaceutical compounds and metals. Furthermore, the aggrupation of the needles in a two-dimensional array is explained due to the patch can withstand higher applied force than individual needles because of the distribution of forces, both normal and shear [23]. Figure 1 presents the base (a) and the needle (b) design and the CAM strategy (c) selected.

Figure 1. Needle features (a) Base setup geometry (b) Needle geometry and (c) CAM strategy for the

needle geometry.

2.2. Experimental setup

The vertical machining center used in this study is a Makino F3 with a ceramic ball bearing and a maximum spindle speed of 30,000 RPM. The tungsten carbide ball nose micro end milling tools were adjusted mechanically to the collet. As first step, the base setup with cylinders was machined using a 0.79 mm square end mill (SGS 30303). As second step, the conical needles were machined with a ball nose micro end milling tool with a cutting diameter of 0.2 mm (Mitsubishi MS2SBR0010S04). This tool is a two-flute ball nose for general use and supplier does not provide any recommendation for the coolant features. Table 2 presents the micro tool features used on each stage. The needles were fabricated on AISI 316L stainless steel material. A comparison between dry cutting and wet cutting was studied to identify the quality differences among needles. Dry cutting was

performed using air cleaning ahead the tip during the machining process. For wet cutting, a bio- based lubricant (MAK kit10 ES-AL) was pumped on the interface between the workpiece and the micro tool. Table 3 presents the cutting conditions used on experimental work and Figure 2 presents the experimental setup to fabricate the AISI 316L stainless steel needles. Table 2. Ball nose micro end milling tool features

Mitsubishi MS2SBR0010S04

SGS 30303

Geometry Ball nose Square Cutting diameter 0.20 mm 0.79 mm Flutes 2 2 Coating (Al, Ti, Cr) N Uncoated Cutting angle 30° 30° Cooling Dry cutting / Wet cutting

Figure 2. Experimental setup.

2.3. Experimental design

Table 3 presents the cutting conditions used on experimental work and Figure 2 presents the experimental setup to fabricate the AISI 316L stainless steel needles. After the fabrication of three needles, the ball nose diameter was measured using a stereo microscope (Carl Zeiss V8, Germany) in order to calculate the percentage of tool wear. This wear was calculated as the difference between the initial diameter measured on the micro tool and the tool diameter after the milling of three needles. Then, the tool diameter reduction was calculated as:

𝐷𝐷𝑟𝑟 = (𝐷𝐷0 − 𝐷𝐷𝑓𝑓𝐷𝐷0

) ∗ 100 (1)

2 E. García-López/ Procedia Manufacturing 00 (2018) 000–000

The transportation of the compounds through the skin is limited by their inability of some drugs to cross this barrier [5]. Some studies have analyzed the influence of the microneedle geometry and quality features on the minimization of pain and the possible risks of infection from blood borne pathogens [6]. These geometrical features (i.e., height, length, top diameter, surface roughness) are achieved through the manufacturing process according to their functional attributes. Table 1 presents a review of the geometrical needle features and the corresponding manufacturing processes used in microneedle fabrication classified by manufacturing principle. Table 1. Literature review on microneedle manufacturing.

Ref. Manufacturing process (Material)

Geometry; (height (H), Base (B), Tip (T)), Construction: [P]: Positive, [N]: Negative

Principle: Chip formation process [7] CNC micro milling

(PMMA) Pyramidal [P], Conical [P] and dwarf conical [P].

Principle: Dissolution [8] Reactive Ion Etching

(Silicon) Cantilevered hollow [P] (H: 60 µm, B: 80µm)

[9] Dry etching (Polyamide and Silicon)

Conical [P]

[10] Photolithography (Silicon and PLA)

Planar tip with hole [P] (H: 100µm, B: 230 µm, T: 10 µm)

Principles: Deposition and lamination. [11] LIGA and hot embossing

(PMMA) Pyramidal [P] (H: 700 µm -900 μm; B: 200 μm – 470 μm)

Principles: Dissolution and chip formation [2] CNC micro milling and

isotropic wet etching (Aluminum)

Pyramidal [N] (H: 500 μm; T:10 μm]

[3] CNC micro milling and lithography (Aluminum)

Pyramidal with beveled edge [N] (H: 645 μm, 950 μm, 510 μm)

Principles: Dissolution and Vaporization [12] Laser micromachining

and LIGA (Glass) Conical [N]

[13] Laser ablation and etching (silicon)

Conical [P] (H: 400 μm)

Principles: Solidification and Vaporization [14] Laser micromachining

and micro-injection molding (Stainless steel 420)

Pyramidal beveled [N] (H: 500 μm, B: 200 μm)

In addition, Table 1 shows the construction type (i.e. a) “negative” when a master mold is used to replicate the needles and b) “positive” when the needles are manufactured directly on the final material). Although there several manufacturing methods have been reported, CNC micro milling process stands out as a viable alternative for the production of complex geometries with a good smoothness, sharp exterior

corners and very vertical sidewalls [15]. When needles are manufactured through mechanical processes, the tool path strategy must be selected carefully due to the choice of the milling strategies programmed in the CAM (Computer aided manufacturing) software which would result in breakage or sharp tips due to the micro-scale [7]. As an alternative, irregularities on surface can be removed through postprocessing techniques such as mechanical or chemical etching in order to clean the surface from debris, slag or burrs. [7,13]. However, ideally the combination of milling strategies and cutting parameters must be properly selected to accomplish the quality features required for the application. There are many studies focused on tool wear and chip formation in micro milling with square end mills geometries. However, only a few studies have investigated the effect of tool wear on ball nose tooling with the purpose of manufacturing sculptured surfaces in the micro-size regimen [16-19]. For example, the cutting tool deterioration was studied for the machining of micro-cavities for die and mold industry with the aim of performing a process calibration avoiding the tool wear which reduces the cutting tool diameter, dulls the cutting edge, decreases the rake angle and diminishes the surface quality of the machined surface [16]. Also, tool flank wear was analyzed in order to compare the MQL (Minimum Quantity Lubrication) cutting with dry cutting resulting in a 60% of tool wear reduction with MQL cutting [20]. This paper presents a study of the feasibility to directly manufacture microneedle arrays on AISI 316L stainless steel using micro milling process. Experimental tests were performed to evaluate tool wear, and its effect on surface finish and the geometrical features on the needles patches.

2. Materials and methods.

2.1. Microneedles design and CAM strategies

A base setup with cylinders was modelled on NX software (Siemens, USA). Separately, a conical needle was modelled with the same software. Two operations were programmed on NX software: The CAM strategy selected for the base setup was a planar mill setup with a path setting of mill finish and for the needles manufacture a streamline strategy was selected with the same path setting and a drive method in spiral. The dimensions of the stock cylinders are 1 mm in diameter by 1 mm in height. The arrays consisted of 9 cylinders separated by 2.5 mm between centers to


The surface roughness on the needle was measured with an Alicona focus variation microscope (Infinite focus XL200, Austria), which works through a noncontact optical principle with a 10x objective lens and a vertical resolution of 0.3 µm. This microscope was selected due to the feasibility to rotate the sample and to measure the surface roughness on the needle in the feed rate direction. Also, the geometrical features as length, tip diameter and base diameter were characterized on a scanning electron microscope (Carl Zeiss Evo Ma25, Germany).

Table 3. Cutting conditions Micro end milling tools Mitsubishi

MS2SBR0010S04 SGS 30303

Spindle speed 𝑁𝑁 (RPM) 30,000 23,000

Feed rate 𝑓𝑓𝑧𝑧 (mm/flute) 0.003 0.001

Radial depth of cut 𝑎𝑎𝑒𝑒 (mm) 0.060 0.200

Axial depth of cut 𝑎𝑎𝑝𝑝 (mm) 0.002 0.100

3. Results and discussion

3.1. Tool wear and surface roughness

The effect of lubrication on microneedles was studied in order to evaluate the influence of cooling on the surface finish and the geometrical accuracy. Figure 3 presents the results of the tool diameter reduction 𝐷𝐷𝑟𝑟 for the comparison between dry and wet cutting based on the calculation of the material removed.

Figure 3. Tool wear comparison between dry and wet cutting

Individual needles were fabricated sequentially until tool deterioration was observed or the surface finished

resulted on high values of surface roughness (see figure 4). With the use of the lubricant around six patches with 9 needles by array were produced before the tool wear affected the surface quality on the needles (i.e., average surface roughness values up to 2.5 µm). However, with dry cutting method a complete patch with 9 needles and 3 needles of a second patch were manufactured before the tool breakage. Also, in dry cutting regime was observed an increase in surface roughness compared with wet cutting. Figure 5 presents the ten-point mean roughness 𝑅𝑅𝑧𝑧 plot to illustrate the mean of five values of the highest profiles peaks and the 5 values of the deepest profile valleys.

Figure 4. Average surface roughness results

Figure 5. Ten-point mean roughness results. Some studies have related the surface quality with tool wear in micro milling. For example, a comparative study was performed on Aluminum 5052 using different chemical lubricants and dry cutting on


straight line paths. Their results reveal that the highest roughness is obtained on dry cutting, as expected, while surface tension on wet cutting has an effect on cleaning out the chips or adhering the chips on surface which increases the surface roughness [24]. In our study, the surface roughness increased in dry cutting after the manufacture of 9 needles while it stays constant until the fabrication of 36 needles using lubricant. For a reduction of 6% in cutting tool diameter, dry condition lasts for 9 needles while the wet condition generates 24 needles.

3.2. Cost evaluation

The total unit cost 𝐶𝐶𝑢𝑢 per patch (9 needles) comprises the costs associated to the use of machine tool, cutting tools and raw material, shown in Equation (2). Table 4 shows the unit cost estimation for dry and wet condition.

Table 4. Needle patch cost estimation

Factor Unit Dry Wet Setup time hr/patch 0.75 0.75

Machining time hr/patch 1.08 1.08 Machine hourly rate

(with labor) USD/hr $100.00 $100.00

Unit cost for machine tool (CM) USD/patch $183.00 $183.00

Ball nose end mill unit cost USD/tool $64.05 $64.05

Patches per tool patches/tool 1 6

Ball nose per patch USD/patch $64.05 $10.68

Flat end mill unit cost USD/tool $5.35 $5.35

Patches per tool patches/tool 3 3

Flat end mill per patch USD/patch $1.78 $1.78 Unit cost for cutting

tools (CT) USD/patch $65.83 $12.46

Unit cost for raw material (Craw) USD/patch $3.30 $3.30

Total unit cost per patch (Cu)

USD/patch $252.13 $198.76

In a related study, A.L. Jauregui et al. prototyped micro-channels, which are the main geometric features of microfluidics devices [25]. Their results reveal that with a simple geometry the micro end milling process provides appropriate quality and a competitive cost compared with using lithography-based master. Also, according to S. Sugiyama et al. [26], when a

microneedle patch is manufactured through LIGA process, the costs are divided into two aspects; the cost of the metal mould fabrication and the cost of the plastic structure. In their study, the total cost of the mould fabrication is about $6,000 USD which is admissible for achieving high productivity in industry.

3.3. Geometrical features

The assessment of the geometrical features on needles is of great importance for their use in the delivery of chemical compounds. The choice of material and the product geometry have a critical role in determining whether the microneedles will effectively penetrate the skin [27]. The tip diameter 𝐷𝐷𝑡𝑡 and base diameter 𝐷𝐷𝑏𝑏were measured one each needle. Figure 6 and figure 7 present the results for the tip diameter and the base diameter respectively. The use of the lubricant to machine the needles has a clear influence on the geometrical accuracy of the needle.

Figure 6. Needle tip diameter measurement. The difference between the geometrical features is

more evident on the needle tip. Values higher than 180 µm were found when needles were machined on dry regimen while values lower than 50 μm were obtained with the use of the lubricant. In particular, the base diameter was not affected by the different treatments. We attribute these dimensional differences to the micro size of the needle tip compared with the base dimension. Figure 8 presents the needle height results measured with the Alicona microscope, these measurements were done with Alicona microscopy based on the unmachined baseline.

𝐶𝐶𝑢𝑢 = 𝐶𝐶𝑀𝑀 + 𝐶𝐶𝑇𝑇 + 𝐶𝐶𝑟𝑟𝑟𝑟𝑟𝑟 (2)


Figure 7. Needle base diameter measurement

Figure 8. Needle height measurement Figure 9 and 10 present the SEM for dry and wet cutting respectively. Also, in parenthesis appears the number of the needle test. For dry cutting, Figure 9 presents some protuberances on the needle surface (i.e., 6 and 12 needle) while for wet cutting, Figure 10 illustrates needles free of burrs or slag.

Figure 9. Scanning electron microscopy of microneedles manufactured with dry cutting.

Further studies will be performed considering a sacrificial material layer to avoid burrs. Kou Z. et al proposed a method to control burrs and prevent plastic deformations by adding material at the edge of the workpiece [28]. Furthermore, studies will be performed on the use of MQL (Minimum quantity of lubrication) to explore the benefits of the treatment and the sequence of postprocessing stages as electropolishing process after machining in order to reduce the surface roughness on the needle surfaces.

Figure 10. Scanning electron microscopy of microneedles manufactured with wet cutting.

4. Conclusions

The conclusions can be summarized as follows:

• Conical needles were manufactured with CNC micro milling process with a base diameter and height of 1 mm by 1 mm.

• The material selection was based on commercial solid needles, which are fabricated through several manufacturing processes. Also, the geometrical features were selected according to our literature review.

• Tool wear, surface roughness and geometrical features were studied to analyze the influence of cutting regime (i.e., dry versus wet cutting).

• Tool breakage occurred after manufacturing 12 needles with dry cutting regimen while 54


needles were machined using a lubricant. Also, a tool diameter reduction was quantified after using both regimes. The results indicated a tool radius reduction of 6% manufacturing 24 needles and 9 needles using the wet and dry regime, respectively.

• Geometrical features resulted affected by the cutting regime, however it was greater in the needle tip diameter than needle height. Our findings reveal a maximum needle tip diameter of 188 μm and 53 μm in dry and wet regime respectively.

• Two surface roughness parameters were studied, the ten-point mean roughness and the average surface roughness. An increase of surface roughness was observed after milling 36 and 9 needles in wet and dry cutting respectively.

• A comparative cost was performed in order to estimate the needle patch cost. Our results indicate a total unit cost of 252 USD/patch and 199 USD/patch for dry and wet condition, respectively. Therefore, the micro milling is a competitive approach for prototyping stage, compared with common manufacturing process based on chemical dissolution and evaporation.

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