Fabrication of Mechanically Tunable and Bioactive Metal Scaffolds for Biomedical Applications

Summary

Bioactive and mechanically reliable metal scaffolds have been fabricated through a method which consists of two processes, dynamic freeze casting for the fabrication of porous Ti, and coating and densification of the Ti scaffolds. The densification process is simple, effective and applicable to the fabrication of functionally graded scaffolds.

Abstract

Biometal systems have been widely used for biomedical applications, in particular, as load-bearing materials. However, major challenges are high stiffness and low bioactivity of metals. In this study, we have developed a new method towards fabricating a new type of bioactive and mechanically reliable porous metal scaffolds-densified porous Ti scaffolds. The method consists of two fabrication processes, 1) the fabrication of porous Ti scaffolds by dynamic freeze casting, and 2) coating and densification of the porous scaffolds. The dynamic freeze casting method to fabricate porous Ti scaffolds allowed the densification of porous scaffolds by minimizing the chemical contamination and structural defects. The densification process is distinctive for three reasons. First, the densification process is simple, because it requires a control of only one parameter (degree of densification). Second, it is effective, as it achieves mechanical enhancement and sustainable release of biomolecules from porous scaffolds. Third, it has broad applications, as it is also applicable to the fabrication of functionally graded porous scaffolds by spatially varied strain during densification.

Introduction

While metallic biomaterials have been widely used as load-bearing implants and internal fixation devices because of their excellent mechanical strength and resilience,^1-3 they involve two critical challenges: 1) mechanical mismatch because metals are much stiffer than biological tissues, causing undesirable damages to the surrounding tissues and 2) low bioactivity that often results in poor interface with biological tissues, often provoking foreign body reactions (e.g., inflammation or thrombosis).^4-6 Porous metallic scaffolds have been proposed to promote bone ingrowth in the structures, improving bone-implant contact while the stress shield effects are suppressed because of their reduced stiffness.^7-9 Moreover, various surface modifications have been applied to enhance the biological activities of metallic implants; such modifications include coating the metal surface with bioactive molecules (e.g., growth factors) or drugs (e.g., vancomycin, tetracycline).^10-12 However, problems such as reduced mechanical properties of porous metal scaffolds, decreased stiffness and the fast release of the bioactive coating layers remain unresolved.^13-16

In particular, titanium (Ti) and Ti alloys are one of the most popular biometal systems because of their excellent mechanical properties, chemical stability, and good biocompatibility.^13,17-19 Their foam-shaped applications have also attracted increasing interest because the 3D porous networks promote bone ingrowth in addition to bone-like mechanical properties.^20-22 Efforts have been made to improve the mechanical properties by developing new manufacturing techniques including replication of polymeric sponge, sintering of metal particles, rapid prototyping (RP) method, and space holder method in order to control the various features of the pores (e.g., pore fraction, shape, size, distribution, and connectivity) and material properties (e.g., metallic phase and impurity).^23-25 Recently, the freeze casting of water-based metal slurry has gained considerable attention to produce mechanically enhanced Ti forms with well-aligned pore structures by utilizing the unidirectional ice dendrite growth during solidification; however, oxygen contamination caused by contact of metal powders with water requires special care to minimize the embrittlement of Ti scaffolds.^14,15

Therefore, we have developed a new approach towards fabricating bioactive and mechanically tunable porous Ti scaffolds.²⁵ The scaffolds initially have porous structures with a porosity of more than 50%. The fabricated porous scaffolds were coated with bioactive molecules and then compressed using a mechanical press during which the final porosity, mechanical properties and drug release behavior were controlled by the applied strain. The densified porous Ti implants have shown low porosity with good strength in spite of the low stiffness comparable to that of bone (3-20 GPa).² Because of the coating layer, the bioactivity of the densified porous Ti was significantly improved. Moreover, because of the unique flat pore structures induced by the densification process, the coated bioactive molecules were seen to be gradually released from the scaffold, maintaining their efficacy for a prolonged period.

In this study, we introduced our established method to fabricate densified porous Ti scaffolds for potential use in biomedical applications. The protocol includes dynamic freezing casting with metal slurries and densification of porous scaffolds. First, to fabricate porous Ti scaffolds with good ductility the dynamic freeze casting method was introduced as shown in Figure 1A. Ti powder was dispersed in liquid camphene; then, by decreasing the temperature, the liquid phase was solidified, resulting in the phase separation between the Ti powder network and solid camphene crystals. Subsequently, the solidified Ti-camphene green body was sintered in which Ti powders were condensed with continuous Ti struts, and the camphene phase was completely removed to obtain a porous structure. The coating and densification process with the obtained porous scaffolds was employed, varying the degree of densification and initial porosity. The coating layer and its release behavior were visualized and quantified using the green fluorescent protein (GFP)-coated porous Ti with and without densification compared to the GFP-coated dense Ti. Finally, functionally graded Ti scaffolds that have two different porous structures were proposed and demonstrated by varying the degree of densification of the inner and outer parts of the porous scaffolds.

Protocol

1. Fabrication of Porous Metal Scaffolds

1. Prepare Ti-camphene slurries by mixing commercially available Ti powder, camphene, and KD-4 after weighing the appropriate amounts of materials as described in Table 1 for porous Ti scaffolds with four initial porosities (40, 50, 60, and 70). Pour the slurries into 500 ml polyethylene (PE) bottles and rotate the bottles at 55 °C for 30 min in a ball-mill oven at 30 rpm.
2. Pour the slurries from the PE bottles into cylindrical aluminum (Al) molds with a diameter of 60 mm and a height of 60 mm. Seal each Al mold with the corresponding Al cover slip and rotate the molds in a ball-mill oven at a speed of 30 rpm at 55 °C for 10 min.
   1. Subsequently, decrease the temperature of the ball-mill oven to 44 °C, and continuously rotate the molds at a speed of 30 rpm at the constant temperature of 44 °C for 12 hr.
3. Take out the mold from the ball-mill oven after additionally rotating the molds at RT for 1 hr for the cooling process. Remove the solidified titanium/camphene green body from the Al mold using an Al plunger.
4. Place the solidified titanium/camphene green body in a rubber bag by hand and completely seal the rubber bag by tying the mouth of the bag with a string. Place the rubber bag in the water tank of a cold isostatic pressing (CIP) machine and apply an isostatic pressure of 200 MPa for 10 min. Remove the compressed green body from the rubber bag.
5. Transfer the Ti-camphene green body on to an alumina crucible by hand and place the crucible in the freeze-dryer machine. Freeze-dry the green body to sublimate the camphene phase in the green body at - 40 °C for 24 hr.
6. Subsequently, close the crucible with an alumina cover slip and place the closed crucible in a vacuum furnace (below 10^-6 Torr) at RT. Then, increase the temperature of the furnace to 1,300 °C at a heating rate of 5 °C/min and hold the temperature at 1,300 °C for 2 hr.
7. After the heat treatment, keep the sintered porous Ti in the furnace for 6-7 hr until the furnace is fully cooled to RT.
   Note: During 6 hr of the cooling process, the average cooling rate of the furnace above 400 °C is ~15 °C/min and the average cooling rate of the furnace below 400 °C is ~2 °C/min.
8. If necessary, cut the block of sintered porous Ti into disc-shaped samples with a diameter of 16 mm through electrical discharge machining (EDM).²⁷
   Note: Depending on the size of the Al molds, the size of the sintered porous Ti needs to be modified through the machining process (Figure 2A).
9. Place a glass beaker with the porous Ti samples in an autoclave and sterilize the samples at 121 °C for 15 min. Remove the samples from the autoclave. Wash the porous Ti samples with distilled water twice and then with 70% ethanol twice. Finally, leave the porous Ti into a Petri dish and air-dry the samples at RT on a clean bench under UV light.

2. Dip Coating of Scaffolds with Bioactive Agents

1. Dilute the commercial Green Fluorescence Protein (GFP) from 1 mg/ml to 100 μg/ml in a clean bench by mixing 1 ml of GFP with 9 ml of Dulbecco’s Phosphate Buffered Saline (DPBS, pH 7.4) solution in a 10 ml-sterilized polystyrene (PS) tube as indicated in Table 1.
2. Immerse the sterilized dense or porous Ti in 10 ml of diluted GFP solution (100 µg/ml) by placing the Ti samples into the PS tube with the GFP solution at RT and placing on a clean bench.
3. Place the PS tube in a vacuum desiccator and evacuate the desiccator for 10 min to ensure the GFP solution penetrates the pores of the porous Ti more effectively.
4. Remove the porous titanium from the PS tube using tweezers. Place the GFP-coated porous Ti into a 10 cm diameter Petri dish and air-dry O/N at RT on a clean bench.
5. Rinse the porous Ti twice with 10 ml of Dulbecco's Phosphate Buffered Saline (DPBS) in a glass beaker, and move the porous Ti into a 10 cm diameter Petri dish using tweezers and air-dry at RT on a clean bench.

3. Densification of Porous Scaffolds

1. Place the GFP-coated porous Ti samples with various heights in a cylindrical steel die, and insert a set of punches into the top and bottom holes of the steel die (Figure 3A).
2. Compress the porous Ti within the steel die assembly at RT in the z direction of the sample (Figure 3A) using a press machine at intermediate strain rates of 0.05 ~ 0.1 sec^-1 against the predetermined applied strains shown in Table 2. Hold the pressure for 1 min before unloading.
3. Remove the densified Ti samples from the steel die. Wash the densified samples twice with 10 ml of DPBS in a beaker and air-dry O/N at RT on a clean bench.

4. Release Test of GFP-coated Scaffolds

1. Immerse three types of specimens (GFP-coated dense Ti (after steps 2), GFP-coated porous Ti (after steps 1 and 2) and GFP-coated densified porous Ti (after steps 1-3)) in 5 ml DPBS (pH 7.4) solution contained in a 10 ml sterilized PS tube at 37 °C on a clean bench.
2. Suction out all the DPBS solution from each PS tube with the GFP-coated sample and replenish with a new 5 ml DPBS solution (pH 7.4) using a pipette according to the predetermined times of 1, 2, 3, 5, 8, 12, 15, 22 and 29 days after immersion.
3. Take the fluorescence images of the GFP-coated samples before immersion (day 0) and after 22 day-immersion using confocal laser scanning spectroscopy (CLSM).
4. Measure the fluorescence signal intensity of the released GFP in 1 ml solution from a total of 5 ml DPBS solution drawn from each PS tube in section 4.2 using UV spectroscopy at a wavelength of 215 nm. Convert the intensity value into the concentration of the GFP solution using the standard curve.
   Note: Before the measurement, draw the standard curve of GFP solution by measuring the fluorescence signal intensity of the GFP solution in the concentration range of 0 ng/ml - 10 µg/ml.

5. Fabrication of Graded Porous Ti Scaffolds

1. Produce a block of the sintered porous Ti by repeating step 1.1 to step 1.7.
2. Machine the sintered porous Ti block according to the predetermined structure designs (e.g., Figure 5a and 5d) by EDM.
3. Place the machined Ti samples with height distribution in a steel die where the diameter of porous Ti is ~0.1 mm smaller than the diameter of the die and insert a set of punches into the top and bottom holes of the steel die.
4. Perform steps 3.2 and 3.3.

6. Porosity Measurement of Ti Scaffolds

1. Measure the mass (m_s) of Ti scaffolds.
2. Calculate the apparent volume (V_s) by measuring length, width and height of Ti scaffolds.
3. Compute the porosity using the following equation:
    
   where P is the total porosity percentage, ρ_Ti is the theoretical density of the titanium and m_S/V_S is the measured density of the sample.
   Note: The porosity of Ti samples can be directly retrieved from microCT images after microCT imaging is carried out using a micro-computed tomography scanner.

Results

The fabrication process used to produce porous Ti scaffolds is illustrated in Figure 1A. Ti powder is kept dispersed homogeneously in camphene by continuous rotation of the container at 44 °C for 12 hr and, while liquid camphene is fully solidified, any sediments of relatively heavy Ti powder can be minimized. As a result, the homogeneous Ti-camphene green body was produced using the dynamic freeze casting process as shown in Figure 1B, in which 3-dimensionally interconnected large camph...

Discussion

While biometal systems have been widely used for biomedical applications, particularly, as load-bearing materials, high stiffness and low bioactivity of metals have been regarded as major challenges. In this study, we established the fabrication method of a new metal system, a densified porous metal scaffold which has biomimetic mechanical properties as well as bioactive surface with sustainable release behavior. The major advantages of our fabrication method include 1) no change in the previous dynamic freezing casting ...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Titanium powder	Alfa Aesar	#42624	-325 mesh, 99.5% (metals basis)
Camphene	SigmaAldrich	#456055	95%, C10H16
KD-4	Croda	­	Hypermer, polymeric dispersant
Phosphate Buffer Solution (PBS)	Welgene	ML 008-01	­
Green Fluorescent Protein (GFP)	Genoss Co.	-	>98% purity, 1 mg/ml
Ball mill oven	SAMHENUG ENERGY	SH-BDO150	­
Freeze dryer	Ilshin Lab.	PVTFD50A	­
Cold isostatic pressing (CIP) machine	SONGWON SYSTEMS	CIP 42260	­
Vaccum furnace	JEONG MIN INDUSTRIAL	JM-HP20	­
electical chaege machine	FANUC robocut	0iB	External use
Press machine	CG&S	AJP-200	­
Confocal laser scanning spectroscopy (CLSM)	Olympus	FluoView FV1000	External use