Micro-robotic deposition guidelines by a design of experiments approach to maximize fabrication reliability for the bone scaffold application

doi:10.1016/j.actbio.2008.02.018

Acta Biomaterialia

Volume 4, Issue 4, July 2008, Pages 897-912

https://doi.org/10.1016/j.actbio.2008.02.018 Get rights and content

Abstract

This work aims to facilitate the transition of micro-robotic deposition (μRD) technology from the research bench to a mass manufacturing environment. The bone scaffolding application is targeted; however, the evaluation process developed is applicable to multiple colloidal material systems, length scales, and structure architectures. A design of experiments (DoE) approach is used to develop statistical correlations between three manufacturing treatments (material calcination time, nozzle size, and deposition speed) and defined reliability metrics. All three selected treatments have a significant effect on structure quality. A longer material calcination time improves the deposition of internal features. Logically, a larger nozzle size decreases structural defects. However, an unexpected result is revealed by this study. Higher deposition speeds are shown to either significantly improve or have no effect on structure quality, permitting a decrease in manufacturing time without adverse consequences.

Introduction

Micro-robotic deposition (μRD) is a solid freeform fabrication process in which a colloidal suspension, or ink, is extruded through a micron-sized nozzle in a defined trajectory to form a three-dimensional structure (Fig. 1). The term micro-robotic deposition is chosen since the extrusion nozzle is usually positioned by a robotic device with resulting part feature sizes between 1 and 1000 μm. The extruded ink forms semisolid rods that can span distances of up to 2 mm [1], which permits the fabrication of porous structures without lost molds. μRD technology has been applied to composite [2], [3], microfluidic [4], photonic bandgap [5], and tissue engineering [6], [7], [8] structures and is referred to in the literature as robocasting [6], [8], [9], robotic deposition [1], [4], direct-write assembly [4], [7], or slurry micro-extrusion [10]. To date, the majority of the μRD research has focused on developing new materials appropriate for deposition [1], [7], [11], [12], [13] as well as decreasing the feature size [5]. In order to make μRD a reliable and viable manufacturing process the number and severity of fabrication defects must be reduced. However, deposition reliability has received relatively little research attention. The work presented here fills this gap in the literature and will help facilitate the transition of μRD technology from the research bench to a manufacturing environment by developing general guidelines that maximize process reliability.

Although μRD is capable of fabricating structures with submicron resolution [5], many applications require structures that are macro-sized in total dimension with micron-sized features. Of primary interest here are artificial bone scaffolds that are large enough to fill anatomical defects, yet have a controlled porous microstructure appropriate for bone cell proliferation [14]. While the material and structure for this study are chosen specifically for use as artificial bone scaffolds, the procedures and results are relevant to the μRD of colloids in general, including micro-sized structures [9], piezoelectric actuators [2], and dental implants [10]. Commonly, the quality of the finished part is compromised by a variety of defects derived from either a momentary loss in material flow, a clogged nozzle, or a material deformation. Each of these types of defects reduces part quality and must be minimized in order to make μRD technology a reliable manufacturing process. To that end, a design of experiments (DoE) is devised that analyzes which combination of manufacturing treatment levels yields the highest μRD process quality.

There are a number of treatment options that may potentially affect the quality of finished μRD parts. For this study, only treatments that directly affect the rate of manufacture and those that are appropriate for micro-sized features are considered. While deterministic of structure quality, colloidal ink fabrication treatments such as colloidal solids loading, dispersant concentration, and pH, have been researched previously [1], [11], [13] and therefore are kept constant here in order to isolate the effects of the selected manufacturing treatments. A 2 × 2 × 3 full factorial DoE examines the effects of two calcination time (CT), two nozzle size (NS), and three deposition speed (DS) treatment levels on structure quality. The test structure has a lattice architecture, consisting of alternating layers of orthogonal rods, which is a common architecture found in bone scaffolding [15], [16] and has been used as a benchmark in previous μRD research [1], [12].

The specific treatment levels chosen satisfy at least one of two criteria: (1) they must have been proven to work well, either by experience [17] or the published literature [1], [7], to specifically focus on optimizing the μRD process or (2) they must have the potential to decrease fabrication cost. The following arguments justify the chosen treatment levels. Ceramic powder calcination is a time-consuming and expensive manufacturing step that smoothes the particle morphology. However, a smoother particle morphology improves particle consolidation and green body density [18] and the added costs of extending CT may significantly improve the finished product enough to justify the cost. Particle morphology at four CT treatment levels (1/2, 2, 10, and 20 h) is analyzed and of these four, two (1/2 and 10 h) are selected for the DoE to study their effect on deposition defects. The two NS treatment levels (250 and 410 μm internal diameter) chosen reflect the upper and lower limits of feature sizes typically found in bone scaffold structures [7], [15]. The DS affects the part fabrication rate and therefore manufacturing cost. The three DS treatment levels include speeds that have been commonly used in μRD bone scaffold fabrication (5 and 10 mm s⁻¹) [19] as well as one higher speed (15 mm s⁻¹) that, if it could be used successfully, would reduce fabrication time and cost.

To analyze the effects of the treatment variables, it is important to have quantitative metrics for evaluation. Here we define and quantify structural quality using the five weighted cost functions, or dependent variables, that are described in detail in Section 2.3. A multivariate analysis of variance (ANOVA) and three complementary statistical tests are used to evaluate which treatments significantly affect the dependent variables. The DoE presented provides statistical correlations between μRD process inputs (i.e. treatment variables) and outputs (i.e. dependent variables). From these correlations, inferences on the mechanisms governing deposition are made.

The following outlines the content of the paper. Section 2 provides details on the materials and instruments used, the DoE setup, deposition procedure, defect quantification method, and statistical analysis. Section 3 presents material processing, lattice deposition, and statistical results. Section 4 discusses the experimental results, analyzing the correlations between manufacturing treatments and part quality and postulating possible mechanisms that govern the process. Section 5 includes a short experiment summary and concludes with resulting insight gained.

Section snippets

Powder processing and characterization

Hydroxyapatite (HA) powder (Riedel-de Haen lot 50270) was used as the solid phase material in the colloidal ink. To smoothen the surface morphology, HA powder was calcined in batches (Electric furnace, Paragon Industries TNFQ11A) at 1100 °C for 1/2, 2, 10, or 20 h and furnace cooled to room temperature. Calcined powder was subsequently ball milled in ethanol with cylindrical alumina media (9 mm × 14 mm) for 13 h. The morphology of the as-received, calcined, and calcined and ball milled powders were

Powder and ink characterization

As-received HA powder has a rough surface morphology (Fig. 2a) and consequently a relatively high SSA of 67.49 m² g⁻¹ (Fig. 3a). The SEM images (Fig. 2) show the evolution of the surface morphology over the range of CTs tested. Surface morphology improves markedly between the as-received and 1/2 h CT powders (Fig. 2b). The particle surface continues to smooth as CT increases, until a 10 h CT, after which there is little noticeable improvement. Measured morphological data are shown in Fig. 3. Here

Discussion

The primary intention of this research is to develop general deposition guidelines that will aid the transition of μRD technology from the lab bench to a mass manufacturing environment. The general guidelines are as follows: to achieve maximum deposition reliability within the defined ranges, powder CT should be extended to sufficiently smoothen particle morphology, the largest NS allowable by the application should be selected, and DS should be sufficiently high. Although these guidelines are

Summary and conclusions

Research efforts have expanded the number of materials appropriate for μRD and have decreased feature sizes to submicron scales [36]. However, there has been less focus on assessing the process reliability for larger structures fabricated by μRD despite the many important applications. This research utilized a DoE approach to determine which manufacturing treatments maximize μRD process reliability for the fabrication of HA artificial bone scaffolds. Although there are many manufacturing

Acknowledgements

The authors would like to acknowledge Dr. Adam Martinsek for statistics consultation, Ranjeet Rao for his help with rheometry, Danchin Chen for his work with XRD, and Amanda Hilldore for her work with Micro-CT. Additionally, we would like to acknowledge the Beckman Institute, Center for Cement Composite Materials, Colloidal Assembly Group, and the Frederick Seitz Materials Research Laboratory Center for Microanalysis of Materials for the use of their characterization facilities. This work was

References (36)

S. Michna et al.
Concentrated hydroxyapatite inks for direct-write assembly of 3-D periodic scaffolds
Biomaterials
(2005)
P. Miranda et al.
Sintering and robocasting of β-tricalcium phosphate scaffolds for orthopaedic applications
Acta Biomater
(2006)
J.N. Stuecker et al.
Control of the viscous behavior of highly concentrated mullite suspensions for robocasting
J Mater Process Technol
(2003)
J. Wang et al.
Rheological and extrusion behavior of dental porcelain slurries for rapid prototyping applications
Mater Sci Eng A
(2005)
L. Moroni et al.
3D fiber-deposited scaffolds for tissue engineering: influence of pores geometry and architecture on dynamic mechanical properties
Biomaterials
(2006)
I.M. Krieger
Rheology of monodisperse lattices
Adv Colloid Interf Sci
(1972)
J.E. Smay et al.
Colloidal inks for directed assembly of 3-D periodic structures
Langmuir
(2002)
J.E. Smay et al.
Piezoelectric properties of 3-X periodic Pb(Zr_xTi_1 − x)O₃-polymer composites
J Appl Phys
(2002)
G. He et al.
Robocasting and cofiring of functionally graded Si₃N₄–W materials
Ceram Eng Sci Proc
(2001)
D. Therriault et al.
Chaotic mixing in three-dimensional microvascular networks fabricated by direct-write assembly
Nat Mater
(2003)

G.M. Gratson et al.

Direct writing of three-dimensional webs

Nature

(2004)

J. Cesarano et al.

Customization of load-bearing hydroxyapatite lattice scaffolds

Int J Appl Ceramic Technol

(2005)

J. Cesarano et al.

Robocasting provides moldless fabrication from slurry deposition

Ceram Ind

(1998)

J. Wang et al.

Solid freeform fabrication of permanent dental restorations via slurry micro-extrusion

J Am Ceram Soc

(2006)

Q. Li et al.

Nanoparticle inks for directed assembly of three-dimensional periodic structures

Adv Mater

(2003)

K.A. Hing

Bioceramic bone graft substitutes: influence of porosity and chemistry

Int J Appl Ceram Technol

(2005)

T.-M.G. Chu et al.

Hydroxyapatite implants with designed internal architecture

J Mater Sci: Mater Med

(2001)

D.J. Hoelzle

Reliability guidelines and flowrate modulation for a micro robotic deposition system

(2007)

Cited by (33)

Medical applications of polymer/functionalized nanoparticle composite systems, renewable polymers, and polymer-metal oxide composites
2022, Renewable Polymers and Polymer-Metal Oxide Composites: Synthesis, Properties, and Applications
Bone tissue engineering (BTE) approaches are focused on repairing fractured or damaged bones. Exciting technologies have been developed to repair and regenerate damaged tissues and organs by the implantation of biopolymers’ composite scaffolds. The hierarchical nature of bone is an inspiration for nanostructured biomaterials, and nanostructured composite scaffolds are gaining special consideration for BTE. These scaffolds are fabricated from biopolymers and bioactive nanofillers and have biodegradability, good mechanical properties, and ease of formation characteristics necessary for osteogenesis. Biopolymers resemble extracellular matrix (ECM) with different levels of biodegradability and better biological qualities, and fillers like hydroxyapatite play a vital role in enhancing biocompatibility, restoration, and osteoconduction. Biopolymers in microspheres, nanofibers, tough foams, and hydrogels can be used to create a wide range of scaffolds. This review will discuss biopolymer-based nanocomposite materials for their design and properties. A detailed review and discussion of the current status and surface engineering of biopolymers and their applications in BTE and their feasibility of production and future applications have also been presented.
Processes and materials used for direct writing technologies: A review
2021, Results in Engineering
Direct Writing (DW), also known as Robocasting, is an extrusion-based layer-by-layer manufacturing technique suitable for manufacturing complex geometries. Different types of materials such as metals, composites, ceramics, biomaterials, and shape memory alloys can be used for DW. The simplicity and cost-efficiency of DW makes it convenient for different applications, from biomedical to optics. Recent studies on DW show a tendency towards the development of new materials and applications. This represents the necessity of a deep understanding of the principles and parameters of each technique, material, and process challenge. This review highlights the principles of many DW techniques, the recent advancements in material development, applications, process parameters, and challenges in each DW process. Since the quality of the printed parts by DW highly depend on the material extrusion, the focus of this review is mainly on the ceramic extrusion process and its challenges from rheological and material development point of view. This review delivers an insight into DW processes and the challenges to overcome for development of new materials and applications. The main objective of the review is to deliver necessary information for non-specialist and interdisciplinary researchers.
Factors governing the dimensional accuracy and fracture modes under compression of regular and shifted orthogonal scaffolds
2020, Journal of the European Ceramic Society
Direct ink writing allows controlling the structure of tissue engineering scaffolds. The aim of this work was to study the effects of the in silico pattern design on the dimensional accuracy of tricalcium phosphate scaffolds and of the loading direction on the compressive strength and fracture modes. Regular and shifted scaffolds showed anisotropic shrinkage during drying and isotropic shrinkage during sintering. Shifted layers induced bending stresses when compressed longitudinally, resulting in lower compressive strength. The transverse Young’s modulus and compressive strength were higher than the longitudinal ones for both regular and shifted structures. Strand deflections were more pronounced in shifted structures. Finite element modelling suggested that such deflections considerably increased the effective transverse modulus of the shifted scaffolds, thus increasing the related compressive strength. In conclusion, shifted scaffolds performed similarly to regular ones in the transverse direction but were less mechanically reliable under the longitudinal direction.
Mineralization in micropores of calcium phosphate scaffolds
2019, Acta Biomaterialia
Citation Excerpt :
Those traditional techniques offer little control over the pore size or shape, either at the macro- or at the microscale. The recent development of fabrication techniques using additive manufacturing, or 3D printing, and the use of sacrificial porogens allow better control of the size, fraction and interconnectivity of both macropores (Fig. 1b) and micropores (Fig. 1d) [36,37,96]. More control of macro- and micropore characteristics allows for better design and exploitation of scaffold porosity at the macro- and microscale.
With the increasing demand for novel bone repair solutions that overcome the drawbacks of current grafting techniques, the design of artificial bone scaffolds is a central focus in bone regeneration research. Calcium phosphate scaffolds are interesting given their compositional similarity with bone mineral. The majority of studies focus on bone growth in the macropores (>100 µm) of implanted calcium phosphate scaffolds where bone structures such as osteons and trabeculae can form. However, a growing body of research shows that micropores (<50 µm) play an important role not only in improving bone growth in the macropores, but also in providing additional space for bone growth. Bone growth in the micropores of calcium phosphate scaffolds offers major mechanical advantages as it improves the mechanical properties of the otherwise brittle materials, further stabilizes the implant, improves load transfer, and generally enhances osteointegration. In this paper, we review evidence in the literature of bone growth into micropores, emphasizing on identification techniques and conditions under which bone components are observed in the micropores. We also review theories on mineralization and propose mechanisms, mediated by cells or not, by which mineralization may occur in the confined micropore space of calcium phosphate scaffolds. Understanding and validating these mechanisms will allow to better control and enhance mineralization in micropores to improve the design and efficiency of bone implants.
The design of synthetic bone scaffolds remains a major focus for engineering solutions to repair damaged and diseased bone. Most studies focus on the design of and growth in macropores (>100 µm), however research increasingly shows the importance of microporosity (<50 µm). Micropores provide an additional space for bone growth, which provides multiple mechanical advantages to the scaffold/bone composite. Here, we review evidence of bone growth into micropores in calcium phosphate scaffolds and conditions under which growth occurs in micropores, and we propose mechanisms that enable or facilitate growth in these pores. Understanding these mechanisms will allow researchers to exploit them and improve the design and efficiency of bone implants.
Accelerated hardening of nanotextured 3D-plotted self-setting calcium phosphate inks
2018, Acta Biomaterialia
Direct ink writing (DIW) techniques open up new possibilities for the fabrication of patient-specific bone grafts. Self-setting calcium phosphate inks, which harden at low temperature, allow obtaining nanostructured scaffolds with biomimetic properties and enhanced bioactivity. However, the slow hardening kinetics hampers the translation to the clinics. Different hydrothermal treatments for the consolidation of DIW scaffolds fabricated with an α-tricalcium phosphate /pluronic F127 ink were explored, comparing them with a biomimetic treatment. Three different scaffold architectures were analysed. The hardening process, associated to the conversion of α-tricalcium phosphate to hydroxyapatite was drastically accelerated by the hydrothermal treatments, reducing the time for complete reaction from 7 days to 30 minutes, while preserving the scaffold architectural integrity and retaining the nanostructured features. β-tricalcium phosphate was formed as a secondary phase, and a change of morphology from plate-like to needle-like crystals in the hydroxyapatite phase was observed. The binder was largely released during the treatment. The hydrothermal treatment resulted in a 30% reduction of the compressive strength, associated to the residual presence of β-tricalcium phosphate. Biomimetic and hydrothermally treated scaffolds supported the adhesion and proliferation of rat mesenchymal stem cells, indicating a good suitability for bone tissue engineering applications.
3D plotting has opened up new perspectives in the bone regeneration field allowing the customisation of synthetic bone grafts able to fit patient-specific bone defects. Moreover, this technique allows the control of the scaffolds’ architecture and porosity. The present work introduces a new method to harden biomimetic hydroxyapatite 3D-plotted scaffolds which avoids high-temperature sintering. It has two main advantages: i) it is fast and simple, reducing the whole fabrication process from the several days required for the biomimetic processing to a few hours; and ii) it retains the nanostructured character of biomimetic hydroxyapatite and allows controlling the porosity from the nano- to the macroscale. Moreover, the good in vitro cytocompatibility results support its suitability for cell-based bone regeneration therapies.
Microstructural control of modular peptide release from microporous biphasic calcium phosphate
2017, Materials Science and Engineering C
Citation Excerpt :
To make the “ink” for deposition, HA powder is combined with additives in order to tune the rheological properties of the ink for printing. The mass of HA powder in a batch determines the specific amount of additives, which include deionized (DI) water, an anti-foaming reagent, gelling reagents, and surfactants [16,17]. The viscosity of the ink is adjusted by adding the base NH4OH until the ink is viscous enough to hold its shape when extruded.
Drug release from tissue scaffolds is commonly controlled by using coatings and carriers, as well as by varying the binding affinity of molecules being released. This paper considers modulating synthetic peptide incorporation and release through the use of interconnected microporosity in biphasic calcium phosphate (BCP) and identifies the microstructural characteristics important to the release using experiments and a model of relative diffusivity. First, the release of three modular peptides designed to include an osteocalcin-inspired binding sequence based on bone morphogenic protein-2 (BMP-2) was compared and one was selected for further study. Next, the incorporation and release of the peptide from four types of substrates were compared: non-microporous (NMP) substrates had no microporosity; microporous (MP) substrates were either 50% microporous with 5 μm pores (50/5), 60% microporous with 5 μm pores (60/5), or 50% microporous with 50 μm pores (50/50). Results showed that MP substrates incorporated significantly more peptide than NMP ones, but that the three different microporous substrates all incorporated the same total amount of peptide. NMP had a markedly lower release rate compared to each of three of the MP samples, though the initial burst release was the highest. The initial release and the release rate for the 60/5 samples were different from the 50/50, though they were not statistically different from the 50/5. The model indicated that the pore interconnection to pore size ratio, affecting the constriction between pores, had the greatest influence on the calculated relative diffusivity. While the model was consistent with the trends observed experimentally, the quantitative experimental results suggested that to attain an appreciable difference in release characteristics, both pore size and pore fraction should be changed for this system. These results contribute to rational scaffold design by showing that microstructure, specifically microporosity, can be used to modulate drug release.

View all citing articles on Scopus

View full text

Micro-robotic deposition guidelines by a design of experiments approach to maximize fabrication reliability for the bone scaffold application

Abstract

Introduction

Section snippets

Powder processing and characterization

Powder and ink characterization

Discussion

Summary and conclusions

Acknowledgements

Biomaterials

Acta Biomater

J Mater Process Technol

Mater Sci Eng A

Biomaterials

Adv Colloid Interf Sci

Colloidal inks for directed assembly of 3-D periodic structures

Langmuir

Piezoelectric properties of 3-X periodic Pb(ZrxTi1 − x)O3-polymer composites

J Appl Phys

Robocasting and cofiring of functionally graded Si3N4–W materials

Ceram Eng Sci Proc

Chaotic mixing in three-dimensional microvascular networks fabricated by direct-write assembly

Nat Mater